How to extract raw data from a rgb image visualized with known colormap?

Hello, everyone,

I have a rgb image, and it was visualized with a known colormap. Now I want to extract the raw 2D matrix data rather than RGB values of this image before it was mapped to the colormap . How should I do?

Here is an example:

raw_data = rand(256,256);
figure;
imagesc(raw_data);
axis image off;
colormap(jet);
colorbar;
title('saved image');

Then I save this visualized image, names as, for example 'saved image.jpg'.

How should I retrieve 'raw_data' from 'saved_img.jpg' according to the known 'jet' colormap?

Thank you!

Answer 1

[Sameer Pujari]

Hello Xin Liu

From the given info, I understand that you want to get RGB image back after you have visualised with a known colormap.

To do that you have to first get the colormap of orignal rgb image

RGB = imread('image.jpg');
[X,cmap] = rgb2ind(RGB,65536);

Then you can get the original image back using

X = ind2rgb(X,cmap);

The same can be done by using "colormap" function for the figure

RGB = imread('image.jpg');
[X,map] = rgb2ind(RGB,65536);
figure
image(X)
axis off
axis image
colormap(parula) %image will be visualized in parula colormap
colormap(cmap) %image will be visualized in orignal rgb colormap

I hope this information helps you!

[Xin Liu]

Dear Sameer,

Thanks very much for your answer. I've tried your solution, but I did not get the correct result as expected.

Maybe I did not explain my purpose clearly before, so I re-edit the question, and give an example.

I hope it will facilitate your understanding.

[Sameer Pujari]

Hi Xin Liu

I have attached a code according to your revised question.

% Saving current figure as a MATALB figure and saving orignal colormap 

raw_data = rand(5,5);
figure;
imagesc(raw_data);
axis image off;
cmap=colormap; % saves orignal data colormap
colorbar;
title('saved image');
savefig('saved_img.fig');

% Changing colormap and saving the jet colormap figure

colormap(jet); % changes colormap to jet
savefig('saved_img_jet.fig');
close(gcf); % closes current figure

% Opening saved figure and changing the colormap to orignal

openfig('saved_img_jet.fig'); % can open fig in another script file if required
colormap(cmap); % applying orignal colormap to jet

I hope this helps you !

[Xin Liu]

Hi Sameer,

Thank you for your code. You may misunderstand my purpose. My purpose is to get the original 2D MATRIX DATA rather than the original image. I saved this figure as a .jpg file and then I want to get the raw 2D MATRIX DATA from this .jpg file according to its colormap.

For example, a certain element in the raw MATRIX DATA is 0.5, and it was mapped to a rgb value of (0.3,0.5,0.8) in the saved .jpg file according to the 'jet' colormap. My purpose is to obtain 0.5 from the rgb value (0.3,0.5,0.8) according to the jet colormap.