# Computing screen coordinates oddities

I would like to have access to the fragment’s screen position from inside the fragment shader. I assumed I’d be able to take the clip space X/Y in the vertex shader and divide by the W to get a nice clean X/Y from -1 to 1. But this, err, doesn’t seem to work. It’s all distorted, clearly clinging to the shapes somewhat. But for some reason it works if I do the W divide in the fragment. I don’t fancy wasting a lot of cycles (this is multiple additive passes) doing what could be interpolated if I can help it, any ideas why this works the way it does.

(Fragment version)

``````   vec4 v4TransformMe = m4Transform * vec4( ( v3VertexIn * fFalloff ) + v3Position, 1.0 );

v3TexCoordOut = vec3( v4TransformMe.xy, v4TransformMe.w );

gl_Position = v4TransformMe;
``````
``````v3TexCoordOut.xy / v3TexCoordOut.z
``````

(Vertex interpolated version)

``````   vec4 v4TransformMe = m4Transform * vec4( ( v3VertexIn * fFalloff ) + v3Position, 1.0 );

v3TexCoordOut = v4TransformMe.xy / v4TransformMe.w;

gl_Position = v4TransformMe;
``````

(the nasty scale and position in the vertex shader is being replaced with a properly built matrix as soon as this is sorted, I swear ;])

I realise that depth is nonlinear, but I assumed that even so it’s done on a vertex level and interpolated via fragment… is this not the case?

Check this out. They all say the same thing in different ways.

I see, thanks! I do recall that graphical artifact occurring on a number of Playstation games.

gl_FragCoord is a built in GLSL read only variable to get the position of the fragment from within a fragment shader, been there since atleast GLSL 1.1, i.e. OpenGL 2.0. Also available in GLSL of OpenGL ES2.

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