Save, then render depth buffer

In short these should be the essential steps:

glGenTextures(1, &l_ShadowMap);
glBindTexture(GL_TEXTURE_2D, l_ShadowMap);
// Specify the format of the texture!
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, l_Width, l_Height, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, NULL);

// Render your scene here

glBindTexture(GL_TEXTURE_2D, l_ShadowMap);
// Copy the depth values
glCopyTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 0, 0, l_Width, l_Height);

// Now render a screenspace quad or so with ShadowMap applied as texture


There are several details that I have omitted here such as setting the viewport to the ShadowMap size, reversing winding to reduce cracks or disabling writing to the framebuffer when rendering to the depth map and setting up the texture environment.

Note that you will need at least OpenGL 1.5 to use GL_DEPTH_COMPONENT.

glGenTextures(1, &depth);
glBindTexture(GL_TEXTURE_2D, depth);

glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, 256, 256, 0, GL_DEPTH_COMPONENT,GL_UNSIGNED_BYTE, NULL);

glEnable(GL_DEPTH_TEST);

main loop....

glClearDepth(1.0);
//Draw a triangle here.


glBindTexture(GL_TEXTURE_2D, depth);
glCopyTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 0, 0, 256, 256);


glEnable(GL_TEXTURE_2D);
//Draw quad

Here is what the program looks like.



I know what you mean about the shadow map, but I can't do that without actually getting a proper depth buffer saved into memory.