I = imread('rice.png');
Now I includes the pixel values from the image 'rice.png' as a matrix, of size equal to the resolution of the image, and you could then access the values as I(pixel_y,pixel_x). Note that the pixel indices start from the left top, instead of left bottom as we are usually used to.
imshow(I(1:100,1:100)) %to show the first 100 pixels from the left top
figure, imshow(I) %Compare it with the entire image.
you could also cast the data into other types, based on how you want the numbers to be stored. The value in this example is uint8, which ranges from [0 - 255].
