Read an image and display it.

I = imread('rice.png');
figure
imshow(I)

Perform an operation that returns a numeric matrix. This operation looks for edges.

J = filter2(fspecial('sobel'),I);
min_matrix = min(J(:))

min_matrix = -779

max_matrix = max(J(:))

max_matrix = 560

Note that the matrix has data type double with values outside of the range [0,1], including negative values.

Display the result of the operation. Because the data range of the matrix is outside the default display range of imshow, every pixel with a positive value displays as white, and every pixel with a negative or zero value displays as black. It is challenging to see the edges of the grains of rice.

figure
imshow(J)

Convert the matrix into an image. Display the maximum and minimum values of the image.

K = mat2gray(J);
min_image = min(K(:))

min_image = 0

max_image = max(K(:))

max_image = 1

Note that values are still data type double, but that all values are in the range [0, 1].

Display the result of the conversion. Pixels show a range of grayscale colors, which makes the location of the edges more apparent.

figure
imshow(K)