Question

I want to make a pixel shader for Silverlight which will help me to change the Hue/Saturation/Lightness using a slider.
* Hue slider has values in range: [-180, 180]
* Saturation slider has values in range: [-100, 100]
* Lightness slider has values in range: [-100, 100]

I managed to create a pixel shader which can manipulate the Saturation and Lightness values.
But I can find any algorithm for changing the hue value.
Can anyone provide me an algorithm? Thank you.

Here is my HLSL code:

/// <summary>The brightness offset.</summary>
/// <minValue>-180</minValue>
/// <maxValue>180</maxValue>
/// <defaultValue>0</defaultValue>
float Hue : register(C0);

/// <summary>The saturation offset.</summary>
/// <minValue>-100</minValue>
/// <maxValue>100</maxValue>
/// <defaultValue>0</defaultValue>
float Saturation : register(C1);

/// <summary>The lightness offset.</summary>
/// <minValue>-100</minValue>
/// <maxValue>100</maxValue>
/// <defaultValue>0</defaultValue>
float Lightness : register(C2);

sampler2D input : register(S0);

//--------------------------------------------------------------------------------------
// Pixel Shader
//--------------------------------------------------------------------------------------

float4 main(float2 uv : TEXCOORD) : COLOR
{
 // some vars
 float saturation = Saturation / 100 + 1;
 float lightness = Lightness / 100;
 float3  luminanceWeights = float3(0.299,0.587,0.114);

    // input raw pixel
    float4 srcPixel = tex2D(input, uv);

    // Apply saturation
    float luminance = dot(srcPixel, luminanceWeights);
    float4 dstPixel = lerp(luminance, srcPixel, saturation);

    // Apply lightness
    dstPixel.rgb += lightness;

    //retain the incoming alpha
    dstPixel.a = srcPixel.a;

    return dstPixel;
}

Answer 1

The conversions between colour spaces are available at EasyRGB, see this post at nokola.com for a Silverlight implementation of a Hue shift. It may be possible to fit hue, saturation and brightness in one PS 2.0 shader if you take the approach mentioned here but I haven’t tried.