Reading the pixels values from the Frame Buffer Object (FBO) using Pixel Buffer Object (PBO)

Question

Can I use Pixel Buffer Object (PBO) to directly read the pixels values (i.e. using glReadPixels) from the FBO (i.e. while FBO is still attached)?

If yes,

1. What are the advantages and disadvantages of using PBO with FBO?
2. What is the problem with following code

{

//DATA_SIZE = WIDTH * HEIGHT * 3 (BECAUSE I AM USING 3 CHANNELS ONLY)
// FBO and PBO status is good
.
.
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
//Draw the objects

Following glReadPixels works fine

glReadPixels(0, 0, screenWidth, screenHeight, GL_BGR_EXT, GL_UNSIGNED_BYTE,  (uchar*)cvimg->imageData);

Following glReadPixels DOES NOT WORK :(

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboId);
//yes glWriteBuffer has also same target and I also checked with every possible values
glReadBuffer(GL_COLOR_ATTACHMENT0_EXT); 
glReadPixels(0, 0, screenWidth, screenHeight, GL_BGR_EXT, GL_UNSIGNED_BYTE, (uchar*)cvimg->imageData);
.
.
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0); //back to window framebuffer

Answer 1

When using a PBO as target for glReadPixels you have to specify a byte offset into the buffer (0, I suppose) instead of (uchar*)cvimg->imageData as target address. It is similar to using a buffer offset in glVertexPointer when using VBOs.

EDIT: When a PBO is bound to the GL_PIXEL_PACK_BUFFER, the last argument to glReadPixels is not treated as a pointer into system memory but as a byte offset into the bound buffer's memory. So to write the pixels into the buffer just pass a 0 (write them to the start of the buffer memory). You can then later acces the buffer memory (to get the pixels) by means of glMapBuffer. The example link you provided in your comment does that, too, just read it extensively. I also suggest reading the part about vertex buffer objects they mention at the start, as these lay the ground to understand buffer objects.