Write primaries and transfer characteritics info in decoded png. (#1422)

* Write primaries and transfer characteritics info in decoded png.

If there is no ICC profile, the CICP color information is added to the file
using various png chunks: cHRM, gAMA (if possible), sRGB (if srgb), cICP.

apps/shared/avifpng.c

@@ -503,8 +503,42 @@ avifBool avifPNGWrite(const char * outputFilename, const avifImage * avif, uint3
     }
 
     png_set_IHDR(png, info, avif->width, avif->height, rgbDepth, colorType, PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
-    if (avif->icc.data && (avif->icc.size > 0)) {
+
+    const avifBool hasIcc = avif->icc.data && (avif->icc.size > 0);
+    if (hasIcc) {
+        // If there is an ICC profile, the CICP values are irrelevant and only the ICC profile
+        // is written. If we could extract the primaries/transfer curve from the ICC profile,
+        // then they could be written in cHRM/gAMA chunks.
         png_set_iCCP(png, info, "libavif", 0, avif->icc.data, (png_uint_32)avif->icc.size);
+    } else {
+        const avifBool isSrgb = (avif->colorPrimaries == AVIF_COLOR_PRIMARIES_BT709) &&
+                                (avif->transferCharacteristics == AVIF_TRANSFER_CHARACTERISTICS_SRGB);
+        if (isSrgb) {
+            png_set_sRGB_gAMA_and_cHRM(png, info, PNG_sRGB_INTENT_PERCEPTUAL);
+        } else {
+            if (avif->colorPrimaries != AVIF_COLOR_PRIMARIES_UNKNOWN && avif->colorPrimaries != AVIF_COLOR_PRIMARIES_UNSPECIFIED) {
+                float primariesCoords[8];
+                avifColorPrimariesGetValues(avif->colorPrimaries, primariesCoords);
+                png_set_cHRM(png,
+                             info,
+                             primariesCoords[6],
+                             primariesCoords[7],
+                             primariesCoords[0],
+                             primariesCoords[1],
+                             primariesCoords[2],
+                             primariesCoords[3],
+                             primariesCoords[4],
+                             primariesCoords[5]);
+            }
+            float gamma;
+            // Write the transfer characteristics IF it can be represented as a
+            // simple gamma value. Most transfer characteristics cannot be
+            // represented this way. Viewers that support the cICP chunk can use
+            // that instead, but older viewers might show incorrect colors.
+            if (avifTransferCharacteristicsGetGamma(avif->transferCharacteristics, &gamma) == AVIF_RESULT_OK) {
+                png_set_gAMA(png, info, 1.0f / gamma);
+            }
+        }
     }
 
     png_text texts[2];
@@ -543,6 +577,18 @@ avifBool avifPNGWrite(const char * outputFilename, const avifImage * avif, uint3
 
     png_write_info(png, info);
 
+    // Custom chunk writing, must appear after png_write_info.
+    // With AVIF, an ICC profile takes priority over CICP, but with PNG files, CICP takes priority over ICC.
+    // Therefore CICP should only be written if there is no ICC profile.
+    if (!hasIcc) {
+        const png_byte cicp[5] = "cICP";
+        const png_byte cicpData[4] = { (png_byte)avif->colorPrimaries,
+                                       (png_byte)avif->transferCharacteristics,
+                                       AVIF_MATRIX_COEFFICIENTS_IDENTITY,
+                                       1 /*full range*/ };
+        png_write_chunk(png, cicp, cicpData, 4);
+    }
+
     rowPointers = (png_bytep *)malloc(sizeof(png_bytep) * avif->height);
     if (monochrome8bit) {
         uint8_t * yPlane = avif->yuvPlanes[AVIF_CHAN_Y];

include/avif/avif.h

@@ -319,6 +319,10 @@ enum
 };
 typedef uint16_t avifTransferCharacteristics; // AVIF_TRANSFER_CHARACTERISTICS_*
 
+// If the given transfer characteristics can be expressed with a simple gamma value, sets 'gamma'
+// to that value and returns AVIF_RESULT_OK. Returns an error otherwise.
+AVIF_API avifResult avifTransferCharacteristicsGetGamma(avifTransferCharacteristics atc, float * gamma);
+
 enum
 {
     AVIF_MATRIX_COEFFICIENTS_IDENTITY = 0,

src/colr.c

@@ -69,6 +69,23 @@ avifColorPrimaries avifColorPrimariesFind(const float inPrimaries[8], const char
     return AVIF_COLOR_PRIMARIES_UNKNOWN;
 }
 
+avifResult avifTransferCharacteristicsGetGamma(avifTransferCharacteristics atc, float * gamma)
+{
+    switch (atc) {
+        case AVIF_TRANSFER_CHARACTERISTICS_BT470M:
+            *gamma = 2.2f;
+            return AVIF_RESULT_OK;
+        case AVIF_TRANSFER_CHARACTERISTICS_BT470BG:
+            *gamma = 2.8f;
+            return AVIF_RESULT_OK;
+        case AVIF_TRANSFER_CHARACTERISTICS_LINEAR:
+            *gamma = 1.0f;
+            return AVIF_RESULT_OK;
+        default:
+            return AVIF_RESULT_INVALID_ARGUMENT;
+    }
+}
+
 struct avifMatrixCoefficientsTable
 {
     avifMatrixCoefficients matrixCoefficientsEnum;