White 256x256 image gives me 256x256x3

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')

Answer 1

[Image Analyst]

Try this:

whiteImage = 255 * ones(256, 256, 'uint8');
imwrite(whiteImage, 'White Image.png'); % Save to disk.

[Osama Hussein]

Thank you very much. This works fine. After that, I added an empty triangle on this image (using Paint for example). The problem appears again! Do you have any idea why this happens? Thank you in advance.

[Walter Roberson]

Paint is writing as RGB.

[Walter Roberson]

Paint has no provision for saving as grayscale, only as color and as bitmap (black and white, no gray levels.)

Answer 2

[Walter Roberson]

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);

[Osama Hussein]

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]

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.