oxipng transform turns transparent-black part of PNG to transparent-white

[chgi]

Describe the bug
I have a model with an attached PNG emissive texture.
For some reason, the texture has an alpha channel (I think it shouldn't be there, but we got it that way).
Using gltf-transform oxipng seems to turn all pixels that are transparent-black to transparent-white.

To Reproduce

• Create a model in Blender
• attach an emissive texture with alpha channel that has areas with RGBA=(0,0,0,0)
• export to Demo.glb
• execute NODE_OPTIONS="--no-experimental-fetch" gltf-transform oxipng Demo.glb Demo_oxipng.glb
• view result in https://sandbox.babylonjs.com/
• confirm that the areas that were previously RGBA=(0,0,0,0) turned to RGBA=(255,255,255,0)
• since alpha should be ignored for emissive, the parts that were previously black are now white

Expected behavior
The RGB values should not change. Black should stay black.
I realize that that the RGB value of a fully transparent pixel should not matter, but this behaviour breaks the lossless promise of PNG.

Versions:

• Version: cli.js 2.4.4
• Environment: Windows 11, Node.js 18.10.0

Additional context
I tried to put the texture through oxipng standalone using https://squoosh.app/. The problem doesn't seem to occur there.

Before and after using gltf-transform oxipng

Zip containing the source .blend file, the original texture and the glb before and after
Demo.zip

[donmccurdy]

Hi @chgi! Thanks for the bug report.

Unfortunately, I'm getting the same (incorrect) result when running the Squoosh CLI manually:

npm install --global @squoosh/cli
squoosh-cli --oxipng "{}" ~/Downloads/Demo/emit.png -d ~/Desktop/

glTF-Transform relies on the Node.js squoosh builds (@squoosh/lib) so it sounds like this may be an issue affecting only that version of the Squoosh codecs, and not their web app.

[chgi]

Hmm, I didn't think to test with the squoosh lib directly.

I investigated a bit and it looks like squoosh is deliberately enabling the alpha optimization of oxipng here.
This option seem seems to disabled by default exactly because stuff like I wrote above happens (see here).

Do you reckon, I should open a bug over at squoosh and ask them to at least expose the option to turn it off in the lib?

[donmccurdy]

I think it's worth filing the issue! They may not be aware that this makes a difference for WebGL, and perhaps other graphics-related use cases. Unfortunately it has been a while since the last squoosh updates were published.

[donmccurdy]

@chgi could you give this a try on your OS?

npm install --global @gltf-transform/cli@next

It's a prerelease for v3 (with #752), including sharp instead of squoosh. I believe sharp handles the alpha as expected here.

[chgi]

@donmccurdy , thanks for the heads-up!

Just tried it with the attached file on Windows11, Node 18.12.
Using the originally attached files, the output seems correct now.

The output file size is a bit larger than with the old version (as expected), but still smaller than the input.

[donmccurdy]

Thanks! To be fixed in v3 with #752.