5. Implement the Grayscale Logic

Now, let’s modify the PS_Grayscale function to convert the color to grayscale. A common way to do this is to take the average of the red, green, and blue components, or use a weighted average for a more perceptually accurate grayscale. The dot product is used here to perform a weighted sum of the color components, which is a standard method for converting RGB to luminance (grayscale).

#include "ReShade.fxh"

float3 PS_Grayscale(float4 vpos : SV_Position, float2 texcoord : TexCoord) : SV_Target
{
    // Read the original color of the pixel
    float3 originalColor = tex2D(ReShade::BackBuffer, texcoord).rgb;

    // Calculate the grayscale value (weighted average for better perception)
    // The dot product multiplies each color component (R, G, B) by a specific weight
    // and sums the results. These weights (0.299, 0.587, 0.114) are commonly used
    // to approximate human perception of brightness.
    float grayscaleValue = dot(originalColor, float3(0.299, 0.587, 0.114));

    // Return a color where R, G, and B are all set to the grayscale value
    return float3(grayscaleValue, grayscaleValue, grayscaleValue);
}

technique MyGrayscaleEffect
{
    pass
    {
        VertexShader = PostProcessVS;
        PixelShader = PS_Grayscale;
    }
}