Question Chroma Lum encoding and decoding

Hello

I'm trying to do a chroma lum encoding to texture for a complex shader I'm doing. I have used unity formula for YCbCr but it doesn't seem to be reversible. Am I doing something wrong? Is there other way to get chroma lum encoding and decoding?

Testing the reversibility in code:

Code (CSharp):

1. fixed4 frag (v2f i) : SV_Target
2.             {
3.                 // sample the texture
4.                 fixed4 col = tex2D(_MainTex, i.uv);
5.                 float3 c = col;
6.                 c = RgbToYCbCr(c);
7.                 c = YCbCrToRgb(c);
8.  
9.                 col = float4(c,1);
10.                 return col;
11.             }

Left is code result, right is original texture colors:


Encoding and decoding formula lift from core shader render pipeline from unity:

Code (CSharp):

1. //
2. // RGB / Full-range YCbCr conversions (ITU-R BT.601)
3. //
4. float3 RgbToYCbCr(float3 c){
5.     float Y  =  0.299 * c.r + 0.587 * c.g + 0.114 * c.b;
6.     float Cb = -0.169 * c.r - 0.331 * c.g + 0.500 * c.b;
7.     float Cr =  0.500 * c.r - 0.419 * c.g - 0.081 * c.b;
8.     return float3(Y, Cb, Cr);
9. }
10.  
11. float3 YCbCrToRgb(float3 c){
12.     float R = c.x + 0.000 * c.y + 1.403 * c.z;
13.     float G = c.x - 0.344 * c.y - 0.714 * c.z;
14.     float B = c.x - 1.773 * c.y + 0.000 * c.z;
15.     return float3(R, G, B);
16. }

 

YCbCr isn't reversible normally, not without some loss, but that's much worse than usual. It looks like the Cb is offset by 0.5, but I don't know if that's the fault of the encode or decode.

Depending on what you're doing, YCoCg might be a better choice as it is reversible (at least some versions are).

 

YCoCg has some good examples on shadertoy I have referenced in the past.

 

Works great! Thanks!