Drawing and loading a GL_DEPTH_COMPONENT texture

Question

I'm trying to load a depthbuffer from a file and copy it to the depth buffer instead of clearing it every frame.

anyway, i'm a bit new to opengl, so i just tried to load my texture like this:

glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, width, height, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, DepthData);

and i try to draw it like this:

glBindTexture(GL_TEXTURE_2D, DepthTexture);
glColorMask(FALSE, FALSE, FALSE, FALSE);

glBegin(GL_QUADS);

    glTexCoord2f(0.0f, 0.0f); glVertex2f(0.0f, 0.0f);
    glTexCoord2f(0.0f, 1.0f); glVertex2f(0.0f, 1.0f);
    glTexCoord2f(1.0f, 1.0f); glVertex2f(1.0f, 1.0f);
    glTexCoord2f(1.0f, 0.0f); glVertex2f(1.0f, 0.0f);

glEnd();

i tried to load a depthbuffer with every value set to 1.0, and everything i tried to draw got hidden, while it shouldn't.

what should i do? btw, i do NOT want to use glDrawPixels

Answer 1

You are indeed drawing a textured quad that is not updating the color buffer and is still updating the depth buffer. However, the values you are writing are the actual depth values of the polygons, and not the values found in the texture! That's why everything ends up being z-culled.

The easiest way to accomplish what you want is to use a custom shader and write a custom gl_FragDepth value reading it from your texture.