Make only the highest values of grayscale image transparent

For a grayscale image with values from 0 to 255, I would like to make only the highest values transparent, so the rest of the image will be black when I overlay it on another image

Answer 1

[Walter Roberson]

image(BackgroundImage);

imageMax = max(YourGrayscaleImage(:));
alphaMask = double(YourGraycaleImage == imageMax);
hold on
image(YourGrayscaleImage, 'alphadata', alphaMask);
hold off
colormap(gray(256))

Answer 2

[DGM]

I'm going to assume that the goal is to combine two images of the same page geometry and then save the result. If the end goal is to have a composite image to keep, then don't rely on in-figure composition and screenshots. Just compose the image.

Generate a mask by some means -- for example, logical thresholding:

% two images of the same class, depth, and page geometry
BG = imread('cameraman.tif'); % uint8
FG = fliplr(BG); % a second test image

% select everything lighter than 65% gray
mask = FG > (255*0.65);

% assemble the output using logical indexing
outpict = BG;
outpict(mask) = FG(mask);

imshow(outpict,'border','tight')

Of course that's not robust at all. If you want to deal with soft or graduated masks, color images, or mixed numeric classes, then there are plenty of examples:

https://www.mathworks.com/matlabcentral/answers/1723310-masking-multiple-parts-of-image

If your goal is something else, then you'll have to elaborate.