Comb filter shader?

Syh · 16 December 2019 17:24

I was watching a video about Dithering on the Sega Genesis with Composite Video and during it, it discusses comb filters being used frequently.

So I was just curious if we had any shaders available that did comb filtering?

hunterk · 16 December 2019 18:16

Yes, all of the NTSC shaders have such filtering, AFAIK, though it’s not always obvious. You can see a couple of examples here:

github.com

libretro/slang-shaders/blob/master/ntsc/shaders/artifact-colors/artifact-colors2.slang#L142


    float Fs = params.SourceSize.x;
    float Fcol = Fs * params.F_COL;
    float Flumlp = Fs * params.F_LUMA_LP;
    float n = floor(vTexCoord.x * params.OutputSize.x);
    
	vec2 uv = vTexCoord.xy * params.OutputSize.xy;
    
    float luma = sample2D(Source, params.SourceSize.xy, uv).r;
    vec2 chroma = vec2(0.);
    
    //Filtering out unwanted high freqency content from the chroma(IQ) signal.
    for(int i = 0;i < int(params.FIR_SIZE);i++)
    {
        int tpidx = int(params.FIR_SIZE) - i - 1;
        float lp = Lowpass(Flumlp, Fs, int(params.FIR_SIZE), tpidx);
        chroma += sample2D(Source, params.SourceSize.xy, uv - vec2(float(i) - params.FIR_SIZE / 2., 0.)).yz * lp;
    }
    
    chroma *= rotate(tau * params.HUE);
    
    vec3 color = yiq2rgb * vec3(params.BRIGHTNESS * luma, chroma * params.SATURATION);

and here:

github.com

libretro/slang-shaders/blob/master/nes_raw_palette/shaders/cgwg-famicom-geom/adaptive-comb-decode.slang#L56


  vec2 coord = texCoord + params.SourceSize.zw*shift;
  if (coord.x < 0.0 || coord.x > 1.0 || coord.y < 0.0 || coord.y > 1.0) return vec3(0.0);
  return texture(Source, texCoord + params.SourceSize.zw*shift).xyz-vec3(0,0.5,0.5);
}


void main()
{
  vec4 samps[] = vec4[](decode_sample(vec2(-1,0)),
			decode_sample(vec2( 0,0)),			
			decode_sample(vec2( 1,0)));
  // quarter-pixel shift between scanlines needed for comb filter
  vec4 s1, s2, s3, s4;
  s1 = decode_sample(vec2(-2,-1));
  s2 = decode_sample(vec2(-1,-1));
  s3 = decode_sample(vec2( 0,-1));
  s4 = decode_sample(vec2( 1,-1));
  vec4 samps_prev[] = vec4[](vec4(s1.zw,s2.xy),
			     vec4(s2.zw,s3.xy),
			     vec4(s3.zw,s4.xy));
  s1 = decode_sample(vec2(-1,1));
  s2 = decode_sample(vec2(-0,1));

Some further reading: http://www.cockam.com/vidcomb.htm#Comb260