Shader - change colour per position on screen

[kubatp]

Hello everyone,
I might have a very simple question but I am having problems with getting it to work.
In short, I want to be able to change output colour of fragment shader per its position on screen - the more in the center of the screen it is, the more transparent the result colour should be.

Use case is also quite simple - I want fog to be less dense in the middle of the screen and get more dense towards the edges.
I thought I will just use the calculation of output position in vertex shader, but it simply doesnt work well for whatever reason.
Anyone did something similar?
Thank you for your help!

[kubatp]

Noone knows? I thought that it must be something easy for guys working with graphics but maybe it is not?

[paroj]

https://www.khronos.org/registry/OpenGL ... oord.xhtml

[kubatp]

Hello paroj,
thank you for the reply. The link you sent did not help me directly because I use cg, but thanks to it I was able to find a chain or pages from which I got the solution.
I am not sure how good the solution is but here it is

Vertex shader:

Code: Select all

output.screenPosition = output.position; //standard output position from vs

Fragment shader:

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vertex_output.screenPosition.xy /= vertex_output.screenPosition.w;
float2 texCoord = 0.5f * (float2(vertex_output.screenPosition.x, -vertex_output.screenPosition.y) + 1);
float distanceFromCenterOfScreen = 1 - max(abs(texCoord[0] - 0.5), abs(texCoord[1] - 0.5));

or probably even better

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float distanceFromCenterOfScreen = distance(texCoord.xy, float2(0.5, 0.5));

I am not sure how good or efficient the solution is but it works. If anyone has ideas how to make it better or different way, please let me know.