PBO + glReadPixels not so fast?

[brinck]

Hi,

I'm trying to speed up some code that pulls that from the framebuffer using glReadPixels.

I've created two PBO with usage set to GL_STREAM_READ_ARB. My rendering code then alternate between the two PBO and do the following:

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, current);
glReadPixels(..., 0);
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0)

I was under the impression that glReadPixel would return immediately, but these three lines takes about 6 ms for a 1024 x 1024 framebuffer.

Currently the code isn't doing any map/unmap so the data should never leave the GPU.

I'm using a Quadro FX 3500 with the latest drivers (169.96).

Am I doing something wrong or is this to be expected?

/A.B.

[zeoverlord]

glReadPixels has to wait until the rendering is done until it can start reading, so yes it's as expected.

[yooyo]

Use two PBO's. Bind 1st, read pixels, bind 2nd, map and copy data to sysmem, unmap, unbind all PBO's, then swap PBO names. Repeat this every frame.
One more thing. Use GL_BGR or GL_BGRA pixel format.

[Jackis]

Good reading on PBO upload/readback - http://www.songho.ca/opengl/gl_pbo.html

PS: Yooyo seems to be little bit tired by answering the same question several times a month

[brinck]

Use two PBO's. Bind 1st, read pixels, bind 2nd, map and copy data to sysmem, unmap, unbind all PBO's, then swap PBO names. Repeat this every frame.
One more thing. Use GL_BGR or GL_BGRA pixel format.

Exactly what I'm doing. But as I said, it's not as fast as I expected. I don't see much improvement over just doing an ordinary glReadPixels.

/A.B.

[yooyo]

Then you are doing something wrong...
1. Render frame
2. bind pbo1
3. readback
4. bind pbo2
5. copy previous frame from pbo to sysmem... map buffer, copy data to sysmem (use some fast memcpy code)
6. unmap buffer
7. unbind pbos
8. swap pbo1 and pbo2

So.. frame will be in sysmem with one frame delay. With PBO readback call is nonblocking call. But map buggers can be blocking call if there is pending operation related to currently binded. So... if you call map buffer too soon it will be blocking call. If there is no pending operations mapbuffers returns very quickly.

[brinck]

Ok, so the question is what am I doing wrong?

The intitation code looks roughly like this:

SomeClass::initPBO() {
  glGenBuffersARB(2, m_ids);
  for (int i = 0; i < 2; ++i) {
    glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, m_ids[i]);
    glBufferDataARB(GL_PIXEL_PACK_BUFFER_ARB, m_width * m_height * 4, 0, GL_STREAM_READ_ARB);
  }
  glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0);
  m_active = 0;
}

and the capture code looks like this:

SomeClass::capture() {
  glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, m_ids[m_active]);
  glReadPixels(0, 0, m_width, m_height, GL_BGRA, GL_UNSIGNED_BYTE, 0);
  glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0);
  m_active = 1 - m_active;
}

As you can see, right now I'm not even mapping the buffers (eventually I will of course, otherwise this whole excersize would be kind of pointless), and the code in capture still takes about 6 ms. This could still stall I guess if one were rendering at a high enough framerate? However, my rendering is capped at 15 fps so this shouldn't be an issue.

The values for m_width and m_height has not changed since I created the buffers so their sizes are still valid.

/A.B