dolphin/Data/User/Shaders/posterize.txt

uniform samplerRECT samp0 : register(s0);

void main(out float4 ocol0 : COLOR0, in float2 uv0 : TEXCOORD0)
{
  float4 c0 = texRECT(samp0, uv0).rgba;
  float red   = 0.0;
  float green = 0.0;
  float blue  = 0.0;

  if (c0.r > 0.25)
  {
    red = c0.r;
  }

  if (c0.g > 0.25)
  {
    green = c0.g;
  }

  if (c0.b > 0.25)
  {
    blue = c0.b;
  }

  ocol0 = float4(red, green, blue, 1.0);
}