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Monday, September 25th, 2017

Time Event
10:25a
Colorful LEDs
RGB LEDs do not exist according to Linux LED subsystem. They are modeled as three separate LEDs, red, green and blue; that matches the hardware.

Unfortunately, it has problems. Lets begin with inconsistent naming: some drivers use :r suffix, some use :red. There's no explicit grouping of LEDs for one light -- there's no place to store parameters common for the light. (LEDs could be grouped by name.)

RGB colorspace is pretty well defined, and people expect to set specific colors. Unfortunately.... that does not work well with LEDs. First, LEDs are usually not balanced according to human perception system, so full power to the LEDs (255, 255, 255) may not
result in white. Second, monitors normally use gamma correction before displaying color, so (128, 128, 128) does not correspond to 50% of light being produced. But LEDs normally use PWM, so (128, 128, 128) does correspond to 50% light. Result is that colors are completely off.

I tested HSV colorspace for the LEDs. That would have advantage of old triggers being able to use selected colors... Unfortunately, on N900, white color is something like 15% blue, which would result in significantly reducing number of white intensities we can display.

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