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How do I save the original image jpg from figure for colormap?

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Dayangku Nur Faizah Pengiran Mohamad
编辑： DGM 2022-12-6
Hi. I want ask a question.
How do I save the original image jpg from figure for colormap?
Here my codes:
I=imread('M_92_ar_2001743_st01_currentsize_200x200.jpg');
imshow(I);
imfinfo('M_92_ar_2001743_st01_currentsize_200x200.jpg');
I2=I(:,:,1);
imshow(I2);
map = [0, 0.3, 0
0, 0.4, 0
0, 0.5, 0
0, 0.6, 0
0, 0.8, 0
0, 1.0, 0];
imshow(I2);
colormap(map);
imwrite(I2,map,'cmalignant200.jpg');
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Dayangku Nur Faizah Pengiran Mohamad
Dayangku Nur Faizah Pengiran Mohamad 2021-11-16
I cannot save the figure colormap as image jpg. Thats the problem.
The codes imwrite(I2,map,'cmalignant200.jpg'); only save as I2 image jpg not the colormap image jpg in figure
DGM
DGM 2021-11-16
imshow's implicit handling of colormaps is different than how ind2rgb or imwrite handles things, particularly when the image is integer-class. The nominal value range in a uint8 image is 0-255 (256 levels). When given the above syntax, imwrite() treats these pixel values as indices into the given colormap. The colormap you're giving it only has 6 entries; correspondingly, the vast majority of the output image will correspond to the last colormap entry.

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Dayangku Nur Faizah Pengiran Mohamad
Hello, I wanna to ask about other colors.
For example, I want to red colormap, so the codes is:
z = zeros(256,1);
map = [z linspace(256,0.3,0.3).' z]; ?
Is this correct codes for red colormap?
How about blue colormap?
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Dayangku Nur Faizah Pengiran Mohamad
Dayangku Nur Faizah Pengiran Mohamad 2022-12-6
移动：DGM 2022-12-6
Hi. What if I want to test yellow colormap? Is this the correct way to write the code?
map = [linspace(0.3,1,256).' ' z]; ?
DGM
DGM 2022-12-6
编辑：DGM 2022-12-6
Ran in:
% a test image
I = imread('cameraman.tif');
I = I(:,:,1);
% a 256-level yellow colormap from 0.3 to 1
z = zeros(256,1);
map = [linspace(0.3,1,256).' linspace(0.3,1,256).' z];
imshow(I,map)
... or you could generalize
% generate a linear colormap between any two color tuples
colorA = [0.3 0.3 0];
colorB = [1 1 0];
numcolors = 256;
map = interp1([1/numcolors 1],[colorA; colorB],(1:numcolors)/numcolors,'linear','extrap');
% apply it to an image
inpict = imread('cameraman.tif');
imshow(inpict,map)

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DGM
DGM 2021-11-16
编辑：DGM 2021-11-16
Ran in:
A few things.
  • Imwrite doesn't support indexed outputs for JPG files, so if you're trying to save the output as an indexed image, JPG won't work.
  • Your image ostensibly is uint8, and thus has 256 levels. Your colormap only has 6. You either need to quantize the image or use a longer colormap.
  • If all you want is an RGB image (all that JPG supports), and all you want is a strictly green colormap, then you don't even need to mess with a colormap or indexing.
% a test image
I = imread('cameraman.tif');
I = I(:,:,1);
% a 6-level green colormap from 0.3 to 1
z = zeros(6,1);
map = [z linspace(0.3,1,6).' z];
I6 = gray2ind(I,6); % quantize the image to 6 gray levels
imwrite(I6,map,'greencm1.jpg'); % write it
imshow(I6,map)
% a 256-level green colormap from 0.3 to 1
z = zeros(256,1);
map = [z linspace(0.3,1,256).' z];
imwrite(I,map,'greencm2.jpg'); % write it
clf; imshow(I,map)
% using some predefined colormap
imwrite(I,jet(256),'jetcm.jpg'); % write it
clf; imshow(I,jet(256))
% or just don't bother with colormaps if all you want is green
Igr = mat2gray(I)*0.7 + 0.3; % normalize, scale, offset
z = zeros(size(I));
Igr = cat(3,z,Igr,z); % green only
imwrite(Igr,'nocm.jpg'); % write it
imshow(Igr)

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