Matlab For Loop Image

Question

I am trying to open a .pgm image file in MATLAB, run a manipulation with a for loop and then save as another .pgm file. Before doing the manipulation I was testing to see if I could recreate the image:

clear

picture = imread('Picture.pgm');

sizePic = size(picture);

sizeX = sizePic(1);
sizeY = sizePic(2);

newPicture = zeros(sizeX,sizeY);


for i = 1:sizeX
    for j = 1:sizeY
        newPicture(i,j) = picture(i,j);
    end
end

imwrite(newPicture, 'NewPicture.pgm');

However, the new image is almost all white with some black splotches (not the original). Shouldn't this just give me back the original image?

Answer 1

By default, picture created from imread(XXX.pgm) is either a uint8 or uint16 array, meaning the pixel values are in the range of [0 255] or [0 65535]. On the other hand, newPicture created from zeros is a double array, the expected pixel value for which is only [0 1]. Any value greater than 1 will be interpreted as 1 (white) in the saved image. When you assign a [0 255] value to such a double array, since most of the pixel values in picture is 1 and above, of course you will get mostly white pixels

When you work with images, always check the type of the image array. For example, it may be a good idea to always work with double type by explicitly converting the image returned by imread as such: pictures=im2double(imread(xxx)) .