[devel] Fixes to rgb_to_gray and cHRM XYZ APIs

ANNOUNCE

@@ -1,5 +1,5 @@
 
-Libpng 1.5.5beta06 - August 17, 2011
+Libpng 1.5.5beta06 - August 25, 2011
 
 This is not intended to be a public release.  It will be replaced
 within a few weeks by a public version or by another test version.
@@ -62,9 +62,22 @@ Version 1.5.5beta05 [August 17, 2011]
   Added new types and internal functions for CIE RGB end point handling to
     pngpriv.h (functions yet to be implemented).
 
-Version 1.5.5beta06 [August 17, 2011]
+Version 1.5.5beta06 [August 25, 2011]
   Ensure the CMAKE_LIBRARY_OUTPUT_DIRECTORY is set in CMakeLists.txt
     (Clifford Yap)
+  Fixes to rgb_to_gray and cHRM XYZ APIs (John Bowler):
+    The rgb_to_gray code had errors when combined with gamma correction.
+    Some pixels were treated as true grey when they weren't and such pixels
+    and true grey ones were not gamma corrected (the original value of the
+    red component was used instead).  APIs to get and set cHRM using color
+    space end points have been added and the rgb_to_gray code that defaults
+    based on cHRM (introduced in 1.5.4) has been corrected.   A considerable
+    number of tests has been added to pngvalid for the rgb_to_gray transform.
+    Arithmetic errors in rgb_to_gray whereby the calculated gray value was
+    truncated to the bit depth rather than rounded have been fixed except in
+    the 8-bit non-gamma-corrected case (where consistency seems more important
+    than correctness.)  The code still has considerable inaccuracies in the
+    8-bit case because 8-bit linear arithmetic is used.
 
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net:
 (subscription required; visit

CHANGES

@@ -3546,9 +3546,22 @@ Version 1.5.5beta05 [August 17, 2011]
   Added new types and internal functions for CIE RGB end point handling to
     pngpriv.h (functions yet to be implemented).
 
-Version 1.5.5beta06 [August 17, 2011]
+Version 1.5.5beta06 [August 25, 2011]
   Ensure the CMAKE_LIBRARY_OUTPUT_DIRECTORY is set in CMakeLists.txt
     (Clifford Yap)
+  Fixes to rgb_to_gray and cHRM XYZ APIs (John Bowler):
+    The rgb_to_gray code had errors when combined with gamma correction.
+    Some pixels were treated as true grey when they weren't and such pixels
+    and true grey ones were not gamma corrected (the original value of the
+    red component was used instead).  APIs to get and set cHRM using color
+    space end points have been added and the rgb_to_gray code that defaults
+    based on cHRM (introduced in 1.5.4) has been corrected.   A considerable
+    number of tests has been added to pngvalid for the rgb_to_gray transform.
+    Arithmetic errors in rgb_to_gray whereby the calculated gray value was
+    truncated to the bit depth rather than rounded have been fixed except in
+    the 8-bit non-gamma-corrected case (where consistency seems more important
+    than correctness.)  The code still has considerable inaccuracies in the
+    8-bit case because 8-bit linear arithmetic is used.
 
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net
 (subscription required; visit

libpng-manual.txt

@@ -1,6 +1,6 @@
 libpng-manual.txt - A description on how to use and modify libpng
 
- libpng version 1.5.5beta06 - August 17, 2011
+ libpng version 1.5.5beta06 - August 25, 2011
  Updated and distributed by Glenn Randers-Pehrson
  <glennrp at users.sourceforge.net>
  Copyright (c) 1998-2011 Glenn Randers-Pehrson
@@ -11,7 +11,7 @@ libpng-manual.txt - A description on how to use and modify libpng
 
  Based on:
 
- libpng versions 0.97, January 1998, through 1.5.5beta06 - August 17, 2011
+ libpng versions 0.97, January 1998, through 1.5.5beta06 - August 25, 2011
  Updated and distributed by Glenn Randers-Pehrson
  Copyright (c) 1998-2011 Glenn Randers-Pehrson
 
@@ -4509,7 +4509,7 @@ Other rules can be inferred by inspecting the libpng source.
 
 XIV. Y2K Compliance in libpng
 
-August 17, 2011
+August 25, 2011
 
 Since the PNG Development group is an ad-hoc body, we can't make
 an official declaration.

libpng.3

@@ -1,4 +1,4 @@
-.TH LIBPNG 3 "August 17, 2011"
+.TH LIBPNG 3 "August 25, 2011"
 .SH NAME
 libpng \- Portable Network Graphics (PNG) Reference Library 1.5.5beta06
 .SH SYNOPSIS
@@ -128,10 +128,22 @@ libpng \- Portable Network Graphics (PNG) Reference Library 1.5.5beta06
 
 \fI\fB
 
-\fBpng_uint_32 png_get_chunk_cache_max (png_const_structp \fIpng_ptr\fP\fB);\fP
+\fBpng_uint_32 png_get_cHRM_XYZ (png_structp \fIpng_ptr,
+
+\fBpng_const_infop \fP\fIinfo_ptr\fP\fB, double \fP\fI*red_X\fP\fB, double \fP\fI*red_Y\fP\fB, double \fI*red_Z,
+
+\fBdouble \fP\fI*green_X\fP\fB, double \fP\fI*green_Y\fP\fB, double \fP\fI*green_Z\fP\fB, double \fI*blue_X,
+
+\fBdouble \fP\fI*blue_Y\fP\fB, double \fI*blue_Z\fP\fB);\fP
 
 \fI\fB
 
+\fBpng_uint_32 png_get_cHRM_XYZ_fixed (png_structp \fP\fIpng_ptr\fP\fB, png_const_infop \fP\fIinfo_ptr\fP\fB, png_fixed_point \fP\fI*int_red_X\fP\fB, png_fixed_point \fP\fI*int_red_Y\fP\fB, png_fixed_point \fP\fI*int_red_Z\fP\fB, png_fixed_point \fP\fI*int_green_X\fP\fB, png_fixed_point \fP\fI*int_green_Y\fP\fB, png_fixed_point \fP\fI*int_green_Z\fP\fB, png_fixed_point \fP\fI*int_blue_X\fP\fB, png_fixed_point \fP\fI*int_blue_Y\fP\fB, png_fixed_point \fI*int_blue_Z\fP\fB);\fP
+
+\fI\fB
+
+\fBpng_uint_32 png_get_chunk_cache_max (png_const_structp \fIpng_ptr\fP\fB);\fP
+
 \fI\fB
 
 \fBpng_alloc_size_t png_get_chunk_malloc_max (png_const_structp \fIpng_ptr\fP\fB);\fP
@@ -548,6 +560,16 @@ libpng \- Portable Network Graphics (PNG) Reference Library 1.5.5beta06
 
 \fI\fB
 
+\fBvoid png_set_cHRM_XYZ (png_structp \fP\fIpng_ptr\fP\fB, png_infop \fP\fIinfo_ptr\fP\fB, double \fP\fIred_X\fP\fB, double \fP\fIred_Y\fP\fB, double \fP\fIred_Z\fP\fB, double \fP\fIgreen_X\fP\fB, double \fIgreen_Y,
+
+\fBdouble \fP\fIgreen_Z\fP\fB, double \fP\fIblue_X\fP\fB, double \fP\fIblue_Y\fP\fB, double \fIblue_Z\fP\fB);\fP
+
+\fI\fB
+
+\fBvoid png_set_cHRM_XYZ_fixed (png_structp \fP\fIpng_ptr\fP\fB, png_infop \fP\fIinfo_ptr\fP\fB, png_fixed_point \fP\fIint_red_X\fP\fB, png_fixed_point \fP\fIint_red_Y\fP\fB, png_fixed_point \fP\fIint_red_Z\fP\fB, png_fixed_point \fP\fIint_green_X\fP\fB, png_fixed_point \fP\fIint_green_Y\fP\fB, png_fixed_point \fP\fIint_green_Z\fP\fB, png_fixed_point \fP\fIint_blue_X\fP\fB, png_fixed_point \fP\fIint_blue_Y\fP\fB, png_fixed_point \fIint_blue_Z\fP\fB);\fP
+
+\fI\fB
+
 \fBvoid png_set_chunk_cache_max (png_structp \fP\fIpng_ptr\fP\fB, png_uint_32 \fIuser_chunk_cache_max\fP\fB);\fP
 
 \fI\fB
@@ -955,7 +977,7 @@ Following is a copy of the libpng-manual.txt file that accompanies libpng.
 .SH LIBPNG.TXT
 libpng-manual.txt - A description on how to use and modify libpng
 
- libpng version 1.5.5beta06 - August 17, 2011
+ libpng version 1.5.5beta06 - August 25, 2011
  Updated and distributed by Glenn Randers-Pehrson
  <glennrp at users.sourceforge.net>
  Copyright (c) 1998-2011 Glenn Randers-Pehrson
@@ -966,7 +988,7 @@ libpng-manual.txt - A description on how to use and modify libpng
 
  Based on:
 
- libpng versions 0.97, January 1998, through 1.5.5beta06 - August 17, 2011
+ libpng versions 0.97, January 1998, through 1.5.5beta06 - August 25, 2011
  Updated and distributed by Glenn Randers-Pehrson
  Copyright (c) 1998-2011 Glenn Randers-Pehrson
 
@@ -5465,7 +5487,7 @@ Other rules can be inferred by inspecting the libpng source.
 
 .SH XIV. Y2K Compliance in libpng
 
-August 17, 2011
+August 25, 2011
 
 Since the PNG Development group is an ad-hoc body, we can't make
 an official declaration.
@@ -5724,7 +5746,7 @@ possible without all of you.
 
 Thanks to Frank J. T. Wojcik for helping with the documentation.
 
-Libpng version 1.5.5beta06 - August 17, 2011:
+Libpng version 1.5.5beta06 - August 25, 2011:
 Initially created in 1995 by Guy Eric Schalnat, then of Group 42, Inc.
 Currently maintained by Glenn Randers-Pehrson (glennrp at users.sourceforge.net).
 
@@ -5747,7 +5769,7 @@ this sentence.
 
 This code is released under the libpng license.
 
-libpng versions 1.2.6, August 15, 2004, through 1.5.5beta06, August 17, 2011, are
+libpng versions 1.2.6, August 15, 2004, through 1.5.5beta06, August 25, 2011, are
 Copyright (c) 2004,2006-2007 Glenn Randers-Pehrson, and are
 distributed according to the same disclaimer and license as libpng-1.2.5
 with the following individual added to the list of Contributing Authors
@@ -5846,7 +5868,7 @@ certification mark of the Open Source Initiative.
 
 Glenn Randers-Pehrson
 glennrp at users.sourceforge.net
-August 17, 2011
+August 25, 2011
 
 .\" end of man page
 

png.c

@@ -617,13 +617,13 @@ png_get_copyright(png_const_structp png_ptr)
 #else
 #  ifdef __STDC__
    return PNG_STRING_NEWLINE \
-     "libpng version 1.5.5beta06 - August 17, 2011" PNG_STRING_NEWLINE \
+     "libpng version 1.5.5beta06 - August 25, 2011" PNG_STRING_NEWLINE \
      "Copyright (c) 1998-2011 Glenn Randers-Pehrson" PNG_STRING_NEWLINE \
      "Copyright (c) 1996-1997 Andreas Dilger" PNG_STRING_NEWLINE \
      "Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc." \
      PNG_STRING_NEWLINE;
 #  else
-      return "libpng version 1.5.5beta06 - August 17, 2011\
+      return "libpng version 1.5.5beta06 - August 25, 2011\
       Copyright (c) 1998-2011 Glenn Randers-Pehrson\
       Copyright (c) 1996-1997 Andreas Dilger\
       Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.";
@@ -713,18 +713,9 @@ png_access_version_number(void)
 
 
 #if defined(PNG_READ_SUPPORTED) || defined(PNG_WRITE_SUPPORTED)
-#  ifdef PNG_SIZE_T
+/* png_convert_size: a PNGAPI but no longer in png.h, so deleted
-/* Added at libpng version 1.2.6 */
+ * at libpng 1.5.5!
-   PNG_EXTERN png_size_t PNGAPI png_convert_size PNGARG((size_t size));
+ */
-png_size_t PNGAPI
-png_convert_size(size_t size)
-{
-   if (size > (png_size_t)-1)
-      PNG_ABORT();  /* We haven't got access to png_ptr, so no png_error() */
-
-   return ((png_size_t)size);
-}
-#  endif /* PNG_SIZE_T */
 
 /* Added at libpng version 1.2.34 and 1.4.0 (moved from pngset.c) */
 #  ifdef PNG_CHECK_cHRM_SUPPORTED
@@ -798,6 +789,326 @@ png_check_cHRM_fixed(png_structp png_ptr,
 }
 #  endif /* PNG_CHECK_cHRM_SUPPORTED */
 
+#ifdef PNG_cHRM_SUPPORTED
+/* Added at libpng-1.5.5 to support read and write of true CIEXYZ values for
+ * cHRM, as opposed to using chromaticities.  These internal APIs return
+ * non-zero on a parameter error.  The X, Y and Z values are required to be
+ * positive and less than 1.0.
+ */
+int png_xy_from_XYZ(png_xy *xy, png_XYZ XYZ)
+{
+   png_int_32 d, dwhite, whiteX, whiteY;
+
+   d = XYZ.redX + XYZ.redY + XYZ.redZ;
+   if (!png_muldiv(&xy->redx, XYZ.redX, PNG_FP_1, d)) return 1;
+   if (!png_muldiv(&xy->redy, XYZ.redY, PNG_FP_1, d)) return 1;
+   dwhite = d;
+   whiteX = XYZ.redX;
+   whiteY = XYZ.redY;
+
+   d = XYZ.greenX + XYZ.greenY + XYZ.greenZ;
+   if (!png_muldiv(&xy->greenx, XYZ.greenX, PNG_FP_1, d)) return 1;
+   if (!png_muldiv(&xy->greeny, XYZ.greenY, PNG_FP_1, d)) return 1;
+   dwhite += d;
+   whiteX += XYZ.greenX;
+   whiteY += XYZ.greenY;
+
+   d = XYZ.blueX + XYZ.blueY + XYZ.blueZ;
+   if (!png_muldiv(&xy->bluex, XYZ.blueX, PNG_FP_1, d)) return 1;
+   if (!png_muldiv(&xy->bluey, XYZ.blueY, PNG_FP_1, d)) return 1;
+   dwhite += d;
+   whiteX += XYZ.blueX;
+   whiteY += XYZ.blueY;
+
+   /* The reference white is simply the same of the end-point (X,Y,Z) vectors,
+    * thus:
+    */
+   if (!png_muldiv(&xy->whitex, whiteX, PNG_FP_1, dwhite)) return 1;
+   if (!png_muldiv(&xy->whitey, whiteY, PNG_FP_1, dwhite)) return 1;
+
+   return 0;
+}
+
+int png_XYZ_from_xy(png_XYZ *XYZ, png_xy xy)
+{
+   png_fixed_point red_inverse, green_inverse, blue_scale;
+   png_fixed_point left, right, denominator;
+
+   /* Check xy and, implicitly, z.  Note that wide gamut color spaces typically
+    * have end points with 0 tristimulus values (these are impossible end
+    * points, but they are used to cover the possible colors.)
+    */
+   if (xy.redx < 0 || xy.redx > PNG_FP_1) return 1;
+   if (xy.redy < 0 || xy.redy > PNG_FP_1-xy.redx) return 1;
+   if (xy.greenx < 0 || xy.greenx > PNG_FP_1) return 1;
+   if (xy.greeny < 0 || xy.greeny > PNG_FP_1-xy.greenx) return 1;
+   if (xy.bluex < 0 || xy.bluex > PNG_FP_1) return 1;
+   if (xy.bluey < 0 || xy.bluey > PNG_FP_1-xy.bluex) return 1;
+   if (xy.whitex < 0 || xy.whitex > PNG_FP_1) return 1;
+   if (xy.whitey < 0 || xy.whitey > PNG_FP_1-xy.whitex) return 1;
+
+   /* The reverse calculation is more difficult because the original tristimulus
+    * value had 9 independent values (red,green,blue)x(X,Y,Z) however only 8
+    * derived values were recorded in the cHRM chunk;
+    * (red,green,blue,white)x(x,y).  This loses one degree of freedom and
+    * therefore an arbitrary ninth value has to be introduced to undo the
+    * original transformations.
+    *
+    * Think of the original end-points as points in (X,Y,Z) space.  The
+    * chromaticity values (c) have the property:
+    *
+    *           C
+    *   c = ---------
+    *       X + Y + Z
+    *
+    * For each c (x,y,z) from the corresponding original C (X,Y,Z).  Thus the
+    * three chromaticity values (x,y,z) for each end-point obey the
+    * relationship:
+    *
+    *   x + y + z = 1
+    *
+    * This describes the plane in (X,Y,Z) space that intersects each axis at the
+    * value 1.0; call this the chromaticity plane.  Thus the chromaticity
+    * calculation has scaled each end-point so that it is on the x+y+z=1 plane
+    * and chromaticity is the intersection of the vector from the origin to the
+    * (X,Y,Z) value with the chromaticity plane.
+    *
+    * To fully invert the chromaticity calculation we would need the three
+    * end-point scale factors, (red-scale, green-scale, blue-scale), but these
+    * were not recorded.  Instead we calculated the reference white (X,Y,Z) and
+    * recorded the chromaticity of this.  The reference white (X,Y,Z) would have
+    * given all three of the scale factors since:
+    *
+    *    color-C = color-c * color-scale
+    *    white-C = red-C + green-C + blue-C
+    *            = red-c*red-scale + green-c*green-scale + blue-c*blue-scale
+    *
+    * But cHRM records only white-x and white-y, so we have lost the white scale
+    * factor:
+    *
+    *    white-C = white-c*white-scale
+    *
+    * To handle this the inverse transformation makes an arbitrary assumption
+    * about white-scale:
+    *
+    *    Assume: white-Y = 1.0
+    *    Hence:  white-scale = 1/white-y
+    *    Or:     red-Y + green-Y + blue-Y = 1.0
+    *
+    * Notice the last statement of the assumption gives an equation in three of
+    * the nine values we want to calculate.  8 more equations come from the
+    * above routine as summarised at the top above (the chromaticity
+    * calculation):
+    *
+    *    Given: color-x = color-X / (color-X + color-Y + color-Z)
+    *    Hence: (color-x - 1)*color-X + color.x*color-Y + color.x*color-Z = 0
+    *
+    * This is 9 simultaneous equations in the 9 variables "color-C" and can be
+    * solved by Cramer's rule.  Cramer's rule requires calculating 10 9x9 matrix
+    * determinants, however this is not as bad as it seems because only 28 of
+    * the total of 90 terms in the various matrices are non-zero.  Nevertheless
+    * Cramer's rule is notoriously numerically unstable because the determinant
+    * calculation involves the difference of large, but similar, numbers.  It is
+    * difficult to be sure that the calculation is stable for real world values
+    * and it is certain that it becomes unstable where the end points are close
+    * together.
+    *
+    * So this code uses the perhaps slighly less optimal but more understandable
+    * and totally obvious approach of calculating color-scale.
+    *
+    * This algorithm depends on the precision in white-scale and that is
+    * (1/white-y), so we can immediately see that as white-y approaches 0 the
+    * accuracy inherent in the cHRM chunk drops off substantially.
+    *
+    * libpng arithmetic: a simple invertion of the above equations
+    * ------------------------------------------------------------
+    *
+    *    white_scale = 1/white-y
+    *    white-X = white-x * white-scale
+    *    white-Y = 1.0
+    *    white-Z = (1 - white-x - white-y) * white_scale
+    *
+    *    white-C = red-C + green-C + blue-C
+    *            = red-c*red-scale + green-c*green-scale + blue-c*blue-scale
+    *
+    * This gives us three equations in (red-scale,green-scale,blue-scale) where
+    * all the coefficients are now known:
+    *
+    *    red-x*red-scale + green-x*green-scale + blue-x*blue-scale
+    *       = white-x/white-y
+    *    red-y*red-scale + green-y*green-scale + blue-y*blue-scale = 1
+    *    red-z*red-scale + green-z*green-scale + blue-z*blue-scale
+    *       = (1 - white-x - white-y)/white-y
+    *
+    * In the last equation color-z is (1 - color-x - color-y) so we can add all
+    * three equations together to get an alternative third:
+    *
+    *    red-scale + green-scale + blue-scale = 1/white-y = white-scale
+    *
+    * So now we have a Cramer's rule solution where the determinants are just
+    * 3x3 - far more tractible.  Unfortunately 3x3 determinants still involve
+    * multiplication of three coefficients so we can't guarantee to avoid
+    * overflow in the libpng fixed point representation.  Using Cramer's rule in
+    * floating point is probably a good choice here, but it's not an option for
+    * fixed point.  Instead proceed to simplify the first two equations by
+    * eliminating what is likely to be the largest value, blue-scale:
+    *
+    *    blue-scale = white-scale - red-scale - green-scale
+    *
+    * Hence:
+    *
+    *    (red-x - blue-x)*red-scale + (green-x - blue-x)*green-scale =
+    *                (white-x - blue-x)*white-scale
+    *
+    *    (red-y - blue-y)*red-scale + (green-y - blue-y)*green-scale =
+    *                1 - blue-y*white-scale
+    *
+    * And now we can trivially solve for (red-scale,green-scale):
+    *
+    *    green-scale =
+    *                (white-x - blue-x)*white-scale - (red-x - blue-x)*red-scale
+    *                -----------------------------------------------------------
+    *                                  green-x - blue-x
+    *
+    *    red-scale =
+    *                1 - blue-y*white-scale - (green-y - blue-y) * green-scale
+    *                ---------------------------------------------------------
+    *                                  red-y - blue-y
+    *
+    * Hence:
+    *
+    *    red-scale =
+    *          ( (green-x - blue-x) * (white-y - blue-y) -
+    *            (green-y - blue-y) * (white-x - blue-x) ) / white-y
+    * -------------------------------------------------------------------------
+    *  (green-x - blue-x)*(red-y - blue-y)-(green-y - blue-y)*(red-x - blue-x)
+    *
+    *    green-scale =
+    *          ( (red-y - blue-y) * (white-x - blue-x) -
+    *            (red-x - blue-x) * (white-y - blue-y) ) / white-y
+    * -------------------------------------------------------------------------
+    *  (green-x - blue-x)*(red-y - blue-y)-(green-y - blue-y)*(red-x - blue-x)
+    *
+    * Accuracy:
+    * The input values have 5 decimal digits of accuracy.  The values are all in
+    * the range 0 < value < 1, so simple products are in the same range but may
+    * need up to 10 decimal digits to preserve the original precision and avoid
+    * underflow.  Because we are using a 32-bit signed representation we cannot
+    * match this; the best is a little over 9 decimal digits, less than 10.
+    *
+    * The approach used here is to preserve the maximum precision within the
+    * signed representation.  Because the red-scale calculation above uses the
+    * difference between two products of values that must be in the range -1..+1
+    * it is sufficient to divide the product by 7; ceil(100,000/32767*2).  The
+    * factor is irrelevant in the calculation because it is applied to both
+    * numerator and denominator.
+    *
+    * Note that the values of the differences of the products of the
+    * chromaticities in the above equations tend to be small, for example for
+    * the sRGB chromaticities they are:
+    *
+    * red numerator:    -0.04751
+    * green numerator:  -0.08788
+    * denominator:      -0.2241 (without white-y multiplication)
+    *
+    *  The resultant Y coefficients from the chromaticities of some widely used
+    *  color space definitions are (to 15 decimal places):
+    *
+    *  sRGB
+    *    0.212639005871510 0.715168678767756 0.072192315360734
+    *  Kodak ProPhoto
+    *    0.288071128229293 0.711843217810102 0.000085653960605
+    *  Adobe RGB
+    *    0.297344975250536 0.627363566255466 0.075291458493998
+    *  Adobe Wide Gamut RGB
+    *    0.258728243040113 0.724682314948566 0.016589442011321
+    */
+   /* By the argument, above overflow should be impossible here. The return
+    * value of 2 indicates an internal error to the caller.
+    */
+   if (!png_muldiv(&left, xy.greenx-xy.bluex, xy.redy - xy.bluey, 7)) return 2;
+   if (!png_muldiv(&right, xy.greeny-xy.bluey, xy.redx - xy.bluex, 7)) return 2;
+   denominator = left - right;
+
+   /* Now find the red numerator. */
+   if (!png_muldiv(&left, xy.greenx-xy.bluex, xy.whitey-xy.bluey, 7)) return 2;
+   if (!png_muldiv(&right, xy.greeny-xy.bluey, xy.whitex-xy.bluex, 7)) return 2;
+
+   /* Overflow is possible here and it indicates an extreme set of PNG cHRM
+    * chunk values.  This calculation actually returns the reciprocal of the
+    * scale value because this allows us to delay the multiplication of white-y
+    * into the denominator, which tends to produce a small number.
+    */
+   if (!png_muldiv(&red_inverse, xy.whitey, denominator, left-right) ||
+       red_inverse <= xy.whitey /* r+g+b scales = white scale */)
+      return 1;
+
+   /* Similarly for green_inverse: */
+   if (!png_muldiv(&left, xy.redy-xy.bluey, xy.whitex-xy.bluex, 7)) return 2;
+   if (!png_muldiv(&right, xy.redx-xy.bluex, xy.whitey-xy.bluey, 7)) return 2;
+   if (!png_muldiv(&green_inverse, xy.whitey, denominator, left-right) ||
+       green_inverse <= xy.whitey)
+      return 1;
+
+   /* And the blue scale, the checks above guarantee this can't overflow but it
+    * can still produce 0 for extreme cHRM values.
+    */
+   blue_scale = png_reciprocal(xy.whitey) - png_reciprocal(red_inverse) -
+      png_reciprocal(green_inverse);
+   if (blue_scale <= 0) return 1;
+
+
+   /* And fill in the png_XYZ: */
+   if (!png_muldiv(&XYZ->redX, xy.redx, PNG_FP_1, red_inverse)) return 1;
+   if (!png_muldiv(&XYZ->redY, xy.redy, PNG_FP_1, red_inverse)) return 1;
+   if (!png_muldiv(&XYZ->redZ, PNG_FP_1 - xy.redx - xy.redy, PNG_FP_1,
+      red_inverse))
+      return 1;
+
+   if (!png_muldiv(&XYZ->greenX, xy.greenx, PNG_FP_1, green_inverse)) return 1;
+   if (!png_muldiv(&XYZ->greenY, xy.greeny, PNG_FP_1, green_inverse)) return 1;
+   if (!png_muldiv(&XYZ->greenZ, PNG_FP_1 - xy.greenx - xy.greeny, PNG_FP_1,
+      green_inverse))
+      return 1;
+
+   if (!png_muldiv(&XYZ->blueX, xy.bluex, blue_scale, PNG_FP_1)) return 1;
+   if (!png_muldiv(&XYZ->blueY, xy.bluey, blue_scale, PNG_FP_1)) return 1;
+   if (!png_muldiv(&XYZ->blueZ, PNG_FP_1 - xy.bluex - xy.bluey, blue_scale,
+      PNG_FP_1))
+      return 1;
+
+   return 0; /*success*/
+}
+
+int png_XYZ_from_xy_checked(png_structp png_ptr, png_XYZ *XYZ, png_xy xy)
+{
+   switch (png_XYZ_from_xy(XYZ, xy))
+   {
+      case 0: /* success */
+         return 1;
+
+      case 1:
+         /* The chunk may be technically valid, but we got png_fixed_point
+          * overflow while trying to get XYZ values out of it.  This is
+          * entirely benign - the cHRM chunk is pretty extreme.
+          */
+         png_chunk_benign_error(png_ptr,
+            "extreme cHRM chunk cannot be converted to tristimulus values");
+         break;
+
+      default:
+         /* libpng is broken; this should be a warning but if it happens we
+          * want error reports so for the moment it is an error.
+          */
+         png_error(png_ptr, "internal error in png_XYZ_from_xy");
+         break;
+   }
+
+   /* ERROR RETURN */
+   return 0;
+}
+#endif
+
 void /* PRIVATE */
 png_check_IHDR(png_structp png_ptr,
    png_uint_32 width, png_uint_32 height, int bit_depth,

png.h

@@ -1,7 +1,7 @@
 
 /* png.h - header file for PNG reference library
  *
- * libpng version 1.5.5beta06 - August 17, 2011
+ * libpng version 1.5.5beta06 - August 25, 2011
  * Copyright (c) 1998-2011 Glenn Randers-Pehrson
  * (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
  * (Version 0.88 Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.)
@@ -11,7 +11,7 @@
  * Authors and maintainers:
  *   libpng versions 0.71, May 1995, through 0.88, January 1996: Guy Schalnat
  *   libpng versions 0.89c, June 1996, through 0.96, May 1997: Andreas Dilger
- *   libpng versions 0.97, January 1998, through 1.5.5beta06 - August 17, 2011: Glenn
+ *   libpng versions 0.97, January 1998, through 1.5.5beta06 - August 25, 2011: Glenn
  *   See also "Contributing Authors", below.
  *
  * Note about libpng version numbers:
@@ -189,7 +189,7 @@
  *
  * This code is released under the libpng license.
  *
- * libpng versions 1.2.6, August 15, 2004, through 1.5.5beta06, August 17, 2011, are
+ * libpng versions 1.2.6, August 15, 2004, through 1.5.5beta06, August 25, 2011, are
  * Copyright (c) 2004, 2006-2011 Glenn Randers-Pehrson, and are
  * distributed according to the same disclaimer and license as libpng-1.2.5
  * with the following individual added to the list of Contributing Authors:
@@ -301,7 +301,7 @@
  * Y2K compliance in libpng:
  * =========================
  *
- *    August 17, 2011
+ *    August 25, 2011
  *
  *    Since the PNG Development group is an ad-hoc body, we can't make
  *    an official declaration.
@@ -364,7 +364,7 @@
 /* Version information for png.h - this should match the version in png.c */
 #define PNG_LIBPNG_VER_STRING "1.5.5beta06"
 #define PNG_HEADER_VERSION_STRING \
-     " libpng version 1.5.5beta06 - August 17, 2011\n"
+     " libpng version 1.5.5beta06 - August 25, 2011\n"
 
 #define PNG_LIBPNG_VER_SONUM   15
 #define PNG_LIBPNG_VER_DLLNUM  15
@@ -2048,6 +2048,10 @@ PNG_FP_EXPORT(133, png_uint_32, png_get_cHRM, (png_const_structp png_ptr,
    png_const_infop info_ptr, double *white_x, double *white_y, double *red_x,
     double *red_y, double *green_x, double *green_y, double *blue_x,
     double *blue_y));
+PNG_FP_EXPORT(230, png_uint_32, png_get_cHRM_XYZ, (png_structp png_ptr,
+    png_const_infop info_ptr, double *red_X, double *red_Y, double *red_Z,
+    double *green_X, double *green_Y, double *green_Z, double *blue_X,
+    double *blue_Y, double *blue_Z));
 #ifdef PNG_FIXED_POINT_SUPPORTED /* Otherwise not implemented */
 PNG_FIXED_EXPORT(134, png_uint_32, png_get_cHRM_fixed,
     (png_const_structp png_ptr,
@@ -2057,6 +2061,13 @@ PNG_FIXED_EXPORT(134, png_uint_32, png_get_cHRM_fixed,
     png_fixed_point *int_green_y, png_fixed_point *int_blue_x,
     png_fixed_point *int_blue_y));
 #endif
+PNG_FIXED_EXPORT(231, png_uint_32, png_get_cHRM_XYZ_fixed,
+    (png_structp png_ptr, png_const_infop info_ptr,
+    png_fixed_point *int_red_X, png_fixed_point *int_red_Y,
+    png_fixed_point *int_red_Z, png_fixed_point *int_green_X,
+    png_fixed_point *int_green_Y, png_fixed_point *int_green_Z,
+    png_fixed_point *int_blue_X, png_fixed_point *int_blue_Y,
+    png_fixed_point *int_blue_Z));
 #endif
 
 #ifdef PNG_cHRM_SUPPORTED
@@ -2064,12 +2075,22 @@ PNG_FP_EXPORT(135, void, png_set_cHRM,
     (png_structp png_ptr, png_infop info_ptr,
     double white_x, double white_y, double red_x, double red_y, double green_x,
     double green_y, double blue_x, double blue_y));
+PNG_FP_EXPORT(232, void, png_set_cHRM_XYZ, (png_structp png_ptr,
+    png_infop info_ptr, double red_X, double red_Y, double red_Z,
+    double green_X, double green_Y, double green_Z, double blue_X,
+    double blue_Y, double blue_Z));
 PNG_FIXED_EXPORT(136, void, png_set_cHRM_fixed, (png_structp png_ptr,
     png_infop info_ptr, png_fixed_point int_white_x,
     png_fixed_point int_white_y, png_fixed_point int_red_x,
     png_fixed_point int_red_y, png_fixed_point int_green_x,
     png_fixed_point int_green_y, png_fixed_point int_blue_x,
     png_fixed_point int_blue_y));
+PNG_FIXED_EXPORT(233, void, png_set_cHRM_XYZ_fixed, (png_structp png_ptr,
+    png_infop info_ptr, png_fixed_point int_red_X, png_fixed_point int_red_Y,
+    png_fixed_point int_red_Z, png_fixed_point int_green_X,
+    png_fixed_point int_green_Y, png_fixed_point int_green_Z,
+    png_fixed_point int_blue_X, png_fixed_point int_blue_Y,
+    png_fixed_point int_blue_Z));
 #endif
 
 #ifdef PNG_gAMA_SUPPORTED
@@ -2577,7 +2598,7 @@ PNG_EXPORT(207, void, png_save_uint_16, (png_bytep buf, unsigned int i));
  * scripts/symbols.def as well.
  */
 #ifdef PNG_EXPORT_LAST_ORDINAL
-  PNG_EXPORT_LAST_ORDINAL(229);
+  PNG_EXPORT_LAST_ORDINAL(233);
 #endif
 
 #ifdef __cplusplus

pngget.c

@@ -459,6 +459,65 @@ png_get_bKGD(png_const_structp png_ptr, png_infop info_ptr,
 #endif
 
 #ifdef PNG_cHRM_SUPPORTED
+/* The XYZ APIs were added in 1.5.5 to take advantage of the code added at the
+ * same time to correct the rgb grayscale coefficient defaults obtained from the
+ * cHRM chunk in 1.5.4
+ */
+png_uint_32 PNGFAPI
+png_get_cHRM_XYZ_fixed(png_structp png_ptr, png_const_infop info_ptr,
+    png_fixed_point *int_red_X, png_fixed_point *int_red_Y,
+    png_fixed_point *int_red_Z, png_fixed_point *int_green_X,
+    png_fixed_point *int_green_Y, png_fixed_point *int_green_Z,
+    png_fixed_point *int_blue_X, png_fixed_point *int_blue_Y,
+    png_fixed_point *int_blue_Z)
+{
+   if (png_ptr != NULL && info_ptr != NULL && (info_ptr->valid & PNG_INFO_cHRM))
+   {
+      png_xy xy;
+      png_XYZ XYZ;
+
+      png_debug1(1, "in %s retrieval function", "cHRM_XYZ");
+
+      xy.whitex = info_ptr->x_white;
+      xy.whitey = info_ptr->y_white;
+      xy.redx = info_ptr->x_red;
+      xy.redy = info_ptr->y_red;
+      xy.greenx = info_ptr->x_green;
+      xy.greeny = info_ptr->y_green;
+      xy.bluex = info_ptr->x_blue;
+      xy.bluey = info_ptr->y_blue;
+
+      /* The *_checked function handles error reporting, so just return 0 if
+       * there is a failure here.
+       */
+      if (png_XYZ_from_xy_checked(png_ptr, &XYZ, xy))
+      {
+         if (int_red_X != NULL)
+            *int_red_X = XYZ.redX;
+         if (int_red_Y != NULL)
+            *int_red_Y = XYZ.redY;
+         if (int_red_Z != NULL)
+            *int_red_Z = XYZ.redZ;
+         if (int_green_X != NULL)
+            *int_green_X = XYZ.greenX;
+         if (int_green_Y != NULL)
+            *int_green_Y = XYZ.greenY;
+         if (int_green_Z != NULL)
+            *int_green_Z = XYZ.greenZ;
+         if (int_blue_X != NULL)
+            *int_blue_X = XYZ.blueX;
+         if (int_blue_Y != NULL)
+            *int_blue_Y = XYZ.blueY;
+         if (int_blue_Z != NULL)
+            *int_blue_Z = XYZ.blueZ;
+
+         return (PNG_INFO_cHRM);
+      }
+   }
+
+   return (0);
+}
+
 #  ifdef PNG_FLOATING_POINT_SUPPORTED
 png_uint_32 PNGAPI
 png_get_cHRM(png_const_structp png_ptr, png_const_infop info_ptr,
@@ -490,6 +549,42 @@ png_get_cHRM(png_const_structp png_ptr, png_const_infop info_ptr,
 
    return (0);
 }
+
+png_uint_32 PNGAPI
+png_get_cHRM_XYZ(png_structp png_ptr, png_const_infop info_ptr,
+   double *red_X, double *red_Y, double *red_Z, double *green_X,
+   double *green_Y, double *green_Z, double *blue_X, double *blue_Y,
+   double *blue_Z)
+{
+   png_XYZ XYZ;
+
+   if (png_get_cHRM_XYZ_fixed(png_ptr, info_ptr,
+      &XYZ.redX, &XYZ.redY, &XYZ.redZ, &XYZ.greenX, &XYZ.greenY, &XYZ.greenZ,
+      &XYZ.blueX, &XYZ.blueY, &XYZ.blueZ) & PNG_INFO_cHRM)
+   {
+      if (red_X != NULL)
+         *red_X = png_float(png_ptr, XYZ.redX, "cHRM red X");
+      if (red_Y != NULL)
+         *red_Y = png_float(png_ptr, XYZ.redY, "cHRM red Y");
+      if (red_Z != NULL)
+         *red_Z = png_float(png_ptr, XYZ.redZ, "cHRM red Z");
+      if (green_X != NULL)
+         *green_X = png_float(png_ptr, XYZ.greenX, "cHRM green X");
+      if (green_Y != NULL)
+         *green_Y = png_float(png_ptr, XYZ.greenY, "cHRM green Y");
+      if (green_Z != NULL)
+         *green_Z = png_float(png_ptr, XYZ.greenZ, "cHRM green Z");
+      if (blue_X != NULL)
+         *blue_X = png_float(png_ptr, XYZ.blueX, "cHRM blue X");
+      if (blue_Y != NULL)
+         *blue_Y = png_float(png_ptr, XYZ.blueY, "cHRM blue Y");
+      if (blue_Z != NULL)
+         *blue_Z = png_float(png_ptr, XYZ.blueZ, "cHRM blue Z");
+      return (PNG_INFO_cHRM);
+   }
+
+   return (0);
+}
 #  endif
 
 #  ifdef PNG_FIXED_POINT_SUPPORTED

pngpriv.h

@@ -1166,7 +1166,12 @@ typedef struct png_XYZ
    png_fixed_point blueX, blueY, blueZ;
 } png_XYZ;
 
-/* The conversion APIs return 0 on success, non-zero on a parameter error. */
+/* The conversion APIs return 0 on success, non-zero on a parameter error. They
+ * allow conversion between the above representations of a color encoding.  When
+ * converting from XYZ end points to chromaticities the absolute magnitude of
+ * the end points is lost, when converting back the sum of the Y values of the
+ * three end points will be 1.0
+ */
 PNG_EXTERN int png_xy_from_XYZ PNGARG((png_xy *xy, png_XYZ XYZ));
 PNG_EXTERN int png_XYZ_from_xy PNGARG((png_XYZ *XYZ, png_xy xy));
 PNG_EXTERN int png_XYZ_from_xy_checked PNGARG((png_structp png_ptr,

pngrtran.c

@@ -968,13 +968,17 @@ png_set_rgb_to_gray_fixed(png_structp png_ptr, int error_action,
       {
          png_uint_16 red_int, green_int;
 
+         /* NOTE: this calculation does not round, but this behavior is retained
+          * for consistency, the inaccuracy is very small.  The code here always
+          * overwrites the coefficients, regardless of whether they have been
+          * defaulted or set already.
+          */
          red_int = (png_uint_16)(((png_uint_32)red*32768L)/100000L);
          green_int = (png_uint_16)(((png_uint_32)green*32768L)/100000L);
 
          png_ptr->rgb_to_gray_red_coeff   = red_int;
          png_ptr->rgb_to_gray_green_coeff = green_int;
-         png_ptr->rgb_to_gray_blue_coeff  =
+         png_ptr->rgb_to_gray_coefficients_set = 1;
-          (png_uint_16)(32768 - red_int - green_int);
       }
 
       else
@@ -983,17 +987,18 @@ png_set_rgb_to_gray_fixed(png_structp png_ptr, int error_action,
             png_warning(png_ptr,
                "ignoring out of range rgb_to_gray coefficients");
 
-         /* Use the defaults, from the cHRM chunk if set, else the built in Rec
+         /* Use the defaults, from the cHRM chunk if set, else the historical
-          * 709 values (which correspond to sRGB, so we don't have to worry
+          * values which are close to the sRGB/HDTV/ITU-Rec 709 values.  See
-          * about the sRGB chunk!)
+          * png_do_rgb_to_gray for more discussion of the values.  In this case
+          * the coefficients are not marked as 'set' and are not overwritten if
+          * something has already provided a default.
           */
          if (png_ptr->rgb_to_gray_red_coeff == 0 &&
-            png_ptr->rgb_to_gray_green_coeff == 0 &&
+            png_ptr->rgb_to_gray_green_coeff == 0)
-            png_ptr->rgb_to_gray_blue_coeff == 0)
          {
-            png_ptr->rgb_to_gray_red_coeff   = 6968;  /* .212671 * 32768 + .5 */
+            png_ptr->rgb_to_gray_red_coeff   = 6968;
-            png_ptr->rgb_to_gray_green_coeff = 23434; /* .715160 * 32768 + .5 */
+            png_ptr->rgb_to_gray_green_coeff = 23434;
-            png_ptr->rgb_to_gray_blue_coeff  = 2366;
+            /* png_ptr->rgb_to_gray_blue_coeff  = 2366; */
          }
       }
    }
@@ -1401,7 +1406,7 @@ png_init_read_transformations(png_structp png_ptr)
    if (png_ptr->transformations & PNG_BACKGROUND_EXPAND)
    {
       /* PNG_BACKGROUND_EXPAND: the background is in the file color space, so if
-       * the file was greyscale the background value is gray.
+       * the file was grayscale the background value is gray.
        */
       if (!(png_ptr->color_type & PNG_COLOR_MASK_COLOR))
          png_ptr->mode |= PNG_BACKGROUND_IS_GRAY;
@@ -3089,26 +3094,38 @@ png_do_gray_to_rgb(png_row_infop row_info, png_bytep row)
  *
  *     Y = (6966 * R + 23436 * G + 2366 * B)/32768
  *
- *  We use the approximation
+ *  Historically, however, libpng uses numbers derived from the ITU-R Rec 709
+ *  end point chromaticities and the D65 white point.  Depending on the
+ *  precision used for the D65 white point this produces a variety of different
+ *  numbers, however if the four decimal place value used in ITU-R Rec 709 is
+ *  used (0.3127,0.3290) the Y calculation would be:
+ *
+ *     Y = (6968 * R + 23435 * G + 2366 * B)/32768
+ *
+ *  While this is correct the rounding results in an overflow for white, because
+ *  the sum of the rounded coefficients is 32769, not 32768.  Consequently
+ *  libpng uses, instead, the closest non-overflowing approximation:
  *
  *     Y = (6968 * R + 23434 * G + 2366 * B)/32768
  *
- *  Starting with libpng-1.5.4, if the image being converted has the
+ *  Starting with libpng-1.5.5, if the image being converted has a cHRM chunk
- *  sRGB chunk, then the sRGB numbers are used by default:
+ *  (including an sRGB chunk) then the chromaticities are used to calculate the
+ *  coefficients.  See the chunk handling in pngrutil.c for more information.
  *
- *     Y = 0.33000*R + 0.60000*G + 0.06000*B
+ *  In all cases the calculation is to be done in a linear colorspace.  If no
+ *  gamma information is available to correct the encoding of the original RGB
+ *  values this results in an implicit assumption that the original PNG RGB
+ *  values were linear.
  *
- *  The calculation is to be done in a linear colorspace.
+ *  Other integer coefficents can be used via png_set_rgb_to_gray().  Because
- *
+ *  the API takes just red and green coefficients the blue coefficient is
- *  Other integer coefficents can be used via png_set_rgb_to_gray().
+ *  calculated to make the sum 32768.  This will result in different rounding
+ *  to that used above.
  */
 int /* PRIVATE */
 png_do_rgb_to_gray(png_structp png_ptr, png_row_infop row_info, png_bytep row)
 
 {
-   png_uint_32 i;
-
-   png_uint_32 row_width = row_info->width;
    int rgb_error = 0;
 
    png_debug(1, "in png_do_rgb_to_gray");
@@ -3116,234 +3133,179 @@ png_do_rgb_to_gray(png_structp png_ptr, png_row_infop row_info, png_bytep row)
    if (!(row_info->color_type & PNG_COLOR_MASK_PALETTE) &&
        (row_info->color_type & PNG_COLOR_MASK_COLOR))
    {
-      png_uint_32 rc = png_ptr->rgb_to_gray_red_coeff;
+      PNG_CONST png_uint_32 rc = png_ptr->rgb_to_gray_red_coeff;
-      png_uint_32 gc = png_ptr->rgb_to_gray_green_coeff;
+      PNG_CONST png_uint_32 gc = png_ptr->rgb_to_gray_green_coeff;
-      png_uint_32 bc = png_ptr->rgb_to_gray_blue_coeff;
+      PNG_CONST png_uint_32 bc = 32768 - rc - gc;
+      PNG_CONST png_uint_32 row_width = row_info->width;
+      PNG_CONST int have_alpha =
+         (row_info->color_type & PNG_COLOR_MASK_ALPHA) != 0;
 
-      if (row_info->color_type == PNG_COLOR_TYPE_RGB)
+      if (row_info->bit_depth == 8)
       {
-         if (row_info->bit_depth == 8)
-         {
 #if defined(PNG_READ_GAMMA_SUPPORTED) || defined(PNG_READ_BACKGROUND_SUPPORTED)
-            if (png_ptr->gamma_from_1 != NULL && png_ptr->gamma_to_1 != NULL)
+         /* Notice that gamma to/from 1 are not necessarily inverses (if
+          * there is an overall gamma correction).  Prior to 1.5.5 this code
+          * checked the linearized values for equality; this doesn't match
+          * the documentation, the original values must be checked.
+          */
+         if (png_ptr->gamma_from_1 != NULL && png_ptr->gamma_to_1 != NULL)
+         {
+            png_bytep sp = row;
+            png_bytep dp = row;
+            png_uint_32 i;
+
+            for (i = 0; i < row_width; i++)
             {
-               png_bytep sp = row;
+               png_byte red   = *(sp++);
-               png_bytep dp = row;
+               png_byte green = *(sp++);
+               png_byte blue  = *(sp++);
 
-               for (i = 0; i < row_width; i++)
+               if (red != green || red != blue)
                {
-                  png_byte red   = png_ptr->gamma_to_1[*(sp++)];
+                  red = png_ptr->gamma_to_1[red];
-                  png_byte green = png_ptr->gamma_to_1[*(sp++)];
+                  green = png_ptr->gamma_to_1[green];
-                  png_byte blue  = png_ptr->gamma_to_1[*(sp++)];
+                  blue = png_ptr->gamma_to_1[blue];
 
-                  if (red != green || red != blue)
+                  rgb_error |= 1;
-                  {
+                  *(dp++) = png_ptr->gamma_from_1[
-                     rgb_error |= 1;
+                      (rc*red + gc*green + bc*blue + 16384)>>15];
-                     *(dp++) = png_ptr->gamma_from_1[
-                         (rc*red + gc*green + bc*blue)>>15];
-                  }
-
-                  else
-                     *(dp++) = *(sp - 1);
                }
-            }
+
-            else
+               else
-#endif
-            {
-               png_bytep sp = row;
-               png_bytep dp = row;
-               for (i = 0; i < row_width; i++)
                {
-                  png_byte red   = *(sp++);
+                  /* If there is no overall correction the table will not be
-                  png_byte green = *(sp++);
+                   * set.
-                  png_byte blue  = *(sp++);
+                   */
+                  if (png_ptr->gamma_table != NULL)
+                     red = png_ptr->gamma_table[red];
 
-                  if (red != green || red != blue)
+                  *(dp++) = red;
-                  {
-                     rgb_error |= 1;
-                     *(dp++) = (png_byte)((rc*red + gc*green + bc*blue)>>15);
-                  }
-
-                  else
-                     *(dp++) = *(sp - 1);
                }
+
+               if (have_alpha)
+                  *(dp++) = *(sp++);
             }
          }
-
+         else
-         else /* RGB bit_depth == 16 */
+#endif
          {
-#if defined(PNG_READ_GAMMA_SUPPORTED) || defined(PNG_READ_BACKGROUND_SUPPORTED)
+            png_bytep sp = row;
-            if (png_ptr->gamma_16_to_1 != NULL &&
+            png_bytep dp = row;
-                png_ptr->gamma_16_from_1 != NULL)
+            png_uint_32 i;
+
+            for (i = 0; i < row_width; i++)
             {
-               png_bytep sp = row;
+               png_byte red   = *(sp++);
-               png_bytep dp = row;
+               png_byte green = *(sp++);
-               for (i = 0; i < row_width; i++)
+               png_byte blue  = *(sp++);
+
+               if (red != green || red != blue)
                {
-                  png_uint_16 red, green, blue, w;
+                  rgb_error |= 1;
+                  /*NOTE: this is the historical approach which simply
+                   * truncates the results.
+                   */
+                  *(dp++) = (png_byte)((rc*red + gc*green + bc*blue)>>15);
+               }
 
-                  red   = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
+               else
-                  green = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
+                  *(dp++) = red;
-                  blue  = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
 
-                  if (red == green && red == blue)
+               if (have_alpha)
-                     w = red;
+                  *(dp++) = *(sp++);
+            }
+         }
+      }
 
-                  else
+      else /* RGB bit_depth == 16 */
-                  {
+      {
-                     png_uint_16 red_1   = png_ptr->gamma_16_to_1[(red&0xff)
+#if defined(PNG_READ_GAMMA_SUPPORTED) || defined(PNG_READ_BACKGROUND_SUPPORTED)
+         if (png_ptr->gamma_16_to_1 != NULL && png_ptr->gamma_16_from_1 != NULL)
+         {
+            png_bytep sp = row;
+            png_bytep dp = row;
+            png_uint_32 i;
+
+            for (i = 0; i < row_width; i++)
+            {
+               png_uint_16 red, green, blue, w;
+
+               red   = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
+               green = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
+               blue  = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
+
+               if (red == green && red == blue)
+               {
+                  if (png_ptr->gamma_16_table != NULL)
+                     w = png_ptr->gamma_16_table[(red&0xff)
                          >> png_ptr->gamma_shift][red>>8];
-                     png_uint_16 green_1 =
-                         png_ptr->gamma_16_to_1[(green&0xff) >>
-                         png_ptr->gamma_shift][green>>8];
-                     png_uint_16 blue_1  = png_ptr->gamma_16_to_1[(blue&0xff)
-                         >> png_ptr->gamma_shift][blue>>8];
-                     png_uint_16 gray16  = (png_uint_16)((rc*red_1 + gc*green_1
-                         + bc*blue_1)>>15);
-                     w = png_ptr->gamma_16_from_1[(gray16&0xff) >>
-                         png_ptr->gamma_shift][gray16 >> 8];
-                     rgb_error |= 1;
-                  }
-
-                  *(dp++) = (png_byte)((w>>8) & 0xff);
-                  *(dp++) = (png_byte)(w & 0xff);
-               }
-            }
-            else
-#endif
-            {
-               png_bytep sp = row;
-               png_bytep dp = row;
-               for (i = 0; i < row_width; i++)
-               {
-                  png_uint_16 red, green, blue, gray16;
-
-                  red   = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
-                  green = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
-                  blue  = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
-
-                  if (red != green || red != blue)
-                     rgb_error |= 1;
-
-                  gray16  = (png_uint_16)((rc*red + gc*green + bc*blue)>>15);
-                  *(dp++) = (png_byte)((gray16>>8) & 0xff);
-                  *(dp++) = (png_byte)(gray16 & 0xff);
-               }
-            }
-         }
-      }
-      if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
-      {
-         if (row_info->bit_depth == 8)
-         {
-#if defined(PNG_READ_GAMMA_SUPPORTED) || defined(PNG_READ_BACKGROUND_SUPPORTED)
-            if (png_ptr->gamma_from_1 != NULL && png_ptr->gamma_to_1 != NULL)
-            {
-               png_bytep sp = row;
-               png_bytep dp = row;
-               for (i = 0; i < row_width; i++)
-               {
-                  png_byte red   = png_ptr->gamma_to_1[*(sp++)];
-                  png_byte green = png_ptr->gamma_to_1[*(sp++)];
-                  png_byte blue  = png_ptr->gamma_to_1[*(sp++)];
-
-                  if (red != green || red != blue)
-                     rgb_error |= 1;
-
-                  *(dp++) =  png_ptr->gamma_from_1
-                      [(rc*red + gc*green + bc*blue)>>15];
-
-                  *(dp++) = *(sp++);  /* alpha */
-               }
-            }
-            else
-#endif
-            {
-               png_bytep sp = row;
-               png_bytep dp = row;
-               for (i = 0; i < row_width; i++)
-               {
-                  png_byte red   = *(sp++);
-                  png_byte green = *(sp++);
-                  png_byte blue  = *(sp++);
-                  if (red != green || red != blue)
-                     rgb_error |= 1;
-
-                  *(dp++) =  (png_byte)((rc*red + gc*green + bc*blue)>>15);
-                  *(dp++) = *(sp++);  /* alpha */
-               }
-            }
-         }
-         else /* RGBA bit_depth == 16 */
-         {
-#if defined(PNG_READ_GAMMA_SUPPORTED) || defined(PNG_READ_BACKGROUND_SUPPORTED)
-            if (png_ptr->gamma_16_to_1 != NULL &&
-                png_ptr->gamma_16_from_1 != NULL)
-            {
-               png_bytep sp = row;
-               png_bytep dp = row;
-               for (i = 0; i < row_width; i++)
-               {
-                  png_uint_16 red, green, blue, w;
-
-                  red   = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
-                  green = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
-                  blue  = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
-
-                  if (red == green && red == blue)
-                     w = red;
 
                   else
-                  {
+                     w = red;
-                     png_uint_16 red_1   = png_ptr->gamma_16_to_1[(red&0xff) >>
+               }
-                         png_ptr->gamma_shift][red>>8];
 
-                     png_uint_16 green_1 =
+               else
-                         png_ptr->gamma_16_to_1[(green&0xff) >>
+               {
-                         png_ptr->gamma_shift][green>>8];
+                  png_uint_16 red_1   = png_ptr->gamma_16_to_1[(red&0xff)
+                      >> png_ptr->gamma_shift][red>>8];
+                  png_uint_16 green_1 =
+                      png_ptr->gamma_16_to_1[(green&0xff) >>
+                      png_ptr->gamma_shift][green>>8];
+                  png_uint_16 blue_1  = png_ptr->gamma_16_to_1[(blue&0xff)
+                      >> png_ptr->gamma_shift][blue>>8];
+                  png_uint_16 gray16  = (png_uint_16)((rc*red_1 + gc*green_1
+                      + bc*blue_1 + 16384)>>15);
+                  w = png_ptr->gamma_16_from_1[(gray16&0xff) >>
+                      png_ptr->gamma_shift][gray16 >> 8];
+                  rgb_error |= 1;
+               }
 
-                     png_uint_16 blue_1  = png_ptr->gamma_16_to_1[(blue&0xff) >>
+               *(dp++) = (png_byte)((w>>8) & 0xff);
-                         png_ptr->gamma_shift][blue>>8];
+               *(dp++) = (png_byte)(w & 0xff);
 
-                     png_uint_16 gray16  = (png_uint_16)((rc * red_1
+               if (have_alpha)
-                         + gc * green_1 + bc * blue_1)>>15);
+               {
-
+                  *(dp++) = *(sp++);
-                     w = png_ptr->gamma_16_from_1[(gray16&0xff) >>
-                         png_ptr->gamma_shift][gray16 >> 8];
-
-                     rgb_error |= 1;
-                  }
-
-                  *(dp++) = (png_byte)((w>>8) & 0xff);
-                  *(dp++) = (png_byte)(w & 0xff);
-                  *(dp++) = *(sp++);  /* alpha */
                   *(dp++) = *(sp++);
                }
             }
-            else
+         }
+         else
 #endif
+         {
+            png_bytep sp = row;
+            png_bytep dp = row;
+            png_uint_32 i;
+
+            for (i = 0; i < row_width; i++)
             {
-               png_bytep sp = row;
+               png_uint_16 red, green, blue, gray16;
-               png_bytep dp = row;
+
-               for (i = 0; i < row_width; i++)
+               red   = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
+               green = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
+               blue  = (png_uint_16)(((*(sp))<<8) | *(sp + 1)); sp += 2;
+
+               if (red != green || red != blue)
+                  rgb_error |= 1;
+
+               /* From 1.5.5 in the 16 bit case do the accurate convertion even
+                * in the 'fast' case - this is because this is where the code
+                * ends up when handling linear 16 bit data.
+                */
+               gray16  = (png_uint_16)((rc*red + gc*green + bc*blue + 16384) >>
+                  15);
+               *(dp++) = (png_byte)((gray16>>8) & 0xff);
+               *(dp++) = (png_byte)(gray16 & 0xff);
+
+               if (have_alpha)
                {
-                  png_uint_16 red, green, blue, gray16;
+                  *(dp++) = *(sp++);
-                  red   = (png_uint_16)((*(sp)<<8) | *(sp + 1)); sp += 2;
-                  green = (png_uint_16)((*(sp)<<8) | *(sp + 1)); sp += 2;
-                  blue  = (png_uint_16)((*(sp)<<8) | *(sp + 1)); sp += 2;
-
-                  if (red != green || red != blue)
-                     rgb_error |= 1;
-
-                  gray16  = (png_uint_16)((rc*red + gc*green + bc*blue)>>15);
-                  *(dp++) = (png_byte)((gray16>>8) & 0xff);
-                  *(dp++) = (png_byte)(gray16 & 0xff);
-                  *(dp++) = *(sp++);  /* alpha */
                   *(dp++) = *(sp++);
                }
             }
          }
       }
+
       row_info->channels -= 2;
       row_info->color_type = (png_byte)(row_info->color_type &
           ~PNG_COLOR_MASK_COLOR);

pngrutil.c

@@ -1022,27 +1022,80 @@ png_handle_cHRM(png_structp png_ptr, png_infop info_ptr, png_uint_32 length)
 
 #ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
    /* Store the _white values as default coefficients for the rgb to gray
-    * operation if it is supported.
+    * operation if it is supported.  Check if the transform is already set to
+    * avoid destroying the transform values.
     */
-   if ((png_ptr->transformations & PNG_RGB_TO_GRAY) == 0)
+   if (!png_ptr->rgb_to_gray_coefficients_set)
    {
-      /* png_set_background has not been called, the coefficients must be in
+      /* png_set_background has not been called and we haven't seen an sRGB
-       * range for the following to work without overflow.
+       * chunk yet.  Find the XYZ of the three end points.
        */
-      if (y_red <= (1<<17) && y_green <= (1<<17) && y_blue <= (1<<17))
+      png_XYZ XYZ;
-      {
+      png_xy xy;
-         /* The y values are chromaticities: Y/X+Y+Z, the weights for the gray
-          * transformation are simply the normalized Y values for red, green and
-          * blue scaled by 32768.
-          */
-         png_uint_32 w = y_red + y_green + y_blue;
 
-         png_ptr->rgb_to_gray_red_coeff   = (png_uint_16)(((png_uint_32)y_red *
+      xy.redx = x_red;
-            32768)/w);
+      xy.redy = y_red;
-         png_ptr->rgb_to_gray_green_coeff = (png_uint_16)(((png_uint_32)y_green
+      xy.greenx = x_green;
-            * 32768)/w);
+      xy.greeny = y_green;
-         png_ptr->rgb_to_gray_blue_coeff  = (png_uint_16)(((png_uint_32)y_blue *
+      xy.bluex = x_blue;
-            32768)/w);
+      xy.bluey = y_blue;
+      xy.whitex = x_white;
+      xy.whitey = y_white;
+
+      if (png_XYZ_from_xy_checked(png_ptr, &XYZ, xy))
+      {
+         /* The success case, because XYZ_from_xy normalises to a reference
+          * white Y of 1.0 we just need to scale the numbers.  This should
+          * always work just fine. It is an internal error if this overflows.
+          */
+         {
+            png_fixed_point r, g, b;
+            if (png_muldiv(&r, XYZ.redY, 32768, PNG_FP_1) &&
+               r >= 0 && r <= 32768 &&
+               png_muldiv(&g, XYZ.greenY, 32768, PNG_FP_1) &&
+               g >= 0 && g <= 32768 &&
+               png_muldiv(&b, XYZ.blueY, 32768, PNG_FP_1) &&
+               b >= 0 && b <= 32768 &&
+               r+g+b <= 32769)
+            {
+               /* We allow 0 coefficients here.  r+g+b may be 32769 if two or
+                * all of the coefficients were rounded up.  Handle this by
+                * reducing the *largest* coefficient by 1; this matches the
+                * approach used for the default coefficients in pngrtran.c
+                */
+               int add = 0;
+
+               if (r+g+b > 32768)
+                  add = -1;
+               else if (r+g+b < 32768)
+                  add = 1;
+
+               if (add != 0)
+               {
+                  if (g >= r && g >= b)
+                     g += add;
+                  else if (r >= g && r >= b)
+                     r += add;
+                  else
+                     b += add;
+               }
+
+               /* Check for an internal error. */
+               if (r+g+b != 32768)
+                  png_error(png_ptr,
+                     "internal error handling cHRM coefficients");
+
+               png_ptr->rgb_to_gray_red_coeff   = (png_uint_16)r;
+               png_ptr->rgb_to_gray_green_coeff = (png_uint_16)g;
+            }
+
+            /* This is a png_error at present even though it could be ignored -
+             * it should never happen, but it is important that if it does, the
+             * bug is fixed.
+             */
+            else
+               png_error(png_ptr, "internal error handling cHRM->XYZ");
+         }
       }
    }
 #endif
@@ -1135,6 +1188,47 @@ png_handle_sRGB(png_structp png_ptr, png_infop info_ptr, png_uint_32 length)
       }
 #endif /* PNG_READ_cHRM_SUPPORTED */
 
+   /* This is recorded for use when handling the cHRM chunk above.  An sRGB
+    * chunk unconditionally overwrites the coefficients for grayscale conversion
+    * too.
+    */
+   png_ptr->is_sRGB = 1;
+
+#  ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
+      /* Don't overwrite user supplied values: */
+      if (!png_ptr->rgb_to_gray_coefficients_set)
+      {
+         /* These numbers come from the sRGB specification (or, since one has to
+          * pay much money to get a copy, the wikipedia sRGB page) the
+          * chromaticity values quoted have been inverted to get the reverse
+          * transformation from RGB to XYZ and the 'Y' coefficients scaled by
+          * 32768 (then rounded).
+          *
+          * sRGB and ITU Rec-709 both truncate the values for the D65 white
+          * point to four digits and, even though it actually stores five
+          * digits, the PNG spec gives the truncated value.
+          *
+          * This means that when the chromaticities are converted back to XYZ
+          * end points we end up with (6968,23435,2366), which, as described in
+          * pngrtran.c, would overflow.  If the five digit precision and up is
+          * used we get, instead:
+          *
+          *    6968*R + 23435*G + 2365*B
+          *
+          * (Notice that this rounds the blue coefficient down, rather than the
+          * choice used in pngrtran.c which is to round the green one down.)
+          */
+         png_ptr->rgb_to_gray_red_coeff   =  6968; /* 0.212639005871510 */
+         png_ptr->rgb_to_gray_green_coeff = 23434; /* 0.715168678767756 */
+         /* png_ptr->rgb_to_gray_blue_coeff  =  2366; 0.072192315360734	*/
+
+         /* The following keeps the cHRM chunk from destroying the
+          * coefficients again in the event that it follows the sRGB chunk.
+          */
+         png_ptr->rgb_to_gray_coefficients_set = 1;
+      }
+#  endif
+
    png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr, intent);
 }
 #endif /* PNG_READ_sRGB_SUPPORTED */

pngset.c

@@ -64,6 +64,39 @@ png_set_cHRM_fixed(png_structp png_ptr, png_infop info_ptr,
    }
 }
 
+void PNGFAPI
+png_set_cHRM_XYZ_fixed(png_structp png_ptr, png_infop info_ptr,
+    png_fixed_point int_red_X, png_fixed_point int_red_Y,
+    png_fixed_point int_red_Z, png_fixed_point int_green_X,
+    png_fixed_point int_green_Y, png_fixed_point int_green_Z,
+    png_fixed_point int_blue_X, png_fixed_point int_blue_Y,
+    png_fixed_point int_blue_Z)
+{
+   png_XYZ XYZ;
+   png_xy xy;
+
+   png_debug1(1, "in %s storage function", "cHRM XYZ fixed");
+
+   if (png_ptr == NULL || info_ptr == NULL)
+      return;
+
+   XYZ.redX = int_red_X;
+   XYZ.redY = int_red_Y;
+   XYZ.redZ = int_red_Z;
+   XYZ.greenX = int_green_X;
+   XYZ.greenY = int_green_Y;
+   XYZ.greenZ = int_green_Z;
+   XYZ.blueX = int_blue_X;
+   XYZ.blueY = int_blue_Y;
+   XYZ.blueZ = int_blue_Z;
+
+   if (png_xy_from_XYZ(&xy, XYZ))
+      png_error(png_ptr, "XYZ values out of representable range");
+
+   png_set_cHRM_fixed(png_ptr, info_ptr, xy.whitex, xy.whitey, xy.redx, xy.redy,
+      xy.greenx, xy.greeny, xy.bluex, xy.bluey);
+}
+
 #  ifdef PNG_FLOATING_POINT_SUPPORTED
 void PNGAPI
 png_set_cHRM(png_structp png_ptr, png_infop info_ptr,
@@ -80,6 +113,23 @@ png_set_cHRM(png_structp png_ptr, png_infop info_ptr,
       png_fixed(png_ptr, blue_x, "cHRM Blue X"),
       png_fixed(png_ptr, blue_y, "cHRM Blue Y"));
 }
+
+void PNGAPI
+png_set_cHRM_XYZ(png_structp png_ptr, png_infop info_ptr, double red_X,
+    double red_Y, double red_Z, double green_X, double green_Y, double green_Z,
+    double blue_X, double blue_Y, double blue_Z)
+{
+   png_set_cHRM_XYZ_fixed(png_ptr, info_ptr,
+      png_fixed(png_ptr, red_X, "cHRM Red X"),
+      png_fixed(png_ptr, red_Y, "cHRM Red Y"),
+      png_fixed(png_ptr, red_Z, "cHRM Red Z"),
+      png_fixed(png_ptr, green_X, "cHRM Red X"),
+      png_fixed(png_ptr, green_Y, "cHRM Red Y"),
+      png_fixed(png_ptr, green_Z, "cHRM Red Z"),
+      png_fixed(png_ptr, blue_X, "cHRM Red X"),
+      png_fixed(png_ptr, blue_Y, "cHRM Red Y"),
+      png_fixed(png_ptr, blue_Z, "cHRM Red Z"));
+}
 #  endif /* PNG_FLOATING_POINT_SUPPORTED */
 
 #endif /* PNG_cHRM_SUPPORTED */

pngstruct.h

@@ -255,13 +255,20 @@ struct png_struct_def
    png_bytep chunk_list;
 #endif
 
+#ifdef PNG_READ_sRGB_SUPPORTED
+   /* Added in 1.5.5 to record an sRGB chunk in the png. */
+   png_byte is_sRGB;
+#endif
+
 /* New members added in libpng-1.0.3 */
 #ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
    png_byte rgb_to_gray_status;
+   /* Added in libpng 1.5.5 to record setting of coefficients: */
+   png_byte rgb_to_gray_coefficients_set;
    /* These were changed from png_byte in libpng-1.0.6 */
    png_uint_16 rgb_to_gray_red_coeff;
    png_uint_16 rgb_to_gray_green_coeff;
-   png_uint_16 rgb_to_gray_blue_coeff;
+   /* deleted in 1.5.5: rgb_to_gray_blue_coeff; */
 #endif
 
 /* New member added in libpng-1.0.4 (renamed in 1.0.9) */