TIF image being displayed incorrectly

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Nour Aburaed
Nour Aburaed 2019-2-27
回答： Image Analyst 2019-2-28
I am trying to do a very simply task; reading R, G, B, bands from a file (in TIF), and then combining them to produce a colored TIF image. When I read the bands, they appear to be of type uint16. Therefore, I convert them to uint8 before combining them (otherwise the image appears black). But after that, the combined image appears white. Why does this happen? This is my code
[red, rmap] =imread('red.tif'); %uint16
[green, gmap] =imread('green.tif'); %uint16
[blue, bmap] =imread('blue.tif'); %uint16
subplot(2,2,1); imshow(red, rmap)
subplot(2,2,2); imshow(green,gmap)
subplot(2,2,3); imshow(blue,bmap)
rgb = cat(3, uint8(red), uint8(green), uint8(blue));
subplot(2,2,4); imshow(rgb); title('Combined')
This is a sample output:
sample output.png

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回答（2 个）

Jan
Jan 2019-2-27
uint8 converts all values above 255 to 255. So use:
rgb = im2uint8(cat(3, red, green, blue));
which scales the data accordingly.
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Nour Aburaed
Nour Aburaed 2019-2-28
I tried using imshow() with and without colormap. The individual bands can be viewed whether I include a colormap or not. For the combined image, it is always either black or white with or without including the color map. Unfortunately I cannot share the data that I am using. But thanks for your help, I will try to make use of the hints you gave me. So are you suggesting that I work with images of type double instead of uint8?
Jan
Jan 2019-2-28
You can prefer double or uint8 as you like. Using doubles will allow to keep the information of uint16, while this is lost with uint8. The main idea is to avoid troubles with different types and/or index or RGB images. If you mix the types, problems like saturated images (all white or all black) can be expected. So decide for a standard format and convert all input to it:
function Img = myIMRead(File, varargin)
[Img, Map] = imread(File, varargin{:});
if ~isempty(Map) % If it is an indexed image, convert it to 3D RGB array
Img = ind2rgb(Img, Map);
end
Img = im2double(Img); % Convert it to double (or im2uint8, if you like)
end

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Image Analyst
Image Analyst 2019-2-28
Did you try it without a map but using [], like
imshow(red, []); % Use bracket bracket to scale actual data to display's range without altering the data variable.
or do you require the stored pseudocolor color map? If that works, then no need to use ind2rgb() or im2uint() or im2double() or anything else.

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