Kelvin2RGB

Arduino library for converting temperature and brightness to RGB values.

Credentials

This library is based upon an article of Tanner Helland and a related story by Neil Bartlett http://www.tannerhelland.com/4435/convert-temperature-rgb-algorithm-code/ http://www.zombieprototypes.com/?p=210 https://en.wikipedia.org/wiki/Color_temperature#Categorizing_different_lighting

There are more approximation formulas, some claim to be better, however these are not investigated. On request these can be added.

Description

The library converts a temperature in Kelvin and a brightness (0..100%) to 3 numbers red, green and blue. These numbers are weights can be used to correct a colour image for virtual white temperature.

There are 2 convert functions where the convert_NB() is claimed to be the more accurate one.

With the numbers R,G,B calculated one can convert images so they will look more like taken with candle light, sunrise or sunset etc.

pseudo code

Kelvin2RGB KRGB;

KRGB.convert(1850, 100);  // sunrise light factors

for each pixel in image
{
  red   *= KRGB.red();
  green *= KRGB.green();
  blue  *= KRGB.blue();
  drawPixel();
}

The numbers can also be used to reduce the blue channel so it has less effect on "getting sleepy".

The library uses floats for the R,G and B weights to keep values as accurate as possible. Especially with images with more than 8 bits per channel this is preferred. That said it is also possible to use this on a 565 image or to adjust color lookup tables.

Interface

The interface is straightforward:

• Kelvin2RGB() constructor
• void begin() empty function for now.
• void reset() resets all internal values to 0. All colours, brightness and temperature.
• void convert_TH(float temperature, float brightness = 100) temperature = 0..65500 temperature below 1000 is not well defined. brightness = 0..100%,
• void convert_NB(float temperature, float brightness = 100) temperature = 0..65500 temperature below 1000 is not well defined. brightness = 0..100%, Is a bit more accurate and slightly slower (few %). Read link above for more information.
• float temperature() returns temperature, to check the value used.
• float brightness() returns brightness, to check the value used.
• float red() returns red channel weight 0.0 .. 1.0 note this is different from Helland / Bartlett who both use an integer value 0 .. 255
• float green() returns green channel weight 0.0 .. 1.0
• float blue() returns blue channel weight 0.0 .. 1.0
• uint32_t setRGB(float red, float green, float blue, float brightness = 100) sets RGB values red, green, blue should be in 0 .. 1.0 range. brightness should be in 0..100%, Default = 100%. returns a 24 bit RGB value,
• uint32_t RGB() returns a 24 bit RGB value, 0 .. 16777215 more efficient than 3 floats for communication.
• uint16_t RGB565() returns a 16 bit RGB value, 5 bits for red, 6 for green and 5 for blue.
• uint32_t BGR() returns a 24 bit BGR value, 0 .. 16777215
• uint32_t CMYK() returns a 32 bit = 4 byte CMYK value,

Operations

See examples

Future

must

• add examples
  • CMYK()
  • BGR()

should

• define constants like candleLight as parameter.
• investigate other formulas.
• investigate timing and performance
• investigate usability for RGB led strip.
• investigate RGB_10_12_10()

could

• separate brightness per colour channel to mimic "artificial illumination" (0.2.0 ?)
• remove begin() ?
• example Led-strip()