al_color_lch man page
al_color_lch — Allegro 5 API
#include <allegro5/allegro_color.h> ALLEGRO_COLOR al_color_lch(float l, float c, float h)
Return an ALLEGRO_COLOR(3) structure from CIE LCH values. LCH colors are very similar to HSL, with the same meaning of L and H and C corresponding to S. However LCH is more visually uniform. Furthermore, this function expects the angle for H in radians and not in degree.
The CIE LCH color space is a cylindrical representation of the L*a*b* color space. The L component is the same and C and H are computed like this:
C = sqrt(a * a + b * b) H = atan2(b, a)