Converting RGB Image to Grayscale Intensity on arbitrary Colormap

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We have some thermal videos that have a colorscale presented along the side. I would like to run some thermal analysis on video frame regions. If I extract the colorscale from the video, how could I apply that to the image such that the result would be grayscale based on the intensity of the colorscale?
I have obtained a cmap for the custom scale by reading in that area, taking the mean over each column, resulting in a 1601x3 double, but I don't know how to map that onto a grayscale image as intensity values.

Accepted Answer

Image Analyst
Image Analyst on 2 Dec 2021
People ask this all the time. So much so that I have a canned demo for it. See attached.
Adapt as needed, like to specify where in the image your image and colorbar are located.

More Answers (1)

DGM
DGM on 1 Dec 2021
Edited: DGM on 1 Dec 2021
Something like this might be a start:
% you already found a colormap
cmap = parula(64);
% generate false-color test image
A = imread('cameraman.tif');
Aind = gray2ind(A,size(cmap,1)); % uniform quantization
Afalse = ind2rgb(Aind,cmap); % apply colormap
imshow(Afalse)
% convert back to intensity image
Arecovered = double(rgb2ind(Afalse,cmap))/(size(cmap,1)-1);
figure; imshow(Arecovered)
immse(im2double(A),Arecovered) % not perfect, but pretty good
ans = 2.9731e-05
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