How to achieve randomness for 2D noise function

[zdenham]

Hello! Thanks for the wonderful library!

I am implementing a film grain image effect, and it involves a noise function and the below 2D random function (taken from book of shaders). I was able to implement the effect nicely in webgl, but when using react-native-webgl, the randomness and noise functions both had unexpected and very different outputs than webgl. The following shader:

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

void main() {
  float rnd = random(v_position);

  gl_FragColor = vec4(vec3(rnd), 1.0);
}

Outputed:

In webgl, but in react-native-webgl outputed: (which does not appear very random). I am wondering why the output is so inconsistent between the two platforms, if the difference can be remedied, or if it is a hardware problem... Any advice would be greatly appreciated!

[zdenham]

Update: this function outputs correctly on simulator (that is the same as webgl), but if built to any real iOS device it outputs something similar to the second image above.

[gre]

have you tried to define a precision ? like precision highp float;

[zdenham]

ahhh yes, I had been using mediump. This did the trick! Thank you for your help!