DWA and sLegacyChannelRules

[lemmyg]

Hello,
in our company we are facing an issue using DWA compression when the channels are named as '__diffuse.r' with rgb as lowercase.
With lowercase, channels are not compressed. However when we use capital letters (RGB) they are.

Looking https://github.com/AcademySoftwareFoundation/openexr/blob/main/src/lib/OpenEXRCore/internal_dwa_classifier.h
sLegacyChannelRules strucure. Seems like lowercase are supported.

Is lowercase supported? whats the expected standard?

The openexr versions I have tested are 3.1.4, 3.1.11 and 3.2.4.

Regards,
Galder

[peterhillman]

Very old DWA compressed files are "version 1" which use the sLegacyChannelRules that allow lower case letters. The legacy rules exist so those files are still read correctly. I don't think there was a public release which wrote DWA version 1 files. All versions of OpenEXR since version 2.2.0 use DWA version 2, which only treats upper case R,G,B as color channels.

The OpenEXR standard is to use upper case R,G,B for color channel names:
https://openexr.com/en/latest/TechnicalIntroduction.html#image-channels-and-sampling-rates
https://openexr.com/en/latest/TechnicalIntroduction.html#channel-names
That doesn't make it clear, but that also applies to channels within a layer, so __diffuse.R is a red channel, but __diffuse.r and __diffuse.red are not. In particular, the RgbaFile interface requires upper case letters for channel names regardless of layer.

Being too lenient in DWA could be problematic as it could trigger lossy compression in a channel that isn't intended to be a color channel. #1942 describes a case where DWA interprets the Y channel as luminance but it is actually a Y coordinate (so __normal.Y is a luminance channel and should have lossy compression applied, but __normal.y isn't)

[lemmyg]

great thanks for clear it.