The issue originates to the change in default view transform from Filmic
to AgX, which does slightly different clipping, and clips color to black
if there is any negative values.
This change implements an idea of skipping view transform for viewer
node when it is connected to the Pick output of the cryptomatte node.
It actually goes a bit deeper than this and any operation can tag its
result as a non-color data, and the viewer node will respect that.
It is achieved by passing some extra meta-data along the evaluation
pipeline. For the CPU compositor it is done via MetaData, and for the
GPU compositor it is done as part of Result.
Connecting any other node in-between of viewer and Cryptomatte's Pick
will treat the result as color values, and apply color management.
Connecting Pick to the Composite output will also consider it as color,
since there is no concept of non-color-managed render result.
An alternative approaches were tested, including:
- Doing negative value clamping at the viewer node.
It does not work for legacy cryptomatte node, as it needs to have
access to original non-modified Pick result.
- Change the order of components, and store ID in another channel.
Using one of other of Green or Blue channels might work for some view
transforms, but it does not work for AgX.
Using Alpha channel seemingly works better, but it is has different
issues caused by the fact that display transform de-associates alpha,
leading to over-exposed regions which are hard to see in the file from
the report. And might lead to the similar issues as the initial report
with other objects or view transforms.
- Use positive values in the Pick channel.
It does make things visible, but they are all white due to the nature
of how AgX works, making it not so useful as a result.
Pull Request: https://projects.blender.org/blender/blender/pulls/122177
1b18e07232