How to eliminate a colour from RGB LED code? - arduino

Question

I recently adjusted an already existing code for the RGB led (in arduino) to fade from yellow to red. However it still produces green in the beginning, and I can't seem to figure out how to eliminate that. Maybe someone has an idea?
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int redPin = 9;
int greenPin = 10;
int bluePin = 11;
#define COMMON_ANODE
void setup()
{
pinMode(redPin, OUTPUT);
pinMode(greenPin, OUTPUT);
pinMode(bluePin, OUTPUT);
}
int rgb[3];
//Arduino has no prebuilt function for hsi to rgb so we make one:
void hsi_to_rgb(float H, float S, float I) {
int r, g, b;
if (H > 360) {
H = H - 360;
}
// Serial.println("H: "+String(H));
H = fmod(H, 360); // cycle H around to 0-360 degrees
H = 3.14159 * H / (float)180; // Convert to radians.
S = S > 0 ? (S < 1 ? S : 1) : 0; // clamp S and I to interval [0,1]
I = I > 0 ? (I < 1 ? I : 1) : 0;
if (H < 2.09439) {
r = 255 * I / 3 * (1 + S * cos(H) / cos(1.047196667 - H));
g = 255 * I / 3 * (1 + S * (1 - cos(H) / cos(1.047196667 - H)));
b = 255 * I / 3 * (1 - S);
} else if (H < 4.188787) {
H = H - 2.09439;
g = 255 * I / 3 * (1 + S * cos(H) / cos(1.047196667 - H));
b = 255 * I / 3 * (1 + S * (1 - cos(H) / cos(1.047196667 - H)));
r = 255 * I / 3 * (1 - S);
} else {
H = H - 4.188787;
b = 255 * I / 3 * (1 + S * cos(H) / cos(1.047196667 - H));
r = 255 * I / 3 * (1 + S * (1 - cos(H) / cos(1.047196667 - H)));
g = 255 * I / 3 * (1 - S);
}
rgb[2] = b;
}
void setColor(int red, int green, int blue)
{
#ifdef COMMON_ANODE
green = 255 - green;
blue = 255 - blue;
#endif
analogWrite(greenPin, green);
analogWrite(bluePin, blue);
}
void loop()
{
for (int i=0; i<=360;i++){
hsi_to_rgb(i,1,1);
setColor(rgb[0],rgb[1],rgb[2]);
delay(20);
}
}
Also if someone has or can easily write a better code, it would be greatly appreciated!`

Answer 1

Firstly, I think you should work in HSV color space to begin with, if you're having a green channel stay on when you're trying to isolate your colors to red and then sweep to yellow, your math is where things are most probably going wrong.
HSV sweeps through the color wheel naturally, you're probably having some mixing values when you're trying to go from yellow to red. Yellow is also made from combining red and green in RGB, and the human eye is much better at distinguishing greens over reds and blues, which is probably why you're seeing green to begin with. If I were you, I'd start things off by playing with the RGB value start points until you get a color of yellow that you can't see the green in. either that, or stick to HSV color space and use that to change how your LED displays color
https://en.wikipedia.org/wiki/HSL_and_HSV
https://blog.saikoled.com/post/43693602826/why-every-led-light-should-be-using-hsi
all the same I have found a github page that converts RGB to HSV that would hopefully be helpful
https://gist.github.com/postspectacular/2a4a8db092011c6743a7

Answer 2

Here is an illustration of rgb values along with the change of hue value
(assuming saturation and intensity are max):
You'll see both r and g have 255 around hue=60. On the other hand, the result with your hsi_to_rgb() seems to have darker greenish values around there.
Please try the following hsi_to_rgb(), which is an implementation of
the description in Wikipedia.
/*
* convert hue, saturation, intensity values to r, g, b values
* h in [0,360)
* s, v in [0,1]
* rgb in [0,255]
*/
void hsi_to_rgb(float h, float s, float i)
{
float r = i;
float g = i;
float b = i;
if (s > 0.0) {
h *= 6.0 / 360; // 0 <= h < 6
int h2 = (int)h; // integral part of h
float f = h - (float)h2; // fractional part of h
switch (h2) {
default:
case 0:
g *= 1 - s * (1 - f);
b *= 1 - s;
break;
case 1:
r *= 1 - s * f;
b *= 1 - s;
break;
case 2:
r *= 1 - s;
b *= 1 - s * (1 - f);
break;
case 3:
r *= 1 - s;
g *= 1 - s * f;
break;
case 4:
r *= 1 - s * (1 - f);
g *= 1 - s;
break;
case 5:
g *= 1 - s;
b *= 1 - s * f;
break;
}
rgb[0] = 255 * r;
rgb[1] = 255 * g;
rgb[2] = 255 * b;
// clamp the rgb values
for (int j = 0; j < 3; j++) {
if (rgb[j] < 0) rgb[j] = 0;
else if (rgb[j] > 255) rgb[j] = 255;
}
}
}
Then change the hue value gradually from 60 (yellow) to 0 (red).