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A little confused about generic vertex attributes...

4 posts in this topic

Hey everybody, I've been reading a bit more of the 3.3 Spec, and I'm a bit confused as to how generic attributes are bound to their vertex shader counterparts... It looks to me that the generic attributes are used if the corresponding vertex buffer data is undefined. I can understand that, but once I get into the glVertexAttrib* functions I start getting a bit confused. They give a values parameter, and an index parameter. I can guess that the values parameter is used for the default values for the generic attribute, but I don't know how to specify the index parameter. Does it match the generic attribute with the attribute in the vertex shader? Is this how they're bound? If so how do I set the index parameter to point to what I want in the vertex shader? I do see the glGetAttribLocation function, but I thought that I can't use that until the program is linked, and by that time the generic attributes would already be assigned by the GL.

Am I even thinking of all this correctly? Help is greatly appreciated to clear this up!
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Have you used vertex arrays before? If so, this should be helpful....

[source]
// the old way....
glVertexPointer(3, GL_FLOAT, vertexStride, vertexDataPtr);
glEnableClientState(GL_VERTEX_ARRAY);

// the new way.
// glslProgramHandle, is the handle generated from glGenPrograms.
// The compiled shaders must be attached and the program must be linked correctly at this point (otherwise the attrib location will be -1).
// I'm not entirely sure, but I think using "gl_Vertex" as the varying attribute name will cause problems on some GPU's I suspect.
// Best to avoid using the standard names ;)
// I don't think I've ever used the normalise argument though....
GLint vertexAttribId = glGetAttribLocation(glslProgramHandle, "gl_Vertex");
glVertexAttribPointer(vertexAttribId, 3, GL_FLOAT, false, vertexStride, vertexDataPtr);
glEnableVertexAttribArray(vertexAttribId);
[/source]

p.s. vertexDataPtr can either be a pointer to memory, or an offset into a VBO.
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Alright, I can somewhat see how that would work. So does that mean that generic attributes hold the actual data used by the shader? Or are they different? And what's the difference between glVertexAttribPointer and the other glVertexAttrib* functions?
I suppose I still am a little confused. I guess what I'm really asking is, why would you need a generic attribute to bind to in the first place? Wouldn't the vertex shader analyze the VBO's contents and initialize the attribs off of that?
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[quote]So does that mean that generic attributes hold the actual data used by the shader?[/quote]
Not exactly sure what you mean here, but a generic attribute is basically a point you can bind a VBO to such that the VBO supplies data through that attribute.

[quote]
And what's the difference between glVertexAttribPointer and the other glVertexAttrib* functions?
[/quote]
glVertexAttrib* (for example glVertexAttrib3f) are kind of like the 'immediate mode' equivalents of glVertexAttribPointer. Its the same thing as the difference between glColorPointer and glColor3f, if you're familiar with the deprecated non-generic attributes. One specifies a VBO as a data source, and the other just sets a constant value. If you had a shader that had a per-vertex color attribute, but you just wanted to draw a red model, then you can just call glVertexAttrib3f(colorIndex, 1,0,0), and this will set the input to 1,0,0 for all vertices if you don't bind a VBO to it.

[quote]
I guess what I'm really asking is, why would you need a generic attribute to bind to in the first place? Wouldn't the vertex shader analyze the VBO's contents and initialize the attribs off of that?[/quote]

Shaders don't know anything about VBOs, and VBOs don't know anything about shaders. Attributes are how you link VBOs to specific input variables in your shader. How would the program know which array of floats you want to be the position, which is the texcoords, which is the normals unless you tell it what they are? Setting up attributes is how you send which VBOs to which input variables.
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[quote name='karwosts' timestamp='1313651959' post='4850636']
[quote]
And what's the difference between glVertexAttribPointer and the other glVertexAttrib* functions?
[/quote]
glVertexAttrib* (for example glVertexAttrib3f) are kind of like the 'immediate mode' equivalents of glVertexAttribPointer. Its the same thing as the difference between glColorPointer and glColor3f, if you're familiar with the deprecated non-generic attributes. One specifies a VBO as a data source, and the other just sets a constant value. If you had a shader that had a per-vertex color attribute, but you just wanted to draw a red model, then you can just call glVertexAttrib3f(colorIndex, 1,0,0), and this will set the input to 1,0,0 for all vertices if you don't bind a VBO to it.

[quote]
I guess what I'm really asking is, why would you need a generic attribute to bind to in the first place? Wouldn't the vertex shader analyze the VBO's contents and initialize the attribs off of that?[/quote]

Shaders don't know anything about VBOs, and VBOs don't know anything about shaders. Attributes are how you link VBOs to specific input variables in your shader. How would the program know which array of floats you want to be the position, which is the texcoords, which is the normals unless you tell it what they are? Setting up attributes is how you send which VBOs to which input variables.
[/quote]

Ahhh, this clears tons of things up. Never knew that's how the shaders got their information. Now I understand how all these functions really work! :D

Thanks for the awesome help guys!
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