alphaMode Not Set to MASK for PNGs With Transparency

[KermMartian]

What it says on the tin. If an input COLLADA file has a PNG image with transparency as a texture, the alphaMode property of the resulting GLTF texture is not set. Inspecting the source code, I see that the GLTFMaterial class supports alphaMode and alphaCutoff fields, but that nothing in the COLLADA2GLTF::Writer sets the alphaMode to MASK. I understand it would be annoying to scan each input image to determine if it contains any transparency, but I propose that it might be reasonable to assume a PNG is being used for its transparency, and set alphaMode to MASK any time a PNG is encountered.

If you agree, I'm happy to make a PR for this.

Edit: In addition, it appears this stanza in the input COLLADA model:

            <transparent>
              <texture texture="model_png-sampler" texcoord="UVMap"/>
            </transparent>

is not triggering this code in COLLADA2GLTF::Writer:

COLLADA2GLTF/src/COLLADA2GLTFWriter.cpp

Lines 1115 to 1119 in 6e11fe6

	if (transparent.isTexture()) {
	std::cerr << "WARNING: Effect " << effect->getOriginalId() <<
	" contains a transparent texture that will be omitted from" <<
	" the converted model." << std::endl;
	}

[lasalvavida]

Hi @KermMartian. I think I'd prefer to do alphaMode and alphaCutoff by command line flag. Would be happy to take a PR for that.

[lasalvavida]

As for the latter part, could you open up a separate issue and attach the relevant model?

This is likely an OpenCOLLADA issue, but I'd like to be able to triage it.