Save matrix_coefficients when exporting AVIF

[novomesk]

When saving AVIF with ICC profile, RGB_to_YcbCr conversion is done according r*0.299f + g*0.587f + b*0.114f coefficients. This represents matrix coefficients 6 (BT.601).

Unfortunately other AVIF implementations don’t know that these coefficients were used during RGB->YUV conversion, and they might use different coefficients when they open the saved file and perform YUV->RGB conversion. User will get different colors and that’s wrong.

If you tell AOM encoder what coefficients where used, other implementation will use the corresponding coefficients too. I think something like this should work: aom_codec_control(&encoder->codec, AV1E_SET_MATRIX_COEFFICIENTS, 6 /*heif_matrix_coefficients_ITU_R_BT_601_6*/);

I observed there is a possibility to set NCLX profile when saving AVIF: heif_image_set_nclx_color_profile

If user sets matrix_coefficients via heif_color_profile_nclx, they should be used in RGB->YUV conversion too. If only BT.601 coefficients are used during export, value 6 should be forced on output.

[farindk]

Ok. My plan is as follows: when a nclx profile was set, RGB input will be converted with these coefficients. Otherwise, ITU_R_BT_601_6 will be used (and saved) as default. If the input is YCbCr, it will be assumed that these are already in the correct colorspace. No conversion is done.

[farindk]

Should work now. Please test. There are also options in heif-enc to set the nclx parameters.

[novomesk]

I tried heif-enc and the value 6 is saved:

 * Resolution     : 2048x1368
 * Bit Depth      : 8
 * Format         : YUV420
 * Alpha          : Absent
 * Range          : Limited
 * Color Primaries: 0
 * Transfer Char. : 0
 * Matrix Coeffs. : 6

But I think the Color Primaries and Transfer Characteristics should not be zero, better use 2 == unspecified.

BTW, why limited range?

I tried to save AVIF from my GIMP plug-in and the Matrix Coeffs. was still 2.

[farindk]

I changed the defaults. When no nclx profile is set, the default matrix coeffs should be 6. And now primaries and transfer = undefined, full_range = true. According to my tests, this appears to work.

[novomesk]

OK, now we have matrix_coefficients saved when exporting AVIF without ICC profile.

But when we have ICC (stored colour_type="prof" so we can't have colour_type="nclx" now), will you save the coefficients to AV1 stream? (so libavif and other implementations will find it)

Even in situations when you are saving colour_type="nclx" only (no ICC), you can pass the 3 values (prim/transf_char./matrix_coefficients) to AV1 encoder, there is no problem to have identical values in NCLX and in AV1 stream.

[novomesk]

The good news is that now when I use heif_image_set_nclx_color_profile when saving AVIF, the image looks visually OK when decoded via libavif-based SW. BTW there was a bug in libavif (https://github.com/AOMediaCodec/libavif/issues/281), it used full_range_flag from AV1 stream and not from NCLX profile. Libavif thought the image is limited range instead of full range. I think when NCLX is present, same values can be sent to AV1 stream too.

When I save AVIF with ICC profile using heif_image_set_raw_color_profile (h_image, "prof", icc_data, icc_length); I still get different colours when decoding picture (both heif-convert or libavif-based SW). The result looks like decoding with Limited range. That's why there is a need to signal full range in AV1 stream. When I added following line to heif_encoder_aom.cc, the decoded picture looked much better: aom_codec_control(&codec, AV1E_SET_COLOR_RANGE, AOM_CR_FULL_RANGE);