OpenGL: Read Framebuffer's Depth Texture

Question

I'm using LWJGL in Java, which has identical function names to the C++ OpenGL. Note that I'm forced to use OpenGL's "old" Fixed Function Pipeline.

I'm currently successfully drawing a Framebuffer's RGB contents to PixelBufferObjects using the following code:

//the initialization method
public void init(int width, int height) {
    //initializing the PBO
    pbo_id = glGenBuffers();
    // [...]

    //initializing the Framebuffer
    framebufferObject = glGenFramebuffers();
    framebufferTexture = glGenTextures();

    depthBuffer = glGenRenderbuffers();

    framebufferFilter = GL_NEAREST;

    glBindTexture(GL_TEXTURE_2D, framebufferTexture);

    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
    glTexParameterf(GL_TEXTURE_2D, GL11.GL_TEXTURE_WRAP_T, GL_CLAMP);

    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, (ByteBuffer)null);
    glBindFramebuffer(GL_FRAMEBUFFER, framebufferObject);
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, framebufferTexture, 0);

    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24, width, height);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthBuffer);

    glBindTexture(GL_TEXTURE_2D, 0);
}

//this is called after the world was rendered to the framebuffer
public void capture() {
    //binding the PBO
    glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo_id);

    //binding the Framebuffer's texture
    glBindTexture(GL_TEXTURE_2D, framebuffer_id);

    //storing the Framebuffer's contents in the PBO
    glGetTexImage(GL_TEXTURE_2D, 0, GL_RGB, GL_UNSIGNED_BYTE, 0);

    //unbinding the Framebuffer
    glBindTexture(GL_TEXTURE_2D, 0);

    //unbinding the PBO
    glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
}

How would I store the contents of the depth map (which I suppose is being initialized as depthBuffer) in the PBO instead of the "main" framebuffer contents?

UPDATE: This is the code which I'm using to read the depth map's contents to a PBO now:

public void captureDepthMap() {
    //binding the PBO
    glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo_id);

    glPixelStorei(GL_PACK_ALIGNMENT, 1);
    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

    //binding the FBO
    glBindFramebuffer(GL_FRAMEBUFFER, framebufferObject);

    //storing the Depth Map's contents in the PBO
    glReadPixels(0, 0, width, height, GL_DEPTH_COMPONENT, GL_UNSIGNED_INT_24_8, 0);

    //unbinding the FBO
    glBindFramebuffer(GL_FRAMEBUFFER, 0);

    //unbinding the PBO
    glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
}

The resulting image is just black however. What may cause this? Did I possibly mess up some GL formats?

Answer 1

Binding the "framebuffer's texture" does not make a whole lot of sense. This texture only represents a single (color buffer) attachment. You can have multiple textures attached to your framebuffer and the depth buffer can be stored as a texture too.

If you use a depth texture attachment instead of renderbuffer, you can read it back the same way you read the color attachment. The only difference would be the format and data type: GL_DEPTH_COMPONENT and GL_UNSIGNED_INT_24_8 .

Alternatively, you can use glReadPixels (...) to read from the attached renderbuffer. The API is a little more involved because you have to specify a rectangle to read.