Converting depth buffer into depth texture for GLSL

Question

What is the easiest way to get the existing depth buffer into a depth texture, or anything that can be used by GLSL?

Is it possible to make a glsl shader that uses the depth buffer, and use it "as is" without having to modify the the way the original rendering is done (e.g modifying the frame buffer object to also render to an additional depth buffer etc)

Answer 1

Nope, you need to copy the depth buffer to a texture (somehow), or use a FBO to make the depth buffer a texture. There is no other way to read from the depth buffer in a shader.