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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
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  • Arm Employee Badge
    Note: This was originally posted on 23rd November 2012 at http://forums.arm.com

    Hi sanchiski,

    Something to take into account regarding the Mali-400 GPU is that we implement floating point in the fragment processor as FP16, which is conformant with the Khronos specification. I would direct you to the forum post here, which contains a detailed discussion of the considerations of FP16 on Mali 400. Also note that on T6xx platforms, we use FP32 for fragment precision.

    I believe in your case, you are attempting to pass very large numbers to the sin and fract functions, which is resulting in a lack of precision required for the operations. You should consider restricting your numbers to a low range (say, no larger than +-3.14 so that you have the maximum amount of bits for precision).

    Please let me know if you have any more questions.

    Thanks,
    Chris
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  • Arm Employee Badge
    Note: This was originally posted on 23rd November 2012 at http://forums.arm.com

    Hi sanchiski,

    Something to take into account regarding the Mali-400 GPU is that we implement floating point in the fragment processor as FP16, which is conformant with the Khronos specification. I would direct you to the forum post here, which contains a detailed discussion of the considerations of FP16 on Mali 400. Also note that on T6xx platforms, we use FP32 for fragment precision.

    I believe in your case, you are attempting to pass very large numbers to the sin and fract functions, which is resulting in a lack of precision required for the operations. You should consider restricting your numbers to a low range (say, no larger than +-3.14 so that you have the maximum amount of bits for precision).

    Please let me know if you have any more questions.

    Thanks,
    Chris
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