Extracting RGB color value

General
1

Hello there,

I’m using one of the various versions of the Adafruit_WS2801 library, and it uses a uint32_t type to describe the colors of an RGB led.
I also found the following function someone created to help create variables of this type:

uint32_t Color(byte r, byte g, byte b)
{
  uint32_t c;
  c = r;
  c <<= 8;
  c |= g;
  c <<= 8;
  c |= b;
  return c;
}

My question is - how do I extract the red, green and blue values out of such a uint32_t type ?

I tried the following, but I’m encountering issues with the blue or green:

int getRedValueFromColor(uint32_t c) {
    return ((uint32_t)c >> 16);
}

int getGreenValueFromColor(uint32_t c) {
    return ((uint16_t)c >> 8);
}

int getBlueValueFromColor(uint32_t c) {
    return (c);
}

When I tried using these function on the photon RGB led, the red worked fine, but the green gave me blue and the blue gave weird purple…

2

All the colors need to be 8 bits, not 16 or 32 like you’re returning for green and blue. For green, if you shift it 8 places to the right, you’re left with the red and green values with 16 bits of information, so if you cast that to a uint8_t (by having the return type be a uint8_t), you will only get the 8 least significant bits. For blue, just casting c to an uint8_t will do the same.

uint8_t getRedValueFromColor(uint32_t c) {
    return c >> 16;
}

uint8_t getGreenValueFromColor(uint32_t c) {
    return c >> 8;
}

uint8_t getBlueValueFromColor(uint32_t c) {
    return c;
}
2 Likes
3

I do like UNION for that

* Types required for RGB-LED control */
struct ARGB_COLOR {
	byte B;  // blue  channel
	byte G;  // green channel
	byte R;  // red   channel
	byte A;  // alpha channel to indicate RGB.control true (!= 0) / false (== 0)
};

union COLOR {
	ARGB_COLOR argb;
	unsigned int value;
} color;

  // usage
  color.value = value;
  RGB.control(color.argb.A);
  RGB.color(color.argb.R, color.argb.G, color.argb.B);
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4

Thank you !
Ric’s solution worked wonderfully!

5

My response to your question was aimed at showing you how to correctly write those functions, but looking at the larger picture, you might want to consider using a union as @ScruffR suggested. A union with a struct as one of its members is a nice and organized way to access smaller pieces (bits, nibbles, bytes) of a larger entity like a 32 bit int. I can’t tell if this is appropriate for your current project, but it’s something to keep in mind.

2 Likes