How to save an image which only display properly using imshow(grayImg,[])?

Question

ASHWIN JOSHY 2022-5-1

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https://ww2.mathworks.cn/matlabcentral/answers/1709275-how-to-save-an-image-which-only-display-properly-using-imshow-grayimg

回答： yanqi liu 2022-5-7

I am working on an image encryption project, so I first converts color image to gray and do the rest, after encryption I can see the image only using

imshow(grayImg,[])

Not by

imshow(grayImg)

but if I store the image with

imwrite(grayImg,outputFileName);

Then it isn't saving as expected or not as I saw when using imshow(grayImg,[]) Does anyone know what kind of error am I making?

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ASHWIN JOSHY 2022-5-1

The encrypted values are double, i want to save as .tiff

Image Analyst 2022-5-1

在 MATLAB Online 中打开

If you use single or double in imshow() it expects the values to be in the range 0-1. If they're below 0 the pixels show up as black. If more than 1 the pixels show up as white. If you use [] then it scales the min of your data (whatever value it may have) to black and the max to white.

To save a double as a tiff, this should do it:

% Create floating point image.
rgbImage = rand (10, 20, 3);
% Image must be single precision.
rgbImage = single(rgbImage);
% Display it.
imshow(rgbImage, 'InitialMagnification', 1000)
axis('on', 'image');
% Create tiff object.
fileName = '_floatingPointImage.tif';
tiffObject = Tiff(fileName, 'w')
% Set tags.
tagstruct.ImageLength = size(rgbImage,1); 
tagstruct.ImageWidth = size(rgbImage,2);
tagstruct.Compression = Tiff.Compression.None;
tagstruct.SampleFormat = Tiff.SampleFormat.IEEEFP;
tagstruct.Photometric = Tiff.Photometric.MinIsBlack;
tagstruct.BitsPerSample = 32;
tagstruct.SamplesPerPixel = size(rgbImage,3);
tagstruct.PlanarConfiguration = Tiff.PlanarConfiguration.Chunky;
tiffObject.setTag(tagstruct);
% Write the array to disk.
tiffObject.write(rgbImage);
tiffObject.close;
% Recall image.
m2 = imread(fileName)
% Check that it's the same as what we wrote out.
maxDiff = max(max(m2-rgbImage))