MATLAB Answers


Save pixel color value for indexed image (without image Processing Toolobx)

Asked by Matteo Niccoli on 8 Feb 2012
Latest activity Edited by Een Qurota Aini on 7 Oct 2013
With reference to Steve Eddins blog post on pixel colors:
Suppose I create his same image: A = magic(5);
Then I display it with imagesc and colormap HSV: imagesc(A); colormap(hsv);
Is there any way now (without using Image processing Toolbox) to save to a matrix the R,G,B triplets corresponding to the colors of each pixel as produced using HSV?
Thanks, Matteo

  1 Comment

Thanks Walter and Sean.
It will take me some time but i will try both suggestions and then will comment on results.

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2 Answers

Answer by Walter Roberson
on 8 Feb 2012
 Accepted Answer

apply that routine and then get() the CData property and that will be the RGB.


Matteo, There was no Alpha mentioned in your question, and hsv color map does not use Alpha. Which Alpha are you referring to?
Sorry, it took me a bit to get back to this.
The example in my original question was a simple one, and with a reference to Steve's blog, so it was perfect to explain what I wanted to do (I often tent to be a bit vague, typical of non native speakers).
Sean, usually my images would have more than 256 elements, so I will have to look into the suggested method more carefully as I'm not familiar with either histc or numel.
With one of the maps I had in mind I was trying to get to the RGB colors after the application of an overlay with alpha as in this other post by Steve: . Walter's idea of using freezeColors worked in that case too, with a slight adaptation. I'm about to post an example on my blog and I will acknowledge the answer, then post it on this thread.
Hi Walter
Here's my blog post showing what I am doing (I used your suggestion and applied to color with shading)
Thanks again. And thanks to Sean as well.

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Answer by Sean de Wolski
on 8 Feb 2012

Something like:
A = magic(5);
map = hsv;
Av = A(:);
[~, bin] = histc(Av,linspace(min(Av),max(Av),min(numel(Av),size(map,1))));
mapp = permute(map,[1 3 2]);
rgb = reshape(mapp(bin,:,:),size(A,1),size(A,2),3);


Hi Sean
I get this error message:
??? [~, bin] = histc(Av,linspace(min(Av),max(Av),min(numel(Av)),size(map,1)));
Error: Expression or statement is incorrect--possibly unbalanced (, {, or [.
You missed a closing ')' on that line - just like the error message says.
Just copy and paste it.
I'll check it out, thank you Sean.

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