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| """ | |
| Based on: http://www.tannerhelland.com/4435/convert-temperature-rgb-algorithm-code/ | |
| Comments resceived: https://gist.github.com/petrklus/b1f427accdf7438606a6 | |
| Original pseudo code: | |
| Set Temperature = Temperature \ 100 | |
| Calculate Red: | |
| If Temperature <= 66 Then | |
| Red = 255 | |
| Else | |
| Red = Temperature - 60 | |
| Red = 329.698727446 * (Red ^ -0.1332047592) | |
| If Red < 0 Then Red = 0 | |
| If Red > 255 Then Red = 255 | |
| End If | |
| Calculate Green: | |
| If Temperature <= 66 Then | |
| Green = Temperature | |
| Green = 99.4708025861 * Ln(Green) - 161.1195681661 | |
| If Green < 0 Then Green = 0 | |
| If Green > 255 Then Green = 255 | |
| Else | |
| Green = Temperature - 60 | |
| Green = 288.1221695283 * (Green ^ -0.0755148492) | |
| If Green < 0 Then Green = 0 | |
| If Green > 255 Then Green = 255 | |
| End If | |
| Calculate Blue: | |
| If Temperature >= 66 Then | |
| Blue = 255 | |
| Else | |
| If Temperature <= 19 Then | |
| Blue = 0 | |
| Else | |
| Blue = Temperature - 10 | |
| Blue = 138.5177312231 * Ln(Blue) - 305.0447927307 | |
| If Blue < 0 Then Blue = 0 | |
| If Blue > 255 Then Blue = 255 | |
| End If | |
| End If | |
| """ | |
| import math | |
| def convert_K_to_RGB(colour_temperature): | |
| """ | |
| Converts from K to RGB, algorithm courtesy of | |
| http://www.tannerhelland.com/4435/convert-temperature-rgb-algorithm-code/ | |
| """ | |
| #range check | |
| if colour_temperature < 1000: | |
| colour_temperature = 1000 | |
| elif colour_temperature > 40000: | |
| colour_temperature = 40000 | |
| tmp_internal = colour_temperature / 100.0 | |
| # red | |
| if tmp_internal <= 66: | |
| red = 255 | |
| else: | |
| tmp_red = 329.698727446 * math.pow(tmp_internal - 60, -0.1332047592) | |
| if tmp_red < 0: | |
| red = 0 | |
| elif tmp_red > 255: | |
| red = 255 | |
| else: | |
| red = tmp_red | |
| # green | |
| if tmp_internal <=66: | |
| tmp_green = 99.4708025861 * math.log(tmp_internal) - 161.1195681661 | |
| if tmp_green < 0: | |
| green = 0 | |
| elif tmp_green > 255: | |
| green = 255 | |
| else: | |
| green = tmp_green | |
| else: | |
| tmp_green = 288.1221695283 * math.pow(tmp_internal - 60, -0.0755148492) | |
| if tmp_green < 0: | |
| green = 0 | |
| elif tmp_green > 255: | |
| green = 255 | |
| else: | |
| green = tmp_green | |
| # blue | |
| if tmp_internal >=66: | |
| blue = 255 | |
| elif tmp_internal <= 19: | |
| blue = 0 | |
| else: | |
| tmp_blue = 138.5177312231 * math.log(tmp_internal - 10) - 305.0447927307 | |
| if tmp_blue < 0: | |
| blue = 0 | |
| elif tmp_blue > 255: | |
| blue = 255 | |
| else: | |
| blue = tmp_blue | |
| return red, green, blue | |
| if __name__ == "__main__": | |
| print("Preview requires matplotlib") | |
| from matplotlib import pyplot as plt | |
| step_size = 100 | |
| for i in range(0, 15000, step_size): | |
| color = list(map(lambda div: div/255.0, convert_K_to_RGB(i))) + [1] | |
| print(color) | |
| plt.plot((i, i), (0, 1), linewidth=step_size/2.0, linestyle="-", color=color) | |
| plt.show() | |
With increasing temperature, output seems like red becoming less saturated, passing through white, and becoming more saturated blue. I expected red turning to orange, yellow, becoming less saturated and turning into white, then becoming more saturated blue. I don't see that yellow part here. It does match https://en.wikipedia.org/wiki/Color_temperature so I guess it is correct. But I was expecting something more like the heated metal colour discussed at: https://physics.stackexchange.com/questions/304299/how-heated-metal-colors-relate-to-black-body-color-at-the-same-temperature. The heated metal colour matches colour temperature of light bulbs more closely.
Edit: For using an RGB led to approximate colour temperature, https://github.com/carpdiem/Color-Match gave better results. Note that there you need to know the wavelength of LEDs to get accurate results.
Edit: Another source is https://github.com/jonls/redshift/blob/master/src/colorramp.c. It uses a lookup table and interpolation, and is much faster than Color-Match.
@dreamlayers Thx for all the links, especially the redhift one really helped be get this going for my ESPHome project. For anyone interested, see: https://github.com/markusressel/CustomRGBWLight/blob/master/components/ColorTempRGBWLight/ColorTempRGBWLight.h
If you are interested in the
rgb_to_kelvin()I just reversed the values, with a step size of 100K, find it below as a header file to run it in C.