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How do I limit the range of a colormap when using imwrite

Asked by Michael Devereux on 23 Apr 2018
Latest activity Commented on by Michael Devereux on 25 Apr 2018
I want to save an image with imwrite and get the same output as the code below displays on screen
figure,imagesc(rhoi),
colormap(jet)
caxis([.45*min(rhoi(:)) 0*.55*max(rhoi(:))])
Is it possible to set a caxis property for imwrite?
imwrite(rhoi,jet,'out.png')
EDIT: I have included the full code. Basically, I want the last image RGB to look the same as that displayed by imagesc at each stage.
clear all
close all
clc
Inner = 0; % inner radius of the colour ring
rOuter = 150; % outer radius of the colour ring
[x, y] = meshgrid(-rOuter:.1:rOuter);
[theta, rho] = cart2pol(x, y);
rhoi=imcomplement(rho);
cmap=jet(256);
rhoi2=rhoi;
rhoi2=(rhoi2-min(rhoi2(:)))/(max(rhoi2(:))-min(rhoi2(:)));
figure;imagesc(rhoi2),colormap(cmap);
caxis([.55 .95])
rhoi2(rhoi2<.55) = .55;
rhoi2(rhoi2> .95) = .95;
figure,imagesc(rhoi2)
colormap( cmap);
X=uint8(256*rhoi2);
figure;imagesc(X),colormap( cmap);
RGB=ind2rgb(X,cmap);
figure;imshow(RGB)

  3 Comments

Can you confirm that your rhoi matrix is made of positive integers only. Otherwise, attempting to write an indexed image as per your imwrite call makes no sense.

I am scaling the matrix to a range of 0-255 and changing the range accordingly
I have added the full code to demonstrate the problem

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2 Answers

Answer by Guillaume
on 24 Apr 2018
 Accepted Answer

Before the call to ind2rgb (or imwrite), you need to rescale your matrix as colour indices so that the minimum is index 1 (for type double, 0 for type uint8) and maximum is index 256 (255 for uint8):
X = round(1 + (rhoi2 - min(rhoi2(:))) * 255 / (max(rhoi2(:)) - min(rhoi2(:)))); %using double
RGB = ind2rgb(X, cmap);
figure; imshow(RGB);

  1 Comment

Thanks! This did what I need.

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Answer by Ahmet Cecen on 23 Apr 2018

You should get the same effect if you just did:
rhoi(rhoi<.45*min(rhoi(:))) = .45*min(rhoi(:));
rhoi(rhoi> 0.55*max(rhoi(:)) = 0.55*max(rhoi(:));

  1 Comment

Thanks! This works for viewing with imagesc without specifying caxis but does not work with imwrite or ind2rgb.

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