Perceptual Luminance With Conversion

lumWithConvert.cs

static float StandardToLinearChannel (in float x)
{
    return x > 0.04045f ?
        Mathf.Pow ((x + 0.055f) / 1.055f, 2.4f) :
        x / 12.92f;
}
    
static float LinearToStandardChannel (in float x)
{
    return x > 0.0031308f ?
        Mathf.Pow (x, 1.0f / 2.4f) * 1.055f - 0.055f :
        x * 12.92f;
}

static Color StandardToLinear (in Color c)
{
    return new Color (
        StandardToLinearChannel (c.r),
        StandardToLinearChannel (c.g),
        StandardToLinearChannel (c.b),
        c.a);
}

static float GrayLumCorrect (in Color srgb)
{
    Color lrgb = StandardToLinear (srgb);
    float lum = 0.21264935f * lrgb.r + 0.71516913f * lrgb.g + 0.07218152f * lrgb.b;
    return LinearToStandardChannel (lum);
}