How does OpenGL know what type each vertex buffer object is?

Question

I've just read through a tutorial about Vertex Array Objects and Vertex Buffer Objects, and I can't work out from the following code how OpenGL knows the first VBO (vertexBufferObjID[0]) represents vertex coordinates, and the second VBO (vertexBufferObjID[1]) represents colour data?

glGenBuffers(2, vertexBufferObjID);

// VBO for vertex data
glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObjID[0]);
glBufferData(GL_ARRAY_BUFFER, 9*sizeof(GLfloat), vertices, GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)0, 3, GL_FLOAT, GL_FALSE, 0, 0); 
glEnableVertexAttribArray(0);

// VBO for colour data
glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObjID[1]);
glBufferData(GL_ARRAY_BUFFER, 9*sizeof(GLfloat), colours, GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)1, 3, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(1);

Edit: Thanks to Peter's answer, I found the following two lines of code which hook up each VBO with the shaders (indices 0 & 1 correlate to the VBO index):

glBindAttribLocation(programId, 0, "in_Position");
glBindAttribLocation(programId, 1, "in_Color");

hello would you be so kind as to share the tutorial you mentioned? — invisible squirrel, Jan 30 '13 at 23:18
I can't remember it off the top of my head, but I think I googled for something like "OpenGL 3 tutorial". — Mark Ingram, Jan 31 '13 at 13:00

score 1 · Accepted Answer · answered Sep 02 '12 at 16:35

1

It doesn't, you have to tell it which is which in the shader.

layout(location = 0) in vec3 position;
layout(location = 1) in vec3 color;

void main()
{
    /* ... */
}

answered Sep 02 '12 at 16:35

Peter Alexander

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Ah, I've found the calls to glBindAttribLocation, which uses the indices of 0 and 1, for "in_Position" and "in_Colour" respectively. Thanks. – Mark Ingram Sep 02 '12 at 16:37

score 0 · Answer 2 · answered Sep 02 '12 at 16:35

glVertexAttribPointer is only meant to be used with shaders. Therefore you, as the shader writer, always know what each attribute index is supposed to do, as it maps to a certain variable in your vertex shader. You control the index to variable map with commands glBindAttribLocation or glGetAttribLocation, or from special keywords inside GLSL.

If you're just using the fixed function pipeline (default shader), then you don't want to use glVertexAttribPointer. Instead for the fixed function pipe the equivalent commands are glVertexPointer and glColorPointer, etc. Note that these are deprecated commands.

Also note that for fixed function glEnableVertexAttribArray is to be replaced with glEnableClientState.

How does OpenGL know what type each vertex buffer object is?

2 Answers2

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