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

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;