How to set specific color in an image as transparent?

Question

Harald von der Osten 2021-12-28

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1618810-how-to-set-specific-color-in-an-image-as-transparent

评论： Harald von der Osten 2021-12-29

In this image:

I would like to set the blue color rgb = [61 38 168] as transparent and save it to a new png. How can I do this? Thanks a lot

EDIT: additional information

Thanks to your answers. Because you ask yourself, what I want with this transparency I want to give you more information: First, I want to define the blue color as transparent:

to be able to georeference it like this:

Again, thanks for your help (I did this using Photoshop, but I would prefer doing this with Matlab... )

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Answer 1

Adam Danz 2021-12-28

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https://ww2.mathworks.cn/matlabcentral/answers/1618810-how-to-set-specific-color-in-an-image-as-transparent#answer_863805

编辑：Adam Danz 2021-12-28

image.png

Load image

img = 'image.png';
I = imread(img); 
figure()
tiledlayout(1,2)
ax1 = nexttile();
imshow(I,'Parent',ax1)

Isolate target color

targetColor = [61 38 168]; 
isTarget = I(:,:,1) == targetColor(1) & I(:,:,2) == targetColor(2) & I(:,:,3) == targetColor(3);
isTargetArray = repelem(isTarget, 1,1,3);

Updated: Plot isolated section on top of another image

ax2 = nexttile();

hold(ax2, 'on')

imshow(rand(size(I)),'Parent',ax2) % noise image

% Isolate section

img2 = imshow(I,'Parent',ax2);

% Set transparency

set(img2, 'AlphaDataMapping', 'none', 'AlphaData', double(~isTargetArray(:,:,1)));

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Adam Danz 2021-12-28

I've updated my answer to show how to selectively add transparency to sections of the image.

Harald von der Osten 2021-12-29

thanks a lot, dear Adam Danz

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Answer 2

Image Analyst 2021-12-28

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1618810-how-to-set-specific-color-in-an-image-as-transparent#answer_863905

Use imoverlay

maskColor = [61 38 168];
% Create a new image where the mask is the specified color over the original rgb image.
image2 = imoverlay(originalRGBImage, maskImage, 'Color', maskColor);
imshow(image2);

where mask is your image and it's 0 where you want the original image to show through, the mask is 1 where you want the specified color. The color will be solid, 100% opaque.