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fix: Remove cHRM check to accomodate ACES AP1

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ACES AP1 has a red endpoint with a negative Z, this triggers the checks
in libpng that ensure that x, y and z (chromaticities) are all >=0.
This removes the checks on the sign of the chromaticities since it is
valid to use negative values for any of them and converts the "internal"
error code return to external (because the internal cases correspond to
negative x, y or z.)

Reviewed-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>
This commit is contained in:
John Bowler 2024-09-03 17:11:20 -07:00 committed by Cosmin Truta
parent 7e18d14296
commit 20f819c29e
1 changed files with 6 additions and 20 deletions
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26
png.c
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@ -1257,20 +1257,6 @@ png_XYZ_from_xy(png_XYZ *XYZ, const 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). We check
* xy->whitey against 5, not 0, to avoid a possible integer overflow.
*/
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 < 5 || 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;
@ -1451,16 +1437,16 @@ png_XYZ_from_xy(png_XYZ *XYZ, const png_xy *xy)
* value of 2 indicates an internal error to the caller.
*/
if (png_muldiv(&left, xy->greenx-xy->bluex, xy->redy - xy->bluey, 7) == 0)
return 2;
return 1;
if (png_muldiv(&right, xy->greeny-xy->bluey, xy->redx - xy->bluex, 7) == 0)
return 2;
return 1;
denominator = left - right;
/* Now find the red numerator. */
if (png_muldiv(&left, xy->greenx-xy->bluex, xy->whitey-xy->bluey, 7) == 0)
return 2;
return 1;
if (png_muldiv(&right, xy->greeny-xy->bluey, xy->whitex-xy->bluex, 7) == 0)
return 2;
return 1;
/* Overflow is possible here and it indicates an extreme set of PNG cHRM
* chunk values. This calculation actually returns the reciprocal of the
@ -1473,9 +1459,9 @@ png_XYZ_from_xy(png_XYZ *XYZ, const png_xy *xy)
/* Similarly for green_inverse: */
if (png_muldiv(&left, xy->redy-xy->bluey, xy->whitex-xy->bluex, 7) == 0)
return 2;
return 1;
if (png_muldiv(&right, xy->redx-xy->bluex, xy->whitey-xy->bluey, 7) == 0)
return 2;
return 1;
if (png_muldiv(&green_inverse, xy->whitey, denominator, left-right) == 0 ||
green_inverse <= xy->whitey)
return 1;