draw the depth value in opengl using shaders

Question

I want to draw the depth buffer in the fragment shader, I do this:

Vertex shader:

varying vec4 position_;

gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
position_ = gl_ModelViewProjectionMatrix * gl_Vertex;

Fragment shader:

float depth = ((position_.z / position_.w) + 1.0) * 0.5;

gl_FragColor = vec4(depth, depth, depth, 1.0);

But all I print is white, what am I doing wrong?

Answer 1

In what space do you want to draw the depth? If you want to draw the window-space depth, you can do this:

gl_FragColor = vec4(gl_FragCoord.z);

However, this will not be particularly useful, since most of the numbers will be very close to 1.0. Only extremely close objects will be visible. This is the nature of the distribution of depth values for a depth buffer using a standard perspective projection.

Or, to put it another way, that's why you're getting white.

If you want these values in a linear space, you will need to do something like the following:

float ndcDepth = ndcPos.z =
    (2.0 * gl_FragCoord.z - gl_DepthRange.near - gl_DepthRange.far) /
    (gl_DepthRange.far - gl_DepthRange.near);
float clipDepth = ndcDepth / gl_FragCoord.w;
gl_FragColor = vec4((clipDepth * 0.5) + 0.5);