Efficient way of reading depth values from depth buffer

Question

For an algorithm of mine I need to be able to access the depth buffer. I have no problem at all doing this using glReadPixels, but reading an 800x600 window is extremely slow (From 300 fps to 20 fps)

I'm reading a lot about this and I think dumping the depth buffer to a texture would be faster. I know how to create a texture, but how do I get the depth out?

Creating an FBO and creating the texture from there might be even faster, at the moment I am using an FBO (but still in combination with glReadPixels).

So what is the fastest way to do this?

(I'm probably not able to use GLSL because I don't know anything about it and I don't have much time left to learn, deadlines!)

edit: Would a PBO work? As described here: http://www.songho.ca/opengl/gl_pbo.html it can go a lot faster but I can not change buffers all the time as in the example.

Edit2: How would I go about putting the depth data in the PBO? At the moment I do:

glGenBuffersARB(1, &pboId);
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboId);
glBufferDataARB(GL_PIXEL_PACK_BUFFER_ARB, 800*600*sizeof(GLfloat),0,     GL_STREAM_READ_ARB);

and right before my readpixels i call glBindbuffer again. The effect is that I read nothing at all. If I disable the PBO's it all works.

Final edit: I guess I solved it, I had to use:

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboId);
glReadPixels( 0, 0,Engine::fWidth, Engine::fHeight, GL_DEPTH_COMPONENT,GL_FLOAT, BUFFER_OFFSET(0));

GLuint *pixels = (GLuint*)glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY);

This gave me a 20 FPS increase. It's not that much but it's something.

So, I used 2 PBO's but I'm still encountering a problem: My code only gets executed once.

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[index]);  
std::cout << "Reading pixels" << std::endl;
glReadPixels( 0, 0,Engine::fWidth, Engine::fHeight, GL_DEPTH_COMPONENT,GL_FLOAT, BUFFER_OFFSET(0));
std::cout << "Getting pixels" << std::endl;
//  glBufferDataARB(GL_PIXEL_PACK_BUFFER_ARB, 800*600*sizeof(GLfloat), 0, GL_STREAM_DRAW_ARB);
GLfloat *pixels = (GLfloat*)glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY);
int count = 0;
for(int i = 0; i != 800*600; ++i){
    std::cout << pixels[i] << std::endl;
}

The last line executes once, but only once, after that it keeps on calling the method (which is normal) but stops at the call to pixels.

---

I apparently forgot to load glUnMapBuffers, that kinda solved it, though my framerate is slower again..

---

I decided giving FBO's a go, but I stumbled across a problem: Initialising FBO:

glGenFramebuffersEXT(1, framebuffers);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffers[0]);
std::cout << "framebuffer generated, id: " << framebuffers[0] << std::endl;
glDrawBuffer(GL_NONE);
glReadBuffer(GL_NONE);

glGenRenderbuffersEXT(1,renderbuffers);
glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, renderbuffers[0]);
glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT, 800, 600);
glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, 0);
glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, renderbuffers[0]);
bool status = checkFramebufferStatus();
    if(!status)
        std::cout << "Could not initialise FBO" << std::endl;
    else
        std::cout << "FBO ready!" << std::endl;

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

My drawing loop:

GLenum errCode; const GLubyte *errString;

if ((errCode = glGetError()) != GL_NO_ERROR) {
    errString = gluErrorString(errCode);
   fprintf (stderr, "OpenGL Error: %s\n", errString);
}

++frameCount;
// -----------  First pass to fill the depth buffer  -------------------
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffers[0]);

std::cout << "FBO bound" << std::endl;
//Enable depth testing
glEnable(GL_DEPTH_TEST);
glDisable(GL_STENCIL_TEST);
glDepthMask( GL_TRUE );
//Disable stencil test, we don't need that for this pass
glClearStencil(0);
glEnable(GL_STENCIL_TEST);

//Disable drawing to the color buffer
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);

//We clear all buffers and reset the modelview matrix
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
glLoadIdentity();

//We set our viewpoint
gluLookAt(eyePoint[0],eyePoint[1], eyePoint[2], 0.0,0.0,0.0,0.0,1.0,0.0);
//std::cout << angle << std::endl;
std::cout << "Writing to FBO depth" << std::endl;
//Draw the VBO's, this does not draw anything to the screen, we are just filling the depth buffer
glDrawElements(GL_TRIANGLES, 120, GL_UNSIGNED_SHORT, BUFFER_OFFSET(0));

After this I call a function that calls glReadPixels() The function does not even get called. The loop restarts at the function call.

---

Apparently I solved this as well: I had to use

glReadPixels( 0, 0,Engine::fWidth, Engine::fHeight, GL_DEPTH_COMPONENT,GL_UNSIGNED_SHORT, pixels);

With GL_UNSIGNED_SHORT instead of GL_FLOAT (or any other format for that matter)

Answer 1

The fastest way of doing this is using asynchronous pixel buffer objects, there's a good explanation here:

http://www.songho.ca/opengl/gl_pbo.html

Answer 2

I would render to a FBO and read its depth buffer after the frame has been rendered. PBOs are outdated technology.