EomMetadataReaderPng: Use built-in SRGB profile for matching cHRM/gAMA

The values used for sRGB in cHRM and gAMA chunks do not compute to a
profile that exactly matches the built-in profile. Catch this case and
use the built-in profile instead to avoid color differences.

upstream commit:
https://gitlab.gnome.org/GNOME/eog/commit/9589430c

1 files changed, 40 insertions, 5 deletions

diff --git a/src/eom-metadata-reader-png.c b/src/eom-metadata-reader-png.c
index ebf2ddf..eadf036 100644
--- a/src/eom-metadata-reader-png.c
+++ b/src/eom-metadata-reader-png.c
@@ -25,6 +25,7 @@
 #include <config.h>
 #endif
 
+#include <math.h>
 #include <string.h>
 #include <zlib.h>
 
@@ -489,6 +490,31 @@ eom_metadata_reader_png_get_xmp_data (EomMetadataReaderPng *emr )
  * A maximum output buffer of 5MB should be enough. */
 #define EOM_ICC_INFLATE_BUFFER_LIMIT (1024*1024*5)
 
+/* Apparently an sRGB profile saved in cHRM and gAMA chunks does not compute
+ * a profile that exactly matches the built-in sRGB profile and thus could
+ * cause a slight color deviation. Try catching this case to allow fallback
+ * to the built-in profile instead.
+ */
+static gboolean
+_chrm_matches_srgb(const cmsCIExyY       *whitepoint,
+		   const cmsCIExyYTRIPLE *primaries,
+		         gdouble          gammaValue)
+{
+	/* PNGs gAMA value for 2.2 is only accurate to the 4th decimal point */
+#define DOUBLE_EQUAL_MAX_DIFF 1e-4
+#define DOUBLE_EQUAL(a,b) (fabs (a - b) < DOUBLE_EQUAL_MAX_DIFF)
+
+	return (DOUBLE_EQUAL(gammaValue, 2.2)
+		&& DOUBLE_EQUAL(whitepoint->x, 0.3127)
+		&& DOUBLE_EQUAL(whitepoint->y, 0.329)
+		&& DOUBLE_EQUAL(primaries->Red.x, 0.64)
+		&& DOUBLE_EQUAL(primaries->Red.y, 0.33)
+		&& DOUBLE_EQUAL(primaries->Green.x, 0.3)
+		&& DOUBLE_EQUAL(primaries->Green.y, 0.6)
+		&& DOUBLE_EQUAL(primaries->Blue.x, 0.15)
+		&& DOUBLE_EQUAL(primaries->Blue.y, 0.06));
+}
+
 static gpointer
 eom_metadata_reader_png_get_icc_profile (EomMetadataReaderPng *emr)
 {
@@ -600,11 +626,20 @@ eom_metadata_reader_png_get_icc_profile (EomMetadataReaderPng *emr)
 		whitepoint.Y = primaries.Red.Y = primaries.Green.Y = primaries.Blue.Y = 1.0;
 
 		gammaValue = (double) 1.0/EXTRACT_DOUBLE_UINT_BLOCK_OFFSET (priv->gAMA_chunk, 0, 100000);
-		gamma[0] = gamma[1] = gamma[2] = cmsBuildGamma (NULL, gammaValue);
-
-		profile = cmsCreateRGBProfile (&whitepoint, &primaries, gamma);
-
-		cmsFreeToneCurve(gamma[0]);
+		eom_debug_message (DEBUG_LCMS, "Gamma %.5lf", gammaValue);
+
+		/* Catch SRGB in cHRM/gAMA chunks and use accurate built-in
+		 * profile instead of computing one that "gets close". */
+		if(_chrm_matches_srgb (&whitepoint, &primaries, gammaValue)) {
+			eom_debug_message (DEBUG_LCMS, "gAMA and cHRM match sRGB");
+			profile = cmsCreate_sRGBProfile ();
+		} else {
+			gamma[0] = gamma[1] = gamma[2] =
+				cmsBuildGamma (NULL, gammaValue);
+			profile = cmsCreateRGBProfile (&whitepoint, &primaries,
+						       gamma);
+			cmsFreeToneCurve(gamma[0]);
+		}
 	}
 
 	return profile;