How to change RGB value without changing index value of an image?

Question

Emre Doruk 2023-2-15

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https://ww2.mathworks.cn/matlabcentral/answers/1912835-how-to-change-rgb-value-without-changing-index-value-of-an-image

评论： Emre Doruk 2023-2-20

Hi,

I have NxM matrix, I want to plot iamge of it and the change specific rgb value without changing index value. How can I do that? I add a sample image.

For example pixel [26,28] and Index 87 have meaningful value for me. I want to change color of this pixel to red without changing index value 87.

Thanks for your help.

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Answer 1

Walter Roberson 2023-2-15

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You would have to change the active colormap at index 87 to be red, which would change all other locations with the same value.

Alternatives:

You could create an overlay image the same size with AlphaData 0.0 except at locations that you wish to recolor. You will probably want to use an rgb image to contain the additional color. You would turn hittest off on the overlay image so that the data cursor would report for the original image.

Or you could change the data cursor callback function so that it outputs your values of interest even though you have changed the image data

I would have to check to see if image objects are eligible to use the newer datatip templates

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DGM 2023-2-15

在 MATLAB Online 中打开

An example of doing it the other way around. This applies alpha to the indexed image and uses a red underlay, that way the datatip always picks up the indexed image as normal without needing to set hittest on an overlay.

% an indexed image and a colormap

A = magic(4);

CT = parula(16);

% create a red underlay

% this needs to be an RGB image

bg = repmat(permute([1 0 0],[1 3 2]),[4 4]);

imshow(bg); hold on

% overlay the indexed image and set its alpha

hi = imshow(A,CT);

alpha = ones(4);

alpha(2,2) = 0;

hi.AlphaData = alpha;

% programmatically insert a datatip for demonstration

datatip(hi,2,2);

Either way has the limitation that the location will be red, the index will be unchanged, but the reported RGB value will also be unchanged. The way I read the question, it kind of sounds like the reported and displayed color should be red, but the index should be preserved. I might be wrong, but I figured an example would be the quickest way to prompt clarification if necessary.

Walter Roberson 2023-2-17

@DGM

Nice idea to use an underlay instead.

Emre Doruk 2023-2-20

Thanks for your help.Your ideas was very helpful to understand concept.

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Answer 2

Image Analyst 2023-2-19

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https://ww2.mathworks.cn/matlabcentral/answers/1912835-how-to-change-rgb-value-without-changing-index-value-of-an-image#answer_1174925

在 MATLAB Online 中打开

Try imoverlay

% Make sample data.

rows = 10;

columns = 40;

indexedImage = randi(255, [rows, columns], 'uint8');

% Show original image.

subplot(2, 1, 1);

imshow(indexedImage, [])

% Create mask of where the image is exactly 87.

mask = indexedImage == 87;

% overlay the mask onto the original image, creating an RGB image.

rgbImage = imoverlay(indexedImage, mask, 'r');

% Display the overlay image.

subplot(2, 1, 2);

imshow(rgbImage)

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Image Analyst 2023-2-20

They said "I want to change color of this pixel to red without changing index value 87." Walter gave one way - to apply a colormap. I gave an alternative way - using an overlay. I did not change the original pixel value. I just created a new image with red in those pixel locations. imoverlay is a function worth knowing about. The original poster can use the method they like best.

Emre Doruk 2023-2-20