OpenGL: Draw a depth texture to depth buffer

I am programming a 2D videogame which must be suitable for almost any Windows system, including older ones. I have my scene drawn and in some point, I am retrieving the color and depth buffer each to its own texture with glCopyTexSubImage2D. I can draw the color buffer again in subsequent frames, but when I try to draw the depth buffer it is drawn as a grayscale color texture. I want it to be written to the screen depth buffer.

I have tried a lot of things. I tried with a FrameBuffer Object. It worked on some systems, but not on older ones. I want the solution to be suitable for all systems, because I am doing this precisely to boost performance on slower systems, so it doesn’t make sense to use a FBO.

I have tried with readpixels and writepixels. Again, on newer systems it works, but it is incredibly slow on older systems.

I need simply to draw that depth texture to the screen depth buffer. Is this really so hard?

Please, help! Thank you very much.

Did you try binding the depth texture as the depth buffer of an FBO?