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_ModelVie         


        

Answer 1

Indeed, the "depth" value of a fragment can be read from it's z value in clip space (that is, after all matrix transformations). That much is correct.

However, your problem is in the division by w. Division by w is called perspective divide. Yes, it is necessary for perspective projection to work correctly.

However. Division by w in this case "bunches up" all your values (as you have seen), to being very close to 1.0. There is a good reason for this: in a perspective projection, w= (some multiplier) *z. That is, you are dividing the z value (whatever it was computed out to be) by the (some factor of) original z. No wonder you always get values near 1.0. You're almost dividing z by itself.

As a very simple fix for this, try dividing z just by the farPlane, and send that to the fragment shader as depth.

Vertex shader

varying float DEPTH ;

uniform float FARPLANE ;  // send this in as a uniform to the shader

gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
DEPTH = gl_Position.z / FARPLANE ; // do not divide by w

Fragment shader:

varying float DEPTH ;
// far things appear white, near things black
gl_Color.rgb=vec3(DEPTH,DEPTH,DEPTH) ;

The result is a not-bad, very linear-looking fade.