Fast, optimized and accurate RGB <-> HSB conversion code in C - c

Question

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.

Answer 1

Here is a straightforward implementation in standard C.
This is - without further context - as good as it can get. Perhaps you care to shed some more light on
how you store your RGB samples (bits/pixel to begin with !?)
how you store your pixel data (do you want to efficiently transform larger buffers, if so, what is the organization)
how you want to represent the output (I assumed floats for now)
I could come up with a further optimized version (perhaps one that utilisze SSE4 instructions nicely...)
All that said, when compiled with optimizations, this doesn't work too badly:
#include <stdio.h>
#include <math.h>
typedef struct RGB_t { unsigned char red, green, blue; } RGB;
typedef struct HSB_t { float hue, saturation, brightness; } HSB;
/*
* Returns the hue, saturation, and brightness of the color.
*/
void RgbToHsb(struct RGB_t rgb, struct HSB_t* outHsb)
{
// TODO check arguments
float r = rgb.red / 255.0f;
float g = rgb.green / 255.0f;
float b = rgb.blue / 255.0f;
float max = fmaxf(fmaxf(r, g), b);
float min = fminf(fminf(r, g), b);
float delta = max - min;
if (delta != 0)
{
float hue;
if (r == max)
{
hue = (g - b) / delta;
}
else
{
if (g == max)
{
hue = 2 + (b - r) / delta;
}
else
{
hue = 4 + (r - g) / delta;
}
}
hue *= 60;
if (hue < 0) hue += 360;
outHsb->hue = hue;
}
else
{
outHsb->hue = 0;
}
outHsb->saturation = max == 0 ? 0 : (max - min) / max;
outHsb->brightness = max;
}
Typical usage and test:
int main()
{
struct RGB_t rgb = { 132, 34, 255 };
struct HSB_t hsb;
RgbToHsb(rgb, &hsb);
printf("RGB(%u,%u,%u) -> HSB(%f,%f,%f)\n", rgb.red, rgb.green, rgb.blue,
hsb.hue, hsb.saturation, hsb.brightness);
// prints: RGB(132,34,255) -> HSB(266.606354,0.866667,1.000000)
return 0;
}

Answer 2

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)

Answer 3

I would suggest using a lookup table to store the HSB and RGB values. First convert the RGB value (which is presumably 8 bits per component) to a 16-bit value (5 bits per component). The HSB values, also 16-bit values, can be converted in the same way, whereby here, the hue component should probably use more bits than the saturation and brightness, probably 8 bits per component, and the saturation and brightness 4 bits each. Of course, the reverse will apply when converting from HSB to RGB.

Answer 4

A fast RGB to HSV floating point conversion from lolengine.net takes a "common" RGB2HSV implementation and makes these observations:
Only the hue offset K changes. The idea now is the following:
Sort the triplet (r,g,b) using comparisons
Build K while sorting the triplet
Perform the final calculation
We notice that the last swap effectively changes the sign of K and the sign of g - b. Since both are then added and passed to fabs(), the sign reversal can actually be omitted.
The step before that last point looks like C code, but their final form is C++ that's trivially convertible to C:
static void RGB2HSV(float r, float g, float b,
float &h, float &s, float &v)
{
float K = 0.f;
if (g < b)
{
std::swap(g, b);
K = -1.f;
}
if (r < g)
{
std::swap(r, g);
K = -2.f / 6.f - K;
}
float chroma = r - std::min(g, b);
h = fabs(K + (g - b) / (6.f * chroma + 1e-20f));
s = chroma / (r + 1e-20f);
v = r;
}