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set NCLX defaults to match sRGB and change handling of unspecified NCLX values #1017

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merged 4 commits into from
Oct 30, 2023
Merged

set NCLX defaults to match sRGB and change handling of unspecified NCLX values #1017

merged 4 commits into from
Oct 30, 2023

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farindk
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@farindk farindk commented Oct 30, 2023

This is an alternative implementation of #1015.

kleisauke
kleisauke reviewed Oct 30, 2023
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libheif/nclx.cc Outdated
m_colour_primaries = heif_color_primaries_ITU_R_BT_709_5;
}

if (m_transfer_characteristics == heif_color_primaries_unspecified) {
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Suggested change
if (m_transfer_characteristics == heif_color_primaries_unspecified) {
if (m_transfer_characteristics == heif_transfer_characteristic_unspecified) {

kmilos
kmilos reviewed Oct 30, 2023
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examples/heif_enc.cc Outdated
uint16_t nclx_matrix_coefficients = 6;
uint16_t nclx_colour_primaries = 2;
uint16_t nclx_transfer_characteristic = 2;
uint16_t nclx_matrix_coefficients = 1;
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Should be 5 for sRGB

libheif/nclx.cc Outdated
m_matrix_coefficients = 6;
m_colour_primaries = 1;
m_transfer_characteristics = 13;
m_matrix_coefficients = 1;
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5 for sRGB

libheif/nclx.cc Outdated
void color_profile_nclx::replace_undefined_values_with_defaults()
{
if (m_matrix_coefficients == heif_matrix_coefficients_unspecified) {
m_matrix_coefficients = heif_matrix_coefficients_ITU_R_BT_709_5;
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heif_matrix_coefficients_ITU_R_BT_470_6_System_B_G (i.e. 5) for sRGB

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farindk commented Oct 30, 2023

Are there any references what the values should be for sRGB? I found this random piece of code: https://observablehq.com/@eeeps/color-space-determiner that also seems to favor 1,13,1.

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kmilos commented Oct 30, 2023 •
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I found this random piece of code: https://observablehq.com/@eeeps/color-space-determiner that also seems to favor 1,13,1.

Plain wrong. See https://www.itu.int/rec/T-REC-H.273 for IEC 61966-2-1 (sRGB/sYCC) legal values.

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farindk commented Oct 30, 2023 •
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Accoding to H273, color_primaries=1. I think we agree here.
It also says to use transfer_characteristics=13.
Then, H273 says to use matrix_coefficients=0 for SRGB. Well ok, but that's not what we're looking for.
How do you derive that matrix_coefficients should be 5 or 1 ?

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I found these two comments:
AOMediaCodec/av1-avif#195 (comment)
MPEGGroup/MIAF#2 (comment)

Which says that matrix_coefficients defaults to 5 or 6 for sRGB.

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kmilos commented Oct 30, 2023 •
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Then, H273 says to use matrix_coefficients=0 of SRGB. Well ok, but that's not what we're looking for.
How do you derive that matrix_coefficients should be 5 or 1 ?

IEC 61966-2-1 mentions sYCC representation that specifically uses matrix coeffs 5 (601).

However, note that the same wikipedia page also mentions sRGB conversion to XYZ using the 709 coeffs earlier...

Also note that the JPEG/JFIF specification uses 601 matrix coeffs.

@kleisauke kleisauke mentioned this pull request Oct 30, 2023
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farindk commented Oct 30, 2023

Ok, that makes sense. Then we'll go for matrix_coefficients=5.

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For reference, libavif seems to default to 13/1/6:
https://github.com/AOMediaCodec/libavif/blob/f9dd31fea43f748ce1a6e49be4cd9e68b916ebe6/apps/avifenc.c#L2269-L2276
https://github.com/AOMediaCodec/libavif/blob/f9dd31fea43f748ce1a6e49be4cd9e68b916ebe6/apps/avifenc.c#L1460

@farindk
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farindk commented Oct 30, 2023

For reference, libavif seems to default to 13/1/6: https://github.com/AOMediaCodec/libavif/blob/f9dd31fea43f748ce1a6e49be4cd9e68b916ebe6/apps/avifenc.c#L2269-L2276 https://github.com/AOMediaCodec/libavif/blob/f9dd31fea43f748ce1a6e49be4cd9e68b916ebe6/apps/avifenc.c#L1460

Yes. But since matrix_coefficients 5 and 6 are exactly the same, this does not matter.

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kmilos commented Oct 30, 2023 •
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For reference, libavif seems to default to 13/1/6

Yeah, IIRC this was because either Chrome or Safari originally only implemented parsing that combo and not (IMHO more canonical) 13/1/5 in addition?!

@farindk farindk merged commit d5a5559 into master Oct 30, 2023
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farindk commented Oct 30, 2023

Thanks, @kleisauke , @kmilos

farindk added a commit that referenced this pull request Oct 30, 2023
@farindk
use MC=6 for sRGB because of potential incompatibility of Safari/iOS …
4809d30 
…with MC=5 (#1017)
farindk added a commit that referenced this pull request Oct 30, 2023
@farindk
use MC=6 for sRGB because of potential incompatibility of Safari/iOS …
7cd873f 
…with MC=5 (#1017)
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