Bind Framebuffer or set ReadBuffer? - opengl

Question

I created a framebuffer:
glBindFramebuffer(GL.GL_FRAMEBUFFER, &fbo);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, rbo);
Is reading pixels from the framebuffer via
glBindFramebuffer(GL_FRAMEBUFFER, &fbo);
glReadPixels(0, 0, w, h, GL_RGBA, GL_FLOAT, &data);
equivalent to
glReadBuffer(GL_COLOR_ATTACHMENT0);
glReadPixels(0, 0, w, h, GL_RGBA, GL_FLOAT, &data);
?

Answer 1

It is not equivalent, reading the GL_COLOR_ATTACHMENT0 will get you data from the currently bound framebuffer, which could be completely different from the one you created.
So basically you need to guarantee you have your framebuffer bound by calling
glBindFramebuffer(GL_FRAMEBUFFER, &fbo);
before any operations using it.
GL_COLOR_ATTACHMENT0 is just an attribute of the Frame buffer object and it is not related to any specific frame buffer. By calling it with another framebuffer bound you are going to read its data which is not what you intend.

Answer 2

Those are completely different calls. Let me provide some background on what FBOs really are to hopefully make this all much clearer.
A framebuffer object (aka FBO) is just a collection of state. The following calls change state tracked in the currently bound FBO:
glFramebufferTexture2D()
glFramebufferRenderbuffer()
glDrawBuffers()
glReadBuffer()
(and a few other variations of similar calls)
This means that anytime you make one of these calls, the state tracked in the currently bound FBO is updated to reflect the change from the call. For example, if you call glDrawBuffers(), the list of draw buffers in the currently bound FBO is updated.
Then, anytime you bind an FBO, the state tracked in the FBO will become active again. So if you previously called glDrawBuffers() while FBO foo was bound, and later bind foo again, the draw buffer setting from the earlier call is active again.
Note that the FBO does not own the renderbuffers/textures that are attached to it. The FBO only contains information on which renderbuffer is attached to the FBO at a given attachment point. In your code, FBO foo stores the fact that renderbuffer rbo is attached to attachment point GL_COLOR_ATTACHMENT0. For example, it is completely legal to attach the same renderbuffer to multiple FBOs.
Now, more specifically on your code:
The glBindFramebuffer() calls have the wrong argument type:
glBindFramebuffer(GL.GL_FRAMEBUFFER, &fbo);
The second argument is the name (id) of the FBO, not an address. So the call is:
glBindFramebuffer(GL.GL_FRAMEBUFFER, fbo);
This call does nothing:
glReadBuffer(GL_COLOR_ATTACHMENT0);
GL_COLOR_ATTACHMENT0 is the default read buffer for FBOs. So unless you previously set it to a different value before, this call is redundant, and only sets the same value that was the default anyway. As the naming suggests, FBOs can have multiple attachments, and you would use glReadBuffer() if you had attached a renderbuffer/texture to an attachment other than ATTACHMENT0, and wanted to read from that one.
As long as you're just using a single attachment for the FBO, the only thing you really need to do is bind the FBO you want to read from:
glBindFramebuffer(GL.GL_FRAMEBUFFER, fbo);
glReadPixels(...);
glReadPixels() always reads from the currently bound FBO, so there is no way around this.

Answer 3

glReadBuffer(GL_COLOR_ATTACHMENT0);
glReadPixels(0, 0, w, h, GL_RGBA, GL_FLOAT, &data);
data over here will be read from current bounded frame buffer and this wont be equivalent to
glBindFramebuffer(GL_FRAMEBUFFER, &fbo);
glReadPixels(0, 0, w, h, GL_RGBA, GL_FLOAT, &data); ( if the framebuffer is not bounded to respective fbo in first case)
I think that is the reason why glNamedFramebufferReadBuffer API is provided,to read data directly from the frame buffer mentioned as the first parameter.