false-/pseudo color coding for temperature

Question

I have some temperature values in a given range, lets say between 0-100 Celsius. Now I want to map these values to colors, so blue=RGB(0,0,255) means 0°C, red=RGB(255,0,0) means 100°C and the colors in between mean some temperatures in between.

Matlab actually has a function called colormap. My stuff is implemented in OpenCV & C++. Is there anyone, who already has some experience in color coding or knows any good mathematical way to do that?

score 2 · Answer 1 · answered Feb 02 '12 at 16:14

For temperatur (t) from 0 to 100 you could calculate each color:

Pseudocode:
  Col(t)=(  0+INT(2.55*t), // Red
            0,             // Green
          255-INT(2.55*t)) // Blue
Col(0)  =(0,0,255)
Col(100)=(255,0,0)

The factor 2.55 comes from the given Max/Min Values of 0° and 100° Celsius. You just have to choose your intervals now.

Hope that helps.

score 2 · Accepted Answer · edited May 23 '17 at 11:48

2

The HSV colourspace might be helpful here.

Hue is periodic, however,, which is to say if you go from red all the way to the other end of the spectrum, you'll be back at red, which isn't that useful in your case. What you'll probably want to do is choose a subset of the hue spectrum that goes from red, through yellow and green, to blue (omitting pink/purple). There's an image in the RGB-HSV section that should show you what I mean.

Update: In fact, this previous answer tells you how to implement exactly the MATLAB Jet palette that you describe.

It seems that Jet is actually a variation on the HSV colourspace anyway!

edited May 23 '17 at 11:48

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answered Feb 02 '12 at 16:15

Will Vousden

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THANKS for this really enlightening answer. [This site](http://paulbourke.net/texture_colour/colourramp/) was especially helpful. I'm still a little stunned that there seemed to be none OpenCV implementation of color coding for information like temperature or distance ... – Fei Feb 03 '12 at 14:51
I came across [this](http://codingmess.blogspot.de/2009/05/conversion-of-wavelength-in-nanometers.html) neat solution yesterday: the algorithm converts the wavelength of the visible light (in nm) to RGB-values. – Fei Oct 22 '12 at 12:38

score -1 · Answer 3 · answered Feb 02 '12 at 18:44

It sounds like you want a JET colormap, which begins at dark blue and ends at dark red.

Assuming you have a set of temperatures from 0 to 100, incrementing by one:

temps = 0:1:100;

Use the JET function to create the set of colors over N points, where N is the length of your temperature vector:

colors = jet(length(temps));

Now, to access the color for the first temperature (this will give blue):

colors(1,:)

Or the colors for the last temperature (this would be red):

colors(end,:)

false-/pseudo color coding for temperature

3 Answers3