Converting temperature to RGB value

Question

I am calculating the current temperature to RGB related to a maximum temperature:

val = round(temp / tempMax * 255, 0);
if (val > 255) val = 255;

In maths the function is v=temp/tempMax*255. This works for a temperature between 0 and tempMax.

I want to do the same but in an area between minTemp and maxTemp-5. That means if the temp is exactly minTemp, val should be 0 and if it is maxTemp-5 it should be 255. all temperature that are lower than minTemp set val to zero and there will be special if statement for the area between maxTemp-5 and maxTemp. All the ifs are no problem but how do I need to modify my function that it calculating proportionally to the given borders?

`v=temp/(tempMax-5)*255` – Robby Cornelissen Jul 29 '19 at 06:46 — Robby Cornelissen, Jul 29 '19 at 06:46
No that’s not the solution as there is no minimum value – T. Karter Jul 29 '19 at 06:53 — T. Karter, Jul 29 '19 at 06:53

score 1 · Answer 1 · answered Jul 29 '19 at 07:42

val = round(((temp - minTemp) * 255) / ((maxTemp - 5) - minTemp), 0);
if (val < 0) val = 0;
else if (val > 255) val = 255;

You subtract minTemp from temp so that the range starts at zero. Then multiply by 255 / ((maxTemp - 5) - minTemp) to scale the range of minTemp -> maxTemp - 5 to 0 -> 255. Finally clamp the result to the allowable range.

I rearranged the formula to do the multiply first, so that it also works using integers. If you wanted it to work with unsigned numbers you could clamp to minTemp before applying the formula and remove the check for v < 0.

score 0 · Accepted Answer · answered Jul 29 '19 at 11:14

RGB color of black body at some temperature

is directly related to B-V index see:
- Star B-V color index to apparent RGB color
you have the equation t=f(B-V) in the OP just reverse it.
what you asking is just linear interpolation

simply:
```
i=i0 + (t-t0)*(i1-i0)/(t1-t0)
```
where t0,t1 is input temp range and i0,i1 is output intensity range.

Converting temperature to RGB value

2 Answers2