Issue: shader won't replace darker colors?

**Solgryn** · 2nd February 2013, 12:34 PM

Based upon Looki's "Keep Color" shader I wrote a shader that takes 3 colors and replaces them with 3 new ones. (Or how many you put in the code)

It works fine for almost all colors except for darker shades of gray? I noticed this happened with Looki's shader, too.

For example, it will replace RGB (188,188,188), but it won't replace (119,119,119) or black (0,0,0)

Here's the code for the shader:

Code:

sampler2D source;

float4 replaceThis1, replaceWith1,replaceThis2, replaceWith2,replaceThis3, replaceWith3;

float4 ps_main(in float2 In : TEXCOORD0) : COLOR0
{
	float4 color = tex2D(source,In);

	if(color.r == replaceThis1.r && color.g == replaceThis1.g && color.b == replaceThis1.b)
		color.rgb = replaceWith1.rgb;

                      if(color.r == replaceThis2.r && color.g == replaceThis2.g && color.b == replaceThis2.b)
		color.rgb = replaceWith2.rgb;

                      if(color.r == replaceThis3.r && color.g == replaceThis3.g && color.b == replaceThis3.b)
		color.rgb = replaceWith3.rgb;

	return color;
}

technique tech_main
{
	pass P0
	{
		PixelShader = compile ps_2_0 ps_main();
	}
}

**Looki** · 2nd February 2013, 03:10 PM

Ah, I always wondered if the fact that colors are floats would cause any trouble. I'm sure that's the problem here... rounding errors.

Essentially you'll need a threshold to compensate that. There are 255 colors, and color values are from 0 to 1... so if a color is 1 unit brighter, the value increases by 1/255 ~= 0.0039

The straight-foward, unelegant solution would be this: if (abs(color.r - replaceThis1.r) < 1/255.0 && .......same for g and b)

It'd probably be a good idea to decrease the threshold though, because you don't want (0,0,0) to match (1,0,0) which might happen if you use 1/255... instead, 1/100 = 0.01 should be enough I think.

But you can also shorten it by using some vector math. Just compare to the length of the vector that contains the color difference.
if (length(color.rgb - replaceThis.rgb) < 0.015)
No idea if 0.01 is a good threshold tbh, it's close to the length of the vector (0.01, 0.01, 0.01):
sqrt(3 * (0.01^2)) = 0.0173205081

Try it out and tell me if it works