Please explain this color blending mode formula so I can replicate it in PHP/ImageMagick

Question

I\'ve been trying to use ImageMagick to replicate Photoshops Colour Blend Mode. I found the following formulas in an online guide but I don\'t know what they mean. Do I just

Answer 1

These color blending formulas are quite tricky if you need to incorporate also the alpha channel. I was not able to reproduce the blending of Photoshop, but Gimp works like this:

Color mix_hsv(
    ColorMixMode::Enum mode, // blending mode
    Color cd,                // destination color (bottom pixel)
    Color cs)                // source color (top pixel)
{
    // Modify the source color
    float dh, ds, dv; // destination hsv
    float sh, ss, sv; // source hsv
    cd.GetHsv(dh, ds, dv);
    cs.GetHsv(sh, ss, sv);

    switch (mode) {
        case HUE:        cs.InitFromHsv(sh, ds, dv); break;
        case SATURATION: cs.InitFromHsv(dh, ss, dv); break;
        case COLOR:      cs.InitFromHsv(sh, ss, dv); break;
        case LUMINOSITY: cs.InitFromHsv(dh, ds, sv); break;
    }
    cs.A = std::min(cd.A, cs.A);

    // Blend the modified source color onto the destination color
    unsigned char cd_A_orig = cd.A;
    cd = mix(NORMAL, cd, cs); // normal blending
    cd.A = cd_A_orig;
    return cd;
}

If you use premultiplied alpha, don't forget to correctly handle it in the above code. I was not able to find the code for blending in Gimp's source code, but the resulting images are very similar.

Photoshop's color blending is clearly different, so if anyone finds a way to implement it, please let us all know :o)

Miso