Algorithm for Hue/Saturation Adjustment Layer from Photoshop

Question

Does anyone know how adjustment layers work in Photoshop? I need to generate a result image having a source image and HSL values from Hue/Saturation adjustment layer. Conversion

Answer 1

I've reverse-engineered the computation for when the "Colorize" checkbox is checked. All of the code below is pseudo-code.

The inputs are:

• hueRGB, which is an RGB color for HSV(photoshop_hue, 100, 100).ToRGB()
• saturation, which is photoshop_saturation / 100.0 (i.e. 0..1)
• lightness, which is photoshop_lightness / 100.0 (i.e. -1..1)
• value, which is the pixel.ToHSV().Value, scaled into 0..1 range.

The method to colorize a single pixel:

color = blend2(rgb(128, 128, 128), hueRGB, saturation);

if (lightness <= -1)
    return black;
else if (lightness >= 1)
    return white;

else if (lightness >= 0)
    return blend3(black, color, white, 2 * (1 - lightness) * (value - 1) + 1)
else
    return blend3(black, color, white, 2 * (1 + lightness) * (value) - 1)

Where blend2 and blend3 are:

blend2(left, right, pos):
    return rgb(left.R * (1-pos) + right.R * pos, same for green, same for blue)

blend3(left, main, right, pos):
    if (pos < 0)
        return blend2(left, main, pos + 1)
    else if (pos > 0)
        return blend2(main, right, pos)
    else
        return main