[devel] Implementation of premultiplied alpha support: png_set_alpha_mode

(libpng-manual.txt still to be updated, see png.h for documentation.)

libpng-manual.txt

@@ -1472,6 +1472,11 @@ on gamma in the PNG specification for an excellent description of what
 gamma is, and why all applications should support it.  It is strongly
 recommended that PNG viewers support gamma correction.
 
+This API unconditionally sets the screen and file gamma values, so it will
+override the value in the PNG file unless it is called before the PNG file
+reading starts.  For this reason you must always call it with the PNG file
+value when you call it in this position:
+
    if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
       png_set_gamma(png_ptr, screen_gamma, file_gamma);
 

png.c

@@ -586,17 +586,17 @@ png_convert_to_rfc1123(png_structp png_ptr, png_const_timep ptime)
          if (pos < (sizeof png_ptr->time_buffer)-1)\
             png_ptr->time_buffer[pos++] = (ch)
 
-      APPEND_NUMBER(PNG_NUMBER_FORMAT_u, ptime->day % 32);
+      APPEND_NUMBER(PNG_NUMBER_FORMAT_u, (unsigned)ptime->day % 32);
       APPEND(' ');
       APPEND_STRING(short_months[(ptime->month - 1) % 12]);
       APPEND(' ');
       APPEND_NUMBER(PNG_NUMBER_FORMAT_u, ptime->year);
       APPEND(' ');
-      APPEND_NUMBER(PNG_NUMBER_FORMAT_02u, ptime->hour % 24);
+      APPEND_NUMBER(PNG_NUMBER_FORMAT_02u, (unsigned)ptime->hour % 24);
       APPEND(':');
-      APPEND_NUMBER(PNG_NUMBER_FORMAT_02u, ptime->minute % 60);
+      APPEND_NUMBER(PNG_NUMBER_FORMAT_02u, (unsigned)ptime->minute % 60);
       APPEND(':');
-      APPEND_NUMBER(PNG_NUMBER_FORMAT_02u, ptime->second % 61);
+      APPEND_NUMBER(PNG_NUMBER_FORMAT_02u, (unsigned)ptime->second % 61);
       APPEND_STRING(" +0000"); /* This reliably terminates the buffer */
 
 #     undef APPEND
@@ -2322,8 +2322,9 @@ png_build_gamma_table(png_structp png_ptr, int bit_depth)
          png_ptr->screen_gamma) : PNG_FP_1);
 
 #if defined(PNG_READ_BACKGROUND_SUPPORTED) || \
+   defined(PNG_READ_ALPHA_MODE_SUPPORTED) || \
    defined(PNG_READ_RGB_TO_GRAY_SUPPORTED)
-     if (png_ptr->transformations & ((PNG_BACKGROUND) | PNG_RGB_TO_GRAY))
+     if (png_ptr->transformations & (PNG_COMPOSE | PNG_RGB_TO_GRAY))
      {
         png_build_8bit_table(png_ptr, &png_ptr->gamma_to_1,
             png_reciprocal(png_ptr->gamma));
@@ -2332,7 +2333,7 @@ png_build_gamma_table(png_structp png_ptr, int bit_depth)
             png_ptr->screen_gamma > 0 ?  png_reciprocal(png_ptr->screen_gamma) :
             png_ptr->gamma/* Probably doing rgb_to_gray */);
      }
-#endif /* PNG_READ_BACKGROUND_SUPPORTED || PNG_RGB_TO_GRAY_SUPPORTED */
+#endif /* READ_BACKGROUND || READ_ALPHA_MODE || RGB_TO_GRAY */
   }
   else
   {
@@ -2391,7 +2392,12 @@ png_build_gamma_table(png_structp png_ptr, int bit_depth)
      png_ptr->gamma_shift = shift;
 
 #ifdef PNG_16BIT_SUPPORTED
-     if (png_ptr->transformations & (PNG_16_TO_8 | PNG_BACKGROUND))
+     /* NOTE: prior to 1.5.3 this test used to include PNG_BACKGROUND (now
+      * PNG_COMPOSE).  This effectively smashed the background calculation for
+      * 16 bit output because the 8 bit table assumes the result will be reduced
+      * to 8 bits.
+      */
+     if (png_ptr->transformations & PNG_16_TO_8)
 #endif
          png_build_16to8_table(png_ptr, &png_ptr->gamma_16_table, shift,
          png_ptr->screen_gamma > 0 ? png_product2(png_ptr->gamma,
@@ -2405,8 +2411,9 @@ png_build_gamma_table(png_structp png_ptr, int bit_depth)
 #endif
 
 #if defined(PNG_READ_BACKGROUND_SUPPORTED) || \
+   defined(PNG_READ_ALPHA_MODE_SUPPORTED) || \
    defined(PNG_READ_RGB_TO_GRAY_SUPPORTED)
-     if (png_ptr->transformations & (PNG_BACKGROUND | PNG_RGB_TO_GRAY))
+     if (png_ptr->transformations & (PNG_COMPOSE | PNG_RGB_TO_GRAY))
      {
         png_build_16bit_table(png_ptr, &png_ptr->gamma_16_to_1, shift,
             png_reciprocal(png_ptr->gamma));
@@ -2419,7 +2426,7 @@ png_build_gamma_table(png_structp png_ptr, int bit_depth)
             png_ptr->screen_gamma > 0 ? png_reciprocal(png_ptr->screen_gamma) :
             png_ptr->gamma/* Probably doing rgb_to_gray */);
      }
-#endif /* PNG_READ_BACKGROUND_SUPPORTED || PNG_RGB_TO_GRAY_SUPPORTED */
+#endif /* READ_BACKGROUND || READ_ALPHA_MODE || RGB_TO_GRAY */
   }
 }
 #endif /* READ_GAMMA */

png.h

@@ -690,6 +690,8 @@ typedef png_info FAR * FAR * png_infopp;
  */
 #define PNG_FP_1    100000
 #define PNG_FP_HALF  50000
+#define PNG_FP_MAX  ((png_fixed_point)0x7fffffffL)
+#define PNG_FP_MIN  (-PNG_FP_MAX)
 
 /* These describe the color_type field in png_info. */
 /* color type masks */
@@ -1112,6 +1114,202 @@ PNG_EXPORT(35, void, png_build_grayscale_palette, (int bit_depth,
     png_colorp palette));
 #endif
 
+#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+/* How the alpha channel is interpreted - this affects how the color channels of
+ * a PNG file are output when an alpha channel, or tRNS chunk is a palette file,
+ * is present.
+ *
+ * The default is to output data according to the PNG specification: the alpha
+ * channel is a linear measure of the contribution of the pixel to the
+ * corresponding output pixel.  The gamma encoded color channels must be scaled
+ * according to the contribution and to do this it is necessary to undo the
+ * encoding, scale the color values, perform the composition and reencode the
+ * values.  This is the 'PNG' format.
+ *
+ * The alternative is to 'associate' the alpha with the color information by
+ * storing color channel values that have been scaled by the alpha.  The
+ * advantage is that the color channels can be resampled (the image can be
+ * scaled) in this form.  The disadvantage is that normal practice is to store
+ * linear, not (gamma) encoded, values and this requires 16 bit channels for
+ * still images rather than the 8 bit channels that are just about sufficient if
+ * gamma encoding is used.  In addition all non-transparent pixel values,
+ * including completely opaque ones, must be gamma encoded to produce the final
+ * image.  This is the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' format (the
+ * latter being the two common names for associated alpha color channels.)
+ *
+ * Since it is not necessary to perform arithmetic on opaque color values so
+ * long as they are not to be resampled and are in the final color space it is
+ * possible to optimize the handling of alpha by storing the opaque pixels in
+ * the PNG format (adjusted for the output color space) while storing partially
+ * opaque pixels in the standard, linear, format.  The accuracy required for
+ * standard alpha composition is relatively low, because the pixels are
+ * isolated, therefore typically the accuracy loss in storing 8 bit linear
+ * values is acceptable.  (This is not true if the alpha channel is used to
+ * simiulate transparency over large areas - use 16 bits or the PNG format in
+ * this case!)  This is the 'OPTIMIZED' format.  For this format a pixel is
+ * treated as opaque only if the alpha value is equal to the maximum value.
+ *
+ * The final choice is to gamma encode the alpha channel as well.  This is
+ * broken because, in practice, no implementation that uses this choice
+ * correctly undoes the encoding before handling alpha composition.  Use this
+ * choice only if other serious errors in the software or hardware you use
+ * mandate it; the typical serious error is for dark halos to appear around
+ * opaque areas of the composited PNG image because of arithmetic overflow.
+ *
+ * The API function png_set_alpha_mode specifies which of these choices to use
+ * with a enumerated 'mode' value and the gamma of the required output:
+ */
+#define PNG_ALPHA_PNG           0 /* according to the PNG standard */
+#define PNG_ALPHA_STANDARD      1 /* according to Porter/Duff */
+#define PNG_ALPHA_ASSOCIATED    1 /* as above; this is the normal practice */
+#define PNG_ALPHA_PREMULTIPLIED 1 /* as above */
+#define PNG_ALPHA_OPTIMIZED     2 /* 'PNG' for opaque pixels, else 'STANDARD' */
+#define PNG_ALPHA_BROKEN        3 /* the alpha channel is gamma encoded */
+
+PNG_FP_EXPORT(227, void, png_set_alpha_mode, (png_structp png_ptr, int mode,
+    double output_gamma));
+PNG_FIXED_EXPORT(228, void, png_set_alpha_mode_fixed, (png_structp png_ptr,
+    int mode, png_fixed_point output_gamma));
+
+/* The output_gamma value is a screen gamma in libpng terminology: it expresses
+ * how to decode the output values, not how they are encoded.  The values used
+ * correspond to the normal numbers used to describe the overall gamma of a
+ * computer display system; for example 2.2 for an sRGB conformant system.  The
+ * values are scaled by 100000 in the _fixed version of the API (so 220000 for
+ * sRGB.)
+ *
+ * The inverse of the value is always used to provide a default for the PNG file
+ * encoding if it has no gAMA chunk and if png_set_gamma() has not been called
+ * to override the PNG gamma information.
+ *
+ * When the ALPHA_OPTIMIZED mode is selected the output gamma is used to encode
+ * opaque pixels however pixels with lower alpha values are not encoded,
+ * regardless of the output gamma setting.
+ *
+ * When the standard Porter Duff handling is requested with mode 1 the output
+ * encoding is set to be linear and the output_gamma value is only relevant
+ * as a default for input data that has no gamma information.  The linear output
+ * encoding will be overridden if png_set_gamma() is called - the results may be
+ * highly unexpected!
+ *
+ * The following numbers are derived from the sRGB standard and the research
+ * behind it.  sRGB is defined to be approximated by a PNG gAMA chunk value of
+ * 0.45455 (1/2.2) for PNG.  The value implicitly includes any viewing
+ * correction required to take account of any differences in the color
+ * environment of the original scene and the intended display environment; the
+ * value expresses how to *decode* the image for display, not how the original
+ * data was *encoded*.
+ *
+ * sRGB provides a peg for the PNG standard by defining a viewing environment.
+ * sRGB itself, and earlier TV standards, actually use a more complex transform
+ * (a linear portion then a gamma 2.4 power law) than PNG can express.  (PNG is
+ * limited to simple power laws.)  By saying that an image for direct display on
+ * an sRGB conformant system should be stored with a gAMA chunk value of 45455
+ * (11.3.3.2 and 11.3.3.5 of the ISO PNG specification) the PNG specification
+ * makes it possible to derive values for other display systems and
+ * environments.
+ *
+ * The Mac value is deduced from the sRGB based on an assumption that the actual
+ * extra viewing correction used in early Mac display systems was implemented as
+ * a power 1.45 lookup table.
+ *
+ * Any system where a programmable lookup table is used or where the behavior of
+ * the final display device characteristics can be changed requires system
+ * specific code to obtain the current characteristic.  However this can be
+ * difficult and most PNG gamma correction only requires an approximate value.
+ *
+ * By default, if png_set_alpha_mode() is not called, libpng assumes that all
+ * values are unencoded, linear, values and that the output device also has a
+ * linear characteristic.  This is only very rarely correct - it is invariably
+ * better to call png_set_alpha_mode() with PNG_DEFAULT_sRGB than rely on the
+ * default if you don't know what the right answer is!
+ *
+ * NOTE: the following values can be passed to either the fixed or floating
+ * point APIs, but the floating point API will also accept floating point
+ * values.
+ */
+#define PNG_DEFAULT_sRGB 0        /* sRGB gamma and color space */
+#define PNG_GAMMA_sRGB   220000   /* Television standards - matchs sRGB gamma */
+#define PNG_GAMMA_MAC    151724   /* Television with a 1.45 correction table */
+#define PNG_GAMMA_LINEAR PNG_FP_1 /* Linear */
+
+/* The following are examples of calls to png_set_alpha_mode to achieve the
+ * required overall gamma correction and, where necessary, alpha
+ * premultiplication.
+ *
+ * png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
+ *    This is the default libpng handling of the alpha channel - it is not
+ *    pre-multiplied into the color components.  In addition the call states
+ *    that the output is for a sRGB system and causes all PNG files without gAMA
+ *    chunks to be assumed to be encoded using sRGB.
+ *
+ * png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
+ *    In this case the output is assumed to be something like an sRGB conformant
+ *    display preceeded by a power-law lookup table of power 1.45.  This is how
+ *    early Mac systems behaved.
+ *
+ * png_set_alpha_mode(pp, PNG_ALPHA_STANDRAD, PNG_GAMMA_LINEAR);
+ *    This is the classic Jim Blinn approach and will work in academic
+ *    environments where everything is done by the book.  It has the shortcoming
+ *    of assuming that input PNG data with no gamma information is linear - this
+ *    is unlikely to be correct unless the PNG files where generated locally.
+ *    Most of the time the output precision will be so low as to show
+ *    significant banding in dark areas of the image.
+ *
+ * png_set_expand_16(pp);
+ * png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);
+ *    This is a somewhat more realistic Jim Blinn inspired approach.  PNG files
+ *    are assumed to have the sRGB encoding if not marked with a gamma value and
+ *    the output is always 16 bits per component.  This permits accurate scaling
+ *    and processing of the data.  If you know that your input PNG files were
+ *    generated locally you might need to replace PNG_DEFAULT_sRGB with the
+ *    correct value for your system.
+ *
+ * png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);
+ *    If you just need to composite the PNG image onto an existing background
+ *    and if you control the code that does this you can use the optimization
+ *    setting.  In this case you just copy completely opaque pixels to the
+ *    output.  For pixels that are not completely transparent (you just skip
+ *    those) you do the composition math using png_composite or png_composite_16
+ *    below then encode the resultant 8 or 16 bit values to match the output
+ *    encoding.
+ *
+ * Other cases
+ *    If neither the PNG nor the standard linear encoding work for you because
+ *    of the software or hardware you use then you have a big problem.  The PNG
+ *    case will probably result in halos around the image.  The linear encoding
+ *    will probably result in a washed out, too bright, image (it's actually too
+ *    contrasty.)  Try the ALPHA_OPTIMIZED mode above - this will probably
+ *    substantially reduce the halos.  Alternatively try:
+ *
+ * png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);
+ *    This option will also reduce the halos, but there will be slight dark
+ *    halos round the opaque parts of the image where the background is light.
+ *    In the OPTIMIZED mode the halos will be light halos where the background
+ *    is dark.  Take your pick - the halos are unavoidable unless you can get
+ *    your hardware/software fixed!  (The OPTIMIZED approach is slightly
+ *    faster.)
+ *
+ * When the default gamma of PNG files doesn't match the output gamma.
+ *    If you have PNG files with no gamma information png_set_alpha_mode allows
+ *    you to provide a default gamma, but it also sets the ouput gamma to the
+ *    matching value.  If you know your PNG files have a gamma that doesn't
+ *    match the output you can take advantage of the fact that
+ *    png_set_alpha_mode always sets the output gamma but only sets the PNG
+ *    default if it is not already set:
+ *
+ * png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
+ * png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
+ *    The first call sets both the default and the output gamma values, the
+ *    second call overrides the output gamma without changing the default.  This
+ *    is easier than achieving the same effect with png_set_gamma.  You must use
+ *    PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will
+ *    fire if more than one call to png_set_alpha_mode and png_set_background is
+ *    made in the same read operation, however multiple calls with PNG_ALPHA_PNG
+ *    are ignored.
+ */
+#endif
+
 #ifdef PNG_READ_STRIP_ALPHA_SUPPORTED
 PNG_EXPORT(36, void, png_set_strip_alpha, (png_structp png_ptr));
 #endif
@@ -1214,12 +1412,16 @@ PNG_EXPORT(49, void, png_set_quantize,
  */
 #define PNG_GAMMA_THRESHOLD (PNG_GAMMA_THRESHOLD_FIXED*.00001)
 
-/* Handle gamma correction. Screen_gamma=(display_exponent) */
+/* Handle gamma correction. Screen_gamma=(display_exponent).
+ * NOTE: this API simply sets the screen and file gamma values, it will
+ * therefore override the value for gamma in a PNG file if it is called after
+ * the file header has been read - use with care!
+ */
 PNG_FP_EXPORT(50, void, png_set_gamma,
     (png_structp png_ptr, double screen_gamma,
-    double default_file_gamma));
+    double override_file_gamma));
 PNG_FIXED_EXPORT(208, void, png_set_gamma_fixed, (png_structp png_ptr,
-    png_fixed_point screen_gamma, png_fixed_point default_file_gamma));
+    png_fixed_point screen_gamma, png_fixed_point override_file_gamma));
 #endif
 
 #ifdef PNG_WRITE_FLUSH_SUPPORTED
@@ -2328,7 +2530,7 @@ 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(226);
+  PNG_EXPORT_LAST_ORDINAL(228);
 #endif
 
 #ifdef __cplusplus

pngpriv.h

@@ -285,7 +285,7 @@ typedef PNG_CONST png_uint_16p FAR * png_const_uint_16pp;
 #define PNG_SWAP_BYTES          0x0010
 #define PNG_INVERT_MONO         0x0020
 #define PNG_QUANTIZE            0x0040
-#define PNG_BACKGROUND          0x0080
+#define PNG_COMPOSE             0x0080     /* Was PNG_BACKGROUND */
 #define PNG_BACKGROUND_EXPAND   0x0100
 #define PNG_EXPAND_16           0x0200     /* Added to libpng 1.5.2 */
 #define PNG_16_TO_8             0x0400
@@ -302,7 +302,7 @@ typedef PNG_CONST png_uint_16p FAR * png_const_uint_16pp;
 #define PNG_RGB_TO_GRAY_ERR   0x200000L
 #define PNG_RGB_TO_GRAY_WARN  0x400000L
 #define PNG_RGB_TO_GRAY       0x600000L  /* two bits, RGB_TO_GRAY_ERR|WARN */
-                       /*     0x800000L     Unused */
+#define PNG_ENCODE_ALPHA      0x800000L  /* Added to libpng-1.5.3 */
 #define PNG_ADD_ALPHA         0x1000000L  /* Added to libpng-1.2.7 */
 #define PNG_EXPAND_tRNS       0x2000000L  /* Added to libpng-1.2.9 */
                        /*   0x4000000L  unused */
@@ -332,9 +332,9 @@ typedef PNG_CONST png_uint_16p FAR * png_const_uint_16pp;
 #define PNG_FLAG_CRC_ANCILLARY_NOWARN     0x0200
 #define PNG_FLAG_CRC_CRITICAL_USE         0x0400
 #define PNG_FLAG_CRC_CRITICAL_IGNORE      0x0800
-                                /*        0x1000  unused */
+#define PNG_FLAG_ASSUME_sRGB              0x1000  /* Added to libpng-1.5.3 */
-                                /*        0x2000  unused */
+#define PNG_FLAG_OPTIMIZE_ALPHA           0x2000  /* Added to libpng-1.5.3 */
-                                /*        0x4000  unused */
+#define PNG_FLAG_DETECT_UNINITIALIZED     0x4000  /* Added to libpng-1.5.3 */
 #define PNG_FLAG_KEEP_UNKNOWN_CHUNKS      0x8000L
 #define PNG_FLAG_KEEP_UNSAFE_CHUNKS       0x10000L
 #define PNG_FLAG_LIBRARY_MISMATCH         0x20000L
@@ -852,8 +852,9 @@ PNG_EXTERN void png_do_shift PNGARG((png_row_infop row_info,
     png_bytep row, png_const_color_8p bit_depth));
 #endif
 
-#ifdef PNG_READ_BACKGROUND_SUPPORTED
+#if defined(PNG_READ_BACKGROUND_SUPPORTED) ||\
-PNG_EXTERN void png_do_background PNGARG((png_row_infop row_info,
+    defined(PNG_READ_ALPHA_MODE_SUPPORTED)
+PNG_EXTERN void png_do_compose PNGARG((png_row_infop row_info,
     png_bytep row, png_structp png_ptr));
 #endif
 
@@ -862,6 +863,11 @@ PNG_EXTERN void png_do_gamma PNGARG((png_row_infop row_info,
     png_bytep row, png_structp png_ptr));
 #endif
 
+#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+PNG_EXTERN void png_do_encode_alpha PNGARG((png_row_infop row_info,
+   png_bytep row, png_structp png_ptr));
+#endif
+
 #ifdef PNG_READ_EXPAND_SUPPORTED
 PNG_EXTERN void png_do_expand_palette PNGARG((png_row_infop row_info,
     png_bytep row, png_const_colorp palette, png_const_bytep trans,

pngstruct.h

@@ -135,7 +135,8 @@ struct png_struct_def
    png_uint_16 filler;           /* filler bytes for pixel expansion */
 #endif
 
-#ifdef PNG_bKGD_SUPPORTED
+#if defined(PNG_bKGD_SUPPORTED) || defined(PNG_READ_BACKGROUND_SUPPORTED) ||\
+   defined(PNG_READ_ALPHA_MODE_SUPPORTED)
    png_byte background_gamma_type;
    png_fixed_point background_gamma;
    png_color_16 background;   /* background color in screen gamma space */

pngwutil.c

@@ -315,7 +315,7 @@ png_zlib_release(png_structp png_ptr)
                break;
          }
 
-         png_warning_parameter_unsigned(p, 1, PNG_NUMBER_FORMAT_d, ret);
+         png_warning_parameter_signed(p, 1, PNG_NUMBER_FORMAT_d, ret);
          png_warning_parameter(p, 2, err);
 
          if (png_ptr->zstream.msg)
@@ -1549,7 +1549,7 @@ png_check_keyword(png_structp png_ptr, png_const_charp key, png_charpp new_key)
          PNG_WARNING_PARAMETERS(p)
 
          png_warning_parameter_unsigned(p, 1, PNG_NUMBER_FORMAT_02x,
-            *ikp);
+            (png_byte)*ikp);
          png_formatted_warning(png_ptr, p, "invalid keyword character 0x@1");
          *dp = ' ';
       }

scripts/pnglibconf.dfa

@@ -311,11 +311,13 @@ option READ_BGR requires READ_TRANSFORMS
 option READ_SWAP requires READ_TRANSFORMS READ_16BIT
 option READ_PACKSWAP requires READ_TRANSFORMS
 option READ_INVERT requires READ_TRANSFORMS
-option READ_BACKGROUND requires READ_TRANSFORMS
+option READ_BACKGROUND requires READ_TRANSFORMS enables READ_STRIP_ALPHA
 option READ_16_TO_8 requires READ_TRANSFORMS
 option READ_FILLER requires READ_TRANSFORMS
 option READ_GAMMA requires READ_TRANSFORMS enables READ_gAMA
 option READ_GRAY_TO_RGB requires READ_TRANSFORMS
+
+option READ_ALPHA_MODE requires READ_TRANSFORMS enables READ_GAMMA
 option READ_SWAP_ALPHA requires READ_TRANSFORMS
 option READ_INVERT_ALPHA requires READ_TRANSFORMS
 option READ_STRIP_ALPHA requires READ_TRANSFORMS

scripts/pnglibconf.h.prebuilt

@@ -69,6 +69,7 @@
 #define PNG_PROGRESSIVE_READ_SUPPORTED
 #define PNG_READ_16BIT_SUPPORTED
 #define PNG_READ_16_TO_8_SUPPORTED
+#define PNG_READ_ALPHA_MODE_SUPPORTED
 #define PNG_READ_ANCILLARY_CHUNKS_SUPPORTED
 #define PNG_READ_BACKGROUND_SUPPORTED
 #define PNG_READ_BGR_SUPPORTED

scripts/symbols.def

@@ -232,3 +232,5 @@ EXPORTS
  png_set_text_compression_strategy @224
  png_set_text_compression_window_bits @225
  png_set_text_compression_method @226
+ png_set_alpha_mode @227
+ png_set_alpha_mode_fixed @228

test-pngvalid-full.sh

@@ -7,9 +7,18 @@ echo >> pngtest-log.txt
 echo "============ pngvalid-full.sh ==============" >> pngtest-log.txt
 
 echo "Running test-pngvalid-full.sh"
-for gamma in threshold transform sbit 16-to-8
+for gamma in threshold transform sbit 16-to-8 background alpha-mode
 do
-   if ./pngvalid "--gamma-$gamma" >> pngtest-log.txt 2>&1
+   # For the moment the composition calculation is performed with minimal
+   # accuracy, do this to work round the problem:
+   if test $gamma = background -o $gamma = alpha-mode
+   then
+      opts=--use-linear-precision
+   else
+      opts=
+   fi
+
+   if ./pngvalid $opts "--gamma-$gamma" >> pngtest-log.txt 2>&1
    then
       echo "  PASS:" pngvalid "--gamma-$gamma"
    else