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 to RGB and then multiplication with the source color does not work.

Or is it possible to replace Hue/Saturation Adjustment Layer with normal layers with appropriately set blending modes (Mulitiply, Screen, Hue, Saturation, Color, Luminocity,...)? If so then how?

Thanks

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

Answer 2

I have figured out how Lightness works.

The input parameter brightness b is in [0, 2], Output is c (color channel).

if(b<1) c = b * c;
else    c = c + (b-1) * (1-c);

Some tests:

b = 0  >>>  c = 0  // black
b = 1  >>>  c = c  // same color
b = 2  >>>  c = 1  // white

However, if you choose some interval (e.g. Reds instead of Master), Lightness behaves completely differently, more like Saturation.

Answer 3

Photoshop, dunno. But the theory is usually: The RGB image is converted to HSL/HSV by the particular layer's internal methods; each pixel's HSL is then modified according to the specified parameters, and the so-obtained result is being provided back (for displaying) in RGB.

PaintShopPro7 used to split up the H space (assuming a range of 0..360) in discrete increments of 30° (IIRC), so if you bumped only the "yellows", i.e. only pixels whose H component was valued 45-75 would be considered for manipulation.

reds 345..15, oranges 15..45, yellows 45..75, yellowgreen 75..105, greens 105..135, etc.

if (h >= 45 && h < 75)
        s += s * yellow_percent;

There are alternative possibilities, such as applying a falloff filter, as in:

/* For h=60, let m=1... and linearly fall off to h=75 m=0. */
m = 1 - abs(h - 60) / 15;
if (m < 0)
        m = 0;
s += s * yellow_percent * d;