Question

When I try to show a grayscale image using :

Img = imread('tr2.png');

subplot(111);

imshow(Img);

it does not appear as the original image. Where is the problem ?

Answer 1

Try to read colormap together with the image:

[Img, map] = imread('tr2.png');
imshow(Img,map);

EDIT:

I believe you have indexed image and you have to convert it to RGB before any processing.
Use ind2rgb or ind2gray function.

See for example Steve’s blog on indexed images.