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random number with mali 400 mp

Fernando Sanchez
Note: This was originally posted on 22nd November 2012 at http://forums.arm.com

I am having no success into generating random noise with the MALI 400 MP GPU, the code I'm using is next which works with all the other GPU vendors I've tried.
On this specific GPU will return just a few cross screen dotted thin lines with a wide space between them, instead of the expected random noise.

float rand(vec2 co)
{
return fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 43758.5453);
}

void main()
{
vec3 color  = texture2D(u_texture, v_texcoord);
vec3 noise =  vec3( rand(gl_FragCoord.st / 1000.0) );
gl_FragColor = vec4(color + noise, 1.0);
}


Is there something to take into account in this specific GPU hardware?

Thanks
Parents
  • Note: This was originally posted on 4th December 2012 at http://forums.arm.com

    Hi Sanchiski,

    This is a tricky problem to solve in FP16 and we're looking into some options. A few notes however:

    In your first example you use gl_FragCoord as the parameter to your rand function. In such cases, given a value of gl_FragCoord, the result of rand is determinable, and is invariant across frames. In such cases it is probably wiser to either render to a texture or precompute this on the CPU and upload as a texture, and sample this in your fragment shader, saving yourself cycles each frame.

    For the other methods where texture coordinates are used, it might still be worth precomputing this, as you know the range and so a sufficiently high res texture sampled in the correct way will yield the same results. We are looking into some options on how to compute this on the fly, but as the result is invariant on the parameter passed in, this is probably a more correct method.

    Thanks,
    Chris
Reply
  • Note: This was originally posted on 4th December 2012 at http://forums.arm.com

    Hi Sanchiski,

    This is a tricky problem to solve in FP16 and we're looking into some options. A few notes however:

    In your first example you use gl_FragCoord as the parameter to your rand function. In such cases, given a value of gl_FragCoord, the result of rand is determinable, and is invariant across frames. In such cases it is probably wiser to either render to a texture or precompute this on the CPU and upload as a texture, and sample this in your fragment shader, saving yourself cycles each frame.

    For the other methods where texture coordinates are used, it might still be worth precomputing this, as you know the range and so a sufficiently high res texture sampled in the correct way will yield the same results. We are looking into some options on how to compute this on the fly, but as the result is invariant on the parameter passed in, this is probably a more correct method.

    Thanks,
    Chris
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