Reduce rendered images without considering opacity

[biboyd]

PR Summary

Issue

This addresses issue #4871, where volume renders of multiple fields with MPI would only produce a portion of the second field in the final image.

I believe I have found where this bug occurs. It mainly has to do with the combining of image arrays in yt/utilities/amr_kdtree/amr_kdtools.py:receive_and_reduce() and how the alpha channel is set in yt/visualization/volume_rendering/render_source.py:VolumeSource.finialize_image()

After the first source is rendered, the last step of finalize_image() sets the alpha channel of the image array to 1's

https://github.com/yt-project/yt/blob/6d121f58543bedb1f587492cd0601a69f06a736a/yt/visualization/volume_rendering/render_source.py#L535-L537

This causes issues when the second source is combining the image arrays in recieve_and_reduce() as these two arrays are combined by scaling the back image by ta, 1 minus the front alpha channel. Since the alpha channel was already set by the first source, ta is a zeros array. This eliminates any contribution of back to the final image.

https://github.com/yt-project/yt/blob/6d121f58543bedb1f587492cd0601a69f06a736a/yt/utilities/amr_kdtree/amr_kdtools.py#L25-L34

Solution

My proposed solution is to add a pathway to combine image arrays without the consideration of opacity. This would be used when transfer_function.grey_opacity=False. In this case, front and back don't have any relevance and we can simply add the two image arrays.

PR Checklist

• [ ] New features are documented, with docstrings and narrative docs
• [ ] Adds a test for any bugs fixed. Adds tests for new features.

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[chrishavlin]

thanks for the pull request! I'll review in detail soon, but you're getting a lot of real test failures that you could fix by making your new use_opacity argument a keyword argument instead (and ideally the default value would be such that it does not change current behavior if you do not set the keyword argument).

[biboyd]

Thanks! Made the change to a keyword and now reduce_tree_images should default to the original behavior

[cphyc]

@yt-fido test this please

[chrishavlin]

@yt-fido test this please

[neutrinoceros]

Do you mean we don't have any form of continuous testing with MPI yet ? if so, I'd agree to leave it out of this PR, but we should probably open issue for it.

[chrishavlin]

Do you mean we don't have any form of continuous testing with MPI yet ? if so, I'd agree to leave it out of this PR, but we should probably open issue for it.

Ya, that's what I meant (but wasn't positive about it).

So IMO this PR is OK as it is.

[neutrinoceros]

Thanks for clarifying. Feel free to merge then.

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