Order of execution of depth_clamp and depth_test

Question

does depth clamp occurs before depth test or after depth test? I am rendering a primitive with coordinates > 1.0 and <-1.0 and using depth clamping with depth test. But when i enable depth test it does not render any geometry.

Here is my code:

GLfloat vertices[]=
    {
0.5f,0.5f,0.5f,                                   
-0.5f,0.5f,0.5f,
-0.5f,-0.5f,0.5f,
0.5f,-0.5f,0.5f,

0.5f,-0.5f,-0.5f,
-0.5f,-0.5f,-0.5f,
-0.5f,0.5f,-0.5f,
0.5f,0.5f,-0.5f  
}

for(int i=0;i<24;i++)
    vertices[3*i+2]*=25;

    glEnable(GL_DEPTH_CLAMP);

        // when i comment stmt below, it draws triangle strips
        glEnable(GL_DEPTH_TEST); 
        glClearDepth(15.0f);
        glClearColor (1.0, 0.0, 0.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); 
        glDrawArrays(GL_TRIANGLE_STRIP,0,6);

How to use depth test and clamping together?

why does the above code does not draw anything on the screen with depth test enabled?

Nicol Bolas · Accepted Answer · 2013-01-15T08:23:32.203

4

From OpenGL 4.3, 17.3.6:

If depth clamping (see section 13.5) is enabled, before the incoming fragment’s z_w is compared, z_w is clamped to the range [min(n; f ); max(n; f )]

i am assuming that by default it is GL_LEQUAL.

That's why you should post fully working examples when you don't know what's going on. The default depth test is GL_LESS. 1.0 is not less than 1.0, so all of your 0.5 depth vertices fail the depth test.

Also, your loop:

vertices[3*i+2]*=25

This is overwriting memory. Your index is going far off the end of the array, since it only has 24 elements. You probably meant to loop 8.

edited Jan 15 '13 at 08:23

answered Jan 15 '13 at 06:57

Nicol Bolas

449,505
63
781
982

why it does not draw anything on the screen? – debonair Jan 15 '13 at 07:04
2

That's a completely different question from what the order of clamping vs. testing is. And answering it will require a lot more than what you posted. Things like what your depth test is, the matrices you're using, etc. – Nicol Bolas Jan 15 '13 at 07:14
i am not using any matrices at all, just passing above vertices array in shader. About depth_test, as i am not calling any other API for depth comparison, i am assuming that by default it is GL_LEQUAL. – debonair Jan 15 '13 at 07:35
1

@Roshan: Of course you're using a matrix, even if it's just the identity matrix, which can be written in a number of ways. If you're using a shader, then you *must* post the shader source, so that we can make any valuable suggestions, as without the shader we're in the blind. – datenwolf Jan 15 '13 at 11:02

Order of execution of depth_clamp and depth_test

1 Answers1