grayscaleEffect.fx

#if OPENGL
    #define SV_POSITION POSITION
    #define VS_SHADERMODEL vs_3_0
    #define PS_SHADERMODEL ps_3_0
#else
    #define VS_SHADERMODEL vs_4_0_level_9_1
    #define PS_SHADERMODEL ps_4_0_level_9_1
#endif

Texture2D SpriteTexture;

// A value between 0 and 1 that controls the intensity of the grayscale effect.
// 0 = full color, 1 = full grayscale.
float Saturation = 1.0;

sampler2D SpriteTextureSampler = sampler_state
{
    Texture = <SpriteTexture>; 
};

struct VertexShaderOutput
{
    float4 Position : SV_POSITION;
    float4 Color : COLOR0;
    float2 TextureCoordinates : TEXCOORD0;
};

float4 MainPS(VertexShaderOutput input) : COLOR
{
    // Sample the texture
    float4 color = tex2D(SpriteTextureSampler, input.TextureCoordinates) * input.Color;

    // Calculate the grayscale value based on human perception of colors
    float grayscale = dot(color.rgb, float3(0.3, 0.59, 0.11));

    // create a grayscale color vector (same value for R, G, and B)
    float3 grayscaleColor = float3(grayscale, grayscale, grayscale);

    // Linear interpolation between he grayscale color and the original color's
    // rgb values based on the saturation parameter.
    float3 finalColor = lerp(grayscale, color.rgb, Saturation);

    // Return the final color with the original alpha value
    return float4(finalColor, color.a);
}

technique SpriteDrawing
{
    pass P0
    {
        PixelShader = compile PS_SHADERMODEL MainPS();
    }
};