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White 256x256 image gives me 256x256x3

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Osama Hussein
Osama Hussein 2016-11-7
评论： Walter Roberson 2016-11-8
I need a white 256x256 pixels image to use it in an assignment. I tried to make it using Paint and downloaded white images from the internet, but for some reason when I read the image, it stored in a 256x256x3 variable (color image format). I tried different extensions (jpg, png).
What is the reason? I attached the images I used and here is the code to read the image.
img = double(imread('white.jpg'));
[l,w] = size(img);
figure
imshow(uint8(img)) % display the test image
title('Original image')

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采纳的回答

Image Analyst
Image Analyst 2016-11-7
Try this:
whiteImage = 255 * ones(256, 256, 'uint8');
imwrite(whiteImage, 'White Image.png'); % Save to disk.
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Walter Roberson
Walter Roberson 2016-11-8
Paint has no provision for saving as grayscale, only as color and as bitmap (black and white, no gray levels.)

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Walter Roberson
Walter Roberson 2016-11-7
jpg images are almost always RGB images, even if they appear grayscale. It is possible to have a grayscale jpg image, but the program would have to support creating it.
You can use rgb2gray() to convert to the equivalent grayscale image.
Caution: you have
[l,w] = size(img);
which is going to give you confusing results for RGB images. You should be using
[l, w, chan] = size(img);
  2 个评论
Osama Hussein
Osama Hussein 2016-11-7
Thank you for your prompt reply. When I used "img = rgb2gray(image);" the variable for the image "img" will be all ones, but I need a greyscale image with pixels between 0 and 256 as previous. Regarding the format of the image, I tried also to use png, but it gives me same result.
Walter Roberson
Walter Roberson 2016-11-7
编辑：Walter Roberson 2016-11-7
GrayImage = img(:,:,1);
This will not work for general RGB images but it will work for images that are completely tones of gray stored as RGB images.

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