Convering Cg to GLSL + gl_FragCoord

Graphics and GPU Programming Programming

Started by Gobbla December 15, 2008 12:53 PM

4 comments, last by idinev 15 years, 4 months ago

122

Author

December 15, 2008 12:53 PM

Hello I am in the process of converting some Cg code to GLSL, but I am having trouble getting it right. The Cg code i think is the problem looks like this

float2 texc = ((IN.Pos.xy / IN.Pos.w) + 1) / 2; // find the right place to lookup in the backside buffer

where IN.pos is define like

 OUT.Pos =  mul( ModelViewProj, IN.Position );

I have written it like

vec2 texc = ((gl_FragCoord.xy/gl_FragCoord.w)+1.0)/2.0;

I would like to know if that is correct ? Another question i have is what gl_FragCoord is ? From what i understand xy is the fragments location in screenspace and z is the depth. But if i draw a quad over the screen with

 gl_FragColor = vec4(FragCoord.x , FragCoord.y , 0 , 1);

i only get yellow which means that both x y = 1. Should it not be an gradient varying from black to yellow with red and green in the coorners ? Any answer is appreciated! / Andreas

V-man

813

December 15, 2008 04:27 PM

You window might be from 1024 x 700 pixels so any values that are above 1.0 gets clamped to 1.0 if you render to some integer format.
The only way to avoid the clamping is to render to some floating point target.

http://www.opengl.org/wiki/index.php/GL_EXT_framebuffer_object
http://www.opengl.org/wiki/index.php/Floating_point_and_mipmapping_and_filtering

Sig: http://glhlib.sourceforge.net
an open source GLU replacement library. Much more modern than GLU.
float matrix[16], inverse_matrix[16];
glhLoadIdentityf2(matrix);
glhTranslatef2(matrix, 0.0, 0.0, 5.0);
glhRotateAboutXf2(matrix, angleInRadians);
glhScalef2(matrix, 1.0, 1.0, -1.0);
glhQuickInvertMatrixf2(matrix, inverse_matrix);
glUniformMatrix4fv(uniformLocation1, 1, FALSE, matrix);
glUniformMatrix4fv(uniformLocation2, 1, FALSE, inverse_matrix);

idinev

236

December 16, 2008 04:47 AM

float2 ScreenSize=float2(1280,720);
gl_FragCoord.xy = (((IN.Pos.xy / IN.Pos.w) + 1) / 2) * ScreenSize + 0.5;

And also, (gl_FragCoord.xy - 0.5) is always integer. [0..1279][0..719]
You don't need to do that division by gl_FragCoord.w :)

V-man

813

December 16, 2008 10:03 AM

Quote:Original post by idinev
float2 ScreenSize=float2(1280,720);
gl_FragCoord.xy = (((IN.Pos.xy / IN.Pos.w) + 1) / 2) * ScreenSize + 0.5;

And also, (gl_FragCoord.xy - 0.5) is always integer. [0..1279][0..719]
You don't need to do that division by gl_FragCoord.w :)

You should not write to gl_FragCoord. It is read only.
Furthermore, no need to multiply by screen size because gl_FragCooord is already in window space.

All I'm saying is that since gl_FragCoord.xy contains high values, that they will get clamped.
As a result
gl_FragColor = vec4(FragCoord.x , FragCoord.y , 0.0 , 1.0);
will just show yellow.

PS : you can write that as
gl_FragColor = vec4(FragCoord.xy, 0.0 , 1.0);

Gobbla

122

Author

December 16, 2008 01:36 PM

Thanks for the answers !

The problem was that I thought gl_FragCoord where in the interval [0..1].
So gl_FragCoord / screenSize gave the right result.

/ Andreas

idinev

236

December 16, 2008 02:29 PM

V-man, I was showing how gl_FragCoord is computed (what values to expect from it)...

Convering Cg to GLSL + gl_FragCoord

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