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What are the differences between gl_TexCoord[0].s, gl_TexCoord[0].t gl_TexCoord[0].p and gl_TexCoord[0].x, gl_TexCoord[0].y gl_TexCoord[0].z ?

I understand that gl_TexCoord[0].x, gl_TexCoord[0].y gl_TexCoord[0].z are pixel coordinates?

But what do gl_TexCoord[0].s, gl_TexCoord[0].t gl_TexCoord[0].p mean and where must I use them?

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The difference is only semantic. Any GLSL vector can be accessed using x/y/z/w, r/g/b/a or s/t/p/q. Typical use is:

  • xyzw for space coordinates
  • rgba for colours
  • stpq for texture coordinates
  • if none of the above applies, any flavour can be used, but when it could be confusing, var[0]/var[1]/var[2]/var[3] can be used instead.

You can mix them in expressions: foo.x = bar.q, or even foo.xy = bar.gb + baz.st;. The major restriction is that swizzling can only be done using letters of the same group, ie. foo.xya is invalid because it mixes xy from xyzw and a from rgba.

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I'm not familiar with the stpq notation for texture coordinates (have always seen uv instead). I'm guessing p would be used for 3D textures, but in what situation would q be used? Edit: Nevermind I just found my answer. That would be, with TextureCubeArrays. –  David Gouveia Dec 13 '11 at 14:18
    
@DavidGouveia: No, actually. The different texturing functions have different meanings. For example, accessing a sampler2DShadow with texture means that the p coordinate is the comparison value. Accessing a sampler2DShadow with textureProj means that the p coordinate is the comparison value, and the q coordinate is divided into the other three (before comparison, so p is actually comparison * q). They don't have any intrinsic meaning; the meaning changes depending on how they are used. –  Nicol Bolas Dec 13 '11 at 17:59
    
@Nicol Thanks for clearing that up. I've actually only used HLSL before and even there only ever really needed the basic tex2D and texCube intrinsics. I was unaware of the multitude of existing texturing functions. (Unrelated: Is that your real name, or just the dragon from MTG?) –  David Gouveia Dec 13 '11 at 18:09
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