uniform sampler2D tex0;
uniform vec3 from, to;
uniform float threshold, softness;

vec3 hsv2rgb(vec3 c)
{
  vec4 K = vec4(1.0, 2.0 / 3.0, 1.0 / 3.0, 3.0);
  vec3 p = abs(fract(c.xxx + K.xyz) * 6.0 - K.www);
  return c.z * mix(K.xxx, clamp(p - K.xxx, 0.0, 1.0), c.y);
}
vec3 rgb2hsv(vec3 c)
{
    vec4 K = vec4(0.0, -1.0 / 3.0, 2.0 / 3.0, -1.0);
    vec4 p = mix(vec4(c.bg, K.wz), vec4(c.gb, K.xy), step(c.b, c.g));
    vec4 q = mix(vec4(p.xyw, c.r), vec4(c.r, p.yzx), step(p.x, c.r));

    float d = q.x - min(q.w, q.y);
    float e = 1.0e-10;
    return vec3(abs(q.z + (q.w - q.y) / (6.0 * d + e)), d / (q.x + e), q.x);
}

void main(void)
{
threshold = 0.0;
softness = 0.4;

	vec4 texel = texture2D(tex0, gl_TexCoord[0].st);

	float distance = length(texel - from);
	//float vig = smoothstep(outer, inner, distance);

	//if (distance <= threshold)
	if (abs(texel.r - from.r) < threshold &&
	    abs(texel.g - from.g) < threshold &&
	    abs(texel.b - from.b) < threshold)
	{
		vec3 texelHsv = rgb2hsv(texel);
		vec3 toHsv = rgb2hsv(to);
		vec3 newRgb = hsv2rgb(vec3(texelHsv.x, texelHsv.y * 0.0, texelHsv.z));

		float zo = distance/threshold;

		if (zo <= 1.0 - softness)
		{
			zo = 0.0;
		}
		else
		{
			zo = (zo - (1.0 - softness));
		}

		texel.rgb = mix(newRgb, texel.rgb, zo);
	}

	gl_FragData[0] = vec4(texel);
}
