Error in techniques for VC model

[khronosgroup]

Error in techniques for VC model

[pjcozzi]

@javagl thanks for all the research. Is this now duplicate with KhronosGroup/glTF#576? Should we consolidate these issues?

[javagl]

They are related, at least.

This issue only refers to the fact that a technique that eventually refers to a semi-opaque PNG does not contain the GL_BLEND state - but I think that it has to contain this, in order to be rendered properly (and in a previous version, it did contain it).

If it contained the GL_BLEND state, it would require additional techniques.states.functions - and this is the relationship to the other issue:

The issue 576 asked about these boxes. They appear in "non-sopisiticated" renderers (that do not use OIT or other fancy stuff). The reason of why the boxes appear is what I currently consider as an error in the glTF file: I think that that blendFuncSeparate parameters are wrong for transparent materials. At least, using GL_SRC_ALPHA for the source seems to be intuitively reasonable, and at corresponds to the "typical" setting that is also mentioned in https://www.opengl.org/archives/resources/faq/technical/transparency.htm#blen0020

I wondered about where to report this: Here in glTF-Sample-Models, or in COLLADA2GLTF, or the gltf-pipeline? You added the collada2gltf label at the other issue, and that's certainly a reasonable point to start investigating both issues (although one could first check whether the technique is already wrong or ambiguous in the .DAE input file, and only start digging deeper if not)

EDIT: From a quick glance, at least the technique.states.functions part seems to be based on the premultipledAlpha setting ( https://github.com/KhronosGroup/COLLADA2GLTF/blob/master/shaders/commonProfileShaders.cpp#L385 ). I'll have to read more about this. Sorry for bothering you.

[javagl]

So I'll have read more about premultiplied alpha (which is ENabled in WebGL by default, but DISabled in OpenGL usually), find the answer to the question of whether the fragment shader should be

gl_FragColor = color;
gl_FragColor = color * u_transparency;
gl_FragColor = vec4(color.rgb * diffuse.a, diffuse.a * u_transparency);
gl_FragColor = vec4(color.rgb * diffuse.a * u_transparency, diffuse.a * u_transparency);

(or whatever) in conjunction with the premultiplied alpha setting, and figure out how to render two assets at the same time, one with premultiplied alpha and one without. But all this is probably only an issue of the renderer implementation, and not of the sample models. Sorry again for the noise caused by my lack of knowledge here.

The texture is still rendered in "opaque black" in cesium, but maybe this is not an issue.