openglvbobuffer-objects

Can you use multiple targets with a single VBO?

Sample code:

1. glGenBuffers(1, &VboId);
2. glBindBuffer(GL_ARRAY_BUFFER, VboId);
3. glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
4. glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);

So we generate a generic VBO handle, and then we bind it using "GL_ARRAY_BUFFER". Binding it seems to have 2 purposes:

  1. We must bind the buffer before we can copy data to the GPU via glBufferData
  2. We must bind the buffer before we can add attributes to it via glVertexAttribPointer

And I think those are the only 2 times you need to bind the VBO. My question is, is there any scenario in which target (GL_ARRAY_BUFFER, GL_ELEMENT_ARRAY_BUFFER, GL_PIXEL_PACK_BUFFER, or GL_PIXEL_UNPACK_BUFFER) would be different on lines 2 and 3? Or we would want to rebind it to a different target before line 4?

Can we bind multiple buffer targets to a single VBO?

Solution

  • You do not bind targets to a buffer object. Targets are locations in the OpenGL context that you can bind things (like buffer objects) to. So you bind buffer objects to targets, not the other way around.

    A buffer object (there is no such thing as a VBO. There are simply buffer objects) is just a unformatted, linear array of memory owned by the OpenGL driver. You can use it as source for vertex array data, by binding the buffer to GL_ARRAY_BUFFER and calling one of the gl*Pointer functions. These function only work with the buffer currently bound to GL_ARRAY_BUFFER. You can use them as the source for index data by binding them to GL_ELEMENT_ARRAY_BUFFER and calling one of the glDrawElements functions.

    The functions used to modify a buffer objects contents (glBufferData, glMapBuffer, glBufferSubData, etc) all specifically take a target for their operations to work on. So glBufferData(GL_ARRAY_BUFFER, ...) does its stuff to whatever buffer is currently bound to GL_ARRAY_BUFFER.

    So there are two kinds of functions that affect buffer objects: those that modify their contents, and those that use them in operations. The latter are specific to a source; glVertexAttribPointer always uses the buffer currently bound to GL_ARRAY_BUFFER. You can't make it use a different target. Similarly, glReadPixels always uses the buffer bound to GL_PIXEL_PACK_BUFFER. And so forth. If a function does stuff with buffer objects but doesn't take a target as a parameter, then its documentation will tell you which target it looks for its buffer from.

    Note: Vertex arrays are kinda weird. The association between a vertex attribute and a buffer object is made by calling glVertexAttribPointer. What this function does is set the appropriate data for that attribute, using the buffer object that is currently bound to GL_ARRAY_BUFFER. By "currently bound", I mean bound at the time this function is called. So immediately after calling this function, you can call glBindBuffer(GL_ARRAY_BUFFER, 0), and it will change nothing about what happens when you go to render. It will render just fine.

    In this way, you can use different buffer objects for different attributes. The information will be retained until you change it with another glVertexAttribPointer call for that particular attribute.