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# Convert color to value

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Xin on 30 Nov 2017
Commented: Image Analyst on 30 Nov 2017
Hello everyone. I have a jpg image with blue to red color corresponding to different value. There is also a colorbar showing each colorscale correspond to a value. Is there a simple way to convert this color image into matrix? Thanks a lot.
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### Accepted Answer

Jos (10584) on 30 Nov 2017
So you have combinations of red and blue colour values (R,B) that correspond to a some arbitrary single values V according to the colour scale. Perhaps 2D interpolation, will work for you:
Extract all the the (R,B) combinations for each pixel (Rq,Bq)
Vq = interp2(R,B,V,Rq,Bq)
If the green channel is also important, use interp3.
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### More Answers (2)

Image Analyst on 30 Nov 2017
See my attached demo where I used the colormap in a thermal image to convert that into a grayscale temperature image.
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Stephen Cobeldick on 30 Nov 2017
Use rgb2ind.
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Image Analyst on 30 Nov 2017
That's the gist of it, but it's not that simple unless you already have the colormap. In many cases, the user does not and has only some kind of screenshot. My solution also uses rgb2ind() but also does the hard part of figuring out how to obtain the colormap.

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