per pixel fog

OpenGL
1

I’m trying to create radial per pixel fog, but I can’t find a way that works because I need the distance to fragment which I don’t have. I know I could get it from the vertex shader, but that gives me per vertex fog which is unaccurate for large polygons. I could also compute it based on gl_FragCoord.z, but this wouldn’t give me radial fog.

I’m sure there’s a way to get the accurate fragment distance to the eye, but I don’t see it. Anybody does?

Thanks

2

I could also compute it based on gl_FragCoord.z, but this wouldn’t give me radial fog.
i think it would

3

Why would it? The z buffer only record the z component. A R-buffer would do it radialy, but not a z component.

4

Create a (varying) attribute which stores the eye-space position for each vertex, this gets linearly interpolated giving you the eye-space position for each fragment. Radial distance from the eye is then the magnitude of this vector.

5

Originally posted by foobar:
Create a (varying) attribute which stores the eye-space position for each vertex, this gets linearly interpolated giving you the eye-space position for each fragment. Radial distance from the eye is then the magnitude of this vector.
This is per vertex fog. I need per pixel fog. I did it, but it is unrealistic when I have large polygons. It really shows if I have large and small polygons at the same time.

Anything else?

6

I’m sure there’s a way to get the accurate fragment distance to the eye, but I don’t see it.
Use a per-fragment worldspace/eyespace position to calculate the distance to the eye. Or use an eyespace vertex position to tap a volume map or some clever combination of 1D/2D maps.

You can’t linearly interpolate distance. Consider a right triangle at the origin with legs 1 and 1. A linear combination of these leg lengths measured from the origin will always produce 1, which is incorrect (mid way is sqrt(2)/2). You can interpolate directions and positions, but not magnitudes (you introduce a spacial ambiguity in doing so).

The per-vertex distance technique works really well if the triangles are relatively small and/or far away.

7

Something like this should work:

uniform vec3 camPos;
varying vec3 pos;

void main(){
    gl_Position = ftransform();
    pos = gl_Vertex - camPos;
}

varying vec3 pos;

void main(){
    float dist = length(pos);
    ...
    ...
}
8

Originally posted by Humus:
Something like this should work:

I knew there was an obvious solution! Thanks Humus :slight_smile:

9

Originally posted by vince:
Why would it? The z buffer only record the z component. A R-buffer would do it radialy, but not a z component.
yeah sorry i was wrong

10

Originally posted by zed:
yeah sorry i was wrong
No problem :wink:

11

Create a (varying) attribute which stores the eye-space position for each vertex, this gets linearly interpolated giving you the eye-space position for each fragment. Radial distance from the eye is then the magnitude of this vector.
Keep reading this until you realise it is correct.

12

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