How to plot a histogram from 0-255

I use img1=imread function to load my image into matlab. The image is 512by512 and the intensity range is 0-255. However the histogram of my image ( which I use imhist(img1) to plot it) has values only at 0 and 1. How is that? I mean when I take a look at the array of my picture, I see numbers varying from 0-255. Is it because I am using imhist? Any idea would be appreciated. and sorry if the question might be very basic for some of you.

Answer 1

[Thorsten]

I = imread('cameraman.tif');
imhist(I)

shows values from 0 to 255. Do you change I after imread and before imhist?

[Changoleon]

actualy this is how I code: a = double(imread('Wk1_O5_Image017.tif')); is it double?

[Thorsten]

Either use

I = imread('yourfile.tif');
imhist(I);

or as Guillaume suggests,

I = im2double(imread('yourfile.tif'));
imhist(I)

Answer 2

[Guillaume]

See the documentation on image classes.

Most image processing functions expect double images to be in the range 0-1. If the range is 0-255, matlab expects it to be a uint8 image.

a = im2double(imread('yourimage'));
imhist(a);

should solve your problem. Or keep the image as uint8.

Answer 3

[dhafer alhajim]

if you want the value and the range from 0 to 255 a = im2double(imread('your image')); h=uint8(imhist(a));

[Guillaume]

@dhifer alhajm

I don't see the point in answering a question that is over a year old. Particularly as your answer is totally incorrect. Because of the call to im2double, the input range is 0-1. As for the uint8(imhist), that's completely silly, any histogram count above 255 will be clamped to 255.

Here's an example:

a = uint8(repelem([0 63 127 255]', [20 40 60 80], 200));
imshow(a);

This is an image with 4000 counts of intensity 0, 8000 of intensity 63, 12000 of intensity 127 and 16000 counts of intensity 255.

>>h = uint8(imhist(im2double(a)))
>>h(1)  %counts of intensity 0
ans = 
   255
>>h(end)  %count of intensity 255
ans  = 
   255

The count of all intensities present in the image is 255 according to you.