HSV to RGB Conversion Functions

Detailed Description

Functions to convert from the HSV colorspace to the RGB colorspace.

These basically fall into two groups: spectra, and rainbows. pectra and rainbows are not the same thing. Wikipedia has a good llustration that shows a "spectrum" and a "rainbow" side by side:

^{Source: http://en.wikipedia.org/wiki/Rainbow#Number_of_colours_in_spectrum_or_rainbow}

Among other differences, you'll see that a "rainbow" has much more yellow than a plain spectrum. "Classic" LED color washes are spectrum based, and usually show very little yellow.

Take a look Wikipedia's page on HSV color space, with pseudocode for conversion to RGB color space: http://en.wikipedia.org/wiki/HSL_and_HSV

Note that their conversion algorithm, which is (naturally) very popular is in the "maximum brightness at any given hue" style, vs. the "uniform brightness for all hues" style.

You can't have both; either purple is the same brightness as red, e.g:

   red = 0xFF0000
purple = 0x800080

Where you have the same "total light" output. OR purple is "as bright as it can be", e.g.:

   red = 0xFF0000
purple = 0xFF00FF

Where purple is much brighter than red.

The colorspace conversions here try to keep the apparent brightness constant even as the hue varies.

Note

CHSV::val is part of this 8-bit colorspace conversion. Very low values necessarily quantize individual RGB channels and can make a mixed hue appear closer to a primary color. For sensor-controlled output brightness, keep the color at full value and use FastLED.setBrightness() so the output pipeline can preserve the channel ratios with temporal dithering.

Adafruit's "Wheel" function, discussed here is also of the "constant apparent brightness" variety.

Todo

Provide the "maximum brightness no matter what" variation.

See also

Some good, clear Arduino C code from Kasper Kamperman, which in turn was based on Windows C code from "nico80"

Macros
#define	HUE_MAX   191
	Max hue accepted for the hsv2rgb_raw() function.
#define	HUE_MAX_RAINBOW   255
	Max hue accepted for the hsv2rgb_rainbow() function.
#define	HUE_MAX_SPECTRUM   255
	Max hue accepted for the hsv2rgb_spectrum() function.

Typedefs
using	CHSV = fl::hsv8
using	CRGB = fl::CRGB

Functions
CRGB	hsv2rgb_fullspectrum (const CHSV &hsv) FL_NO_EXCEPT
	version of hsv2rgb_fullspectrum which returns a CRGB object.
void	hsv2rgb_fullspectrum (const CHSV &hsv, CRGB &rgb) FL_NO_EXCEPT
	Converts an HSV value to RGB using the algorithm from here: https://en.wikipedia.org/wiki/HSL_and_HSV#HSV_to_RGB_alternative By Shawn Silverman.
void	hsv2rgb_fullspectrum (const CHSV *phsv, CRGB *prgb, int numLeds) FL_NO_EXCEPT
	Converts an HSV value to RGB using the algorithm from here: https://en.wikipedia.org/wiki/HSL_and_HSV#HSV_to_RGB_alternative By Shawn Silverman.
CRGB	hsv2rgb_rainbow (const CHSV &hsv) FL_NO_EXCEPT
void	hsv2rgb_rainbow (const CHSV &hsv, CRGB &rgb) FL_NO_EXCEPT
void	hsv2rgb_rainbow (const CHSV *phsv, CRGB *prgb, int numLeds) FL_NO_EXCEPT
void	hsv2rgb_raw (const CHSV &hsv, CRGB &rgb) FL_NO_EXCEPT
	Convert an HSV value to RGB using a mathematically straight spectrum.
void	hsv2rgb_raw (const CHSV *phsv, CRGB *prgb, int numLeds) FL_NO_EXCEPT
	Convert an HSV value to RGB using a mathematically straight spectrum.
CRGB	hsv2rgb_spectrum (const CHSV &hsv) FL_NO_EXCEPT
	Inline version of hsv2rgb_spectrum which returns a CRGB object.
void	hsv2rgb_spectrum (const CHSV &hsv, CRGB &rgb) FL_NO_EXCEPT
	Convert an HSV value to RGB using a mathematically straight spectrum.
void	hsv2rgb_spectrum (const CHSV *phsv, CRGB *prgb, int numLeds) FL_NO_EXCEPT
	Convert an HSV value to RGB using a mathematically straight spectrum.
CHSV	rgb2hsv_approximate (const CRGB &rgb) FL_NO_EXCEPT
	Recover approximate HSV values from RGB.