I'm looking for a fast, accurate implementation of RGB to HSB and HSB to RGB in pure C. Note that I'm specifically looking for Hue, Saturation, Brightness and not HSL (Luminosity).
Of course I have Googled this extensively, but speed is of the utmost importance here and I am looking for any specific recommendations for solid, fast, reliable code.
First off
HSB and HLS were developed to specify numerical Hue, Saturation and Brightness (or Hue, Lightness and Saturation) in an age when users had to specify colors numerically. The usual formulations of HSB and HLS are flawed with respect to the properties of color vision. Now that users can choose colors visually, or choose colors related to other media (such as PANTONE), or use perceptually-based systems like L*u*v* and L*a*b*, HSB and HLS should be abandoned [source]
Look at the opensource Java implementation here
Boost library (I know, it's C++) seemed to contains conversion to HSB at one time but nowadays I can only find a luminance conversion (here)