Why is ALPHATEST a macro definition?

[joeedh]

This has made texture-mapped transparency pretty much impossible for me (recompiling the shader every frame is a bit much). Before I do anything like maintain two copies of every material, I'd like to know why this decision was made.

Thanks.

[WestLangley]

It was done that way to avoid the if-statement. alphaTest is not set by most users most of the time.

#ifdef ALPHATEST

    if ( gl_FragColor.a < ALPHATEST ) discard;

#endif

[joeedh]

How do people do texture-mapped transparency, then?

[mrdoob]

How do people do texture-mapped transparency, then?

You mean alphaMap?

[joeedh]

Yes. Texture-mapped transparency results in a combination of solid and transparent faces. One way to draw this is to use binary alpha with a low threshold to draw only the solid parts, then reset it to draw the transparent bits.

This requires changing the cutoff threshold, however.

[mrdoob]

material.alphaTest = 0.5;

[joeedh]

Meaning. . .?

[mrdoob]

I may not have understood your question. Do you mind rephrasing it?

[joeedh]

I need to draw transparent stuff. I.e. I need to draw a solid layer first, then a transparent layer (that's the simplest way to do it). That, in turn, requires that alphaTest not be a macro, since I'm dealing with texture-mapped transparency (i.e. faces can have both solid and transparent bits), and cannot simply sort faces into transparent/solid categories.

[mrdoob]

When you say layer, do you mean 2 different objects or you're talking about 2 layers/passes in the same material?

And why alphaTest can't be a macro?

Some screenshots may help...

[joeedh]

Two layers/passes in the same material. Basically, I change all the materials in the scene to have alphaTest=0.9, call WebGLRenderer.render(), change them back to what they were before, then call WebGLRenderer.render() again.

The idea is to render anything solid (including solid parts of transparency-mapped textures) into the zbuffer first, then render the transparent pass over it.

[mrdoob]

I'm sorry but I'm having a hard time to picture what you're trying to do.
Could you share screenshots?

[WestLangley]

@joeedh Actually, a simple demo would be more illustrative. There are too many use cases to comment without knowing exactly what you are doing.

Also show what you would like to do, and what you are doing as a work-around.

Also explain why the work-around is not acceptable to you.

[joeedh]

Unfortunately I can't upload to github from this computer, so I've put a screenshot in a pastebin at:

http://www.pasteall.org/pic/80456

That's rendered with one (distorted) cube. In my actual use case (which I can't show, this is a mock up) there are transparent reflections rendered in the texture. To render something like this, you need to render the solid (100% alpha) bits first, then draw the transparent window panes on top.

[UPDATE] This keeps the zbuffer from getting messed up. The normal method is to have separate opaque and solid faces; rendering becomes simple, just draw opaque triangles before transparent ones. Sadly, that isn't possible in this case.

[joeedh]

Also, see https://www.opengl.org/wiki/Transparency_Sorting#Opaque_first