CODEWITHSUNDEEP
opengl
Graznarak
Graznarak

Reputation: 3734

glBindVertexBuffer vs glBindBuffer

I see that I can bind a vertex buffer using glBindBuffer(GL_ARRAY_BUFFER, vbo);. This works, and I can also bind an element array using glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);. When using these everything draws as desired.

In reading the OpenGL 4.4 specification in section 10.3.1 I find mention of glBindVertexBuffer. It looks like this permits binding multiple vertex buffers and then having them all rendered at once. Is this correct? Also, how does it differ from using glBindBuffer? Can you use an element array buffer when binding vertex buffers with this?

Upvotes: 11

Views: 6914

Answers (2)

Andon M. Coleman
Andon M. Coleman

Reputation: 43369

Actually, the point of glBindVertexBuffer (...) is entirely different.

The idea when it was introduced in GL_ARB_vertex_attrib_binding was to separate the fixed mapping that has always existed between vertex buffer and pointer and replace it with a more flexible system that allows you to setup a format for a vertex attribute, a binding location for the vertex buffer/attribute to use and then simply swap out the buffer bound to that location.

You will not simply be replacing a call to glBindBuffer (...) with this new function, you have at least two other functions you need to call to setup a generic vertex attribute to be able to take advantage of glBindVertexBuffer (...):

  1. glVertexAttribFormat (...) - This is effectively the same thing as glVertexAttribPointer (...) except it does not setup a pointer, it merely establishes how the buffer that will be paired with this attribute is to be interpreted.

  2. glVertexAttribBinding (...) - This associates the generic attribute location with a new type of binding location (vertex buffer binding) so that you can use glBindVertexBuffer (...) to bind your VBO to a location this attribute will use.

  3. glVertexBindingDivisor (...) - This is a replacement for glVertexAttribDivisor(). This replacement respects the binding association between the Vertex Attribute index and the Binding Slot index made by glVertexAttribBinding(...). This is worth pointing out because it's name is somewhat out of form with the other two functions listed above, while the older glVertexAttribDivisor() appears more in-form, but is the wrong function to use.

To put this into perspective, I have included some pseudo-code from the extension specification that shows how glVertexAttribPointer (...) works in relation to this new API addition.

The commands:

void glVertexAttribPointer (GLuint index, GLint size, GLenum type,
                            GLboolean normalized, GLsizei stride, 
                            const GLvoid *pointer);
void glVertexAttribIPointer (GLuint index, GLint size, GLenum type,
                             GLsizei stride, const GLvoid *pointer);
void glVertexAttribLPointer (GLuint index, GLint size, GLenum type,
                             GLsizei stride, const GLvoid *pointer);

Control vertex attribute state, a vertex buffer binding, and the mapping between a vertex attribute and a vertex buffer binding.

They are equivalent to (assuming no errors are generated):

if (no buffer is bound to GL_ARRAY_BUFFER and pointer != NULL) 
{
    generate GL_INVALID_OPERATION;
}
glVertexAttrib*Format (index, size, type, {normalized, }, 0);
glVertexAttribBinding (index, index);
if (stride != 0) {
    effectiveStride = stride;
} else {
    compute effectiveStride based on size/type;
}
GL_VERTEX_ATTRIB_ARRAY_STRIDE[index] = stride;
// GL_VERTEX_BINDING_STRIDE will be set to effectiveStride
// by glBindVertexBuffer.
glBindVertexBuffer (index, <buffer bound to GL_ARRAY_BUFFER>, 
                 (GLchar *)pointer - (GLchar *)NULL, effectiveStride);

It is a little difficult to wrap your head around at first, but what it boils down to is very similar to the separation of sampler state from Texture Objects when Sampler Objects were introduced in GL 3.3. You can continue using the old API, but this alternative solution gives you some more flexibility.

Regarding element array buffers, no. This new part of the API has nothing to do with that, because Vertex Array Objects actually manage the one-and-only element array buffer binding.

Upvotes: 21

Bogdan
Bogdan

Reputation: 51

Not really. glBindVertexBuffers is used for binding multiple vertex buffers. As for glBindVertexBuffer, it is used for binding a buffer to a vertex buffer. You may have to call it many times to bind multiple vertex buffers.

You can check their documentation here: http://www.opengl.org/sdk/docs/man/xhtml/glBindVertexBuffers.xml

Upvotes: -2

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