Difference between hist and imhist

Hi,

I have a fundamental question about the hist and imhist and an explanation will be great as I am a bit lost.

I am running this script:

I = imread('pout.tif');
figure;
x=imhist(I)
imhist(I)
figure(2);
hist(x)

and when I compare the two histograms they do not look alike, why?

How can I save the imhist data so that the hist will show the same plot?

Thanks

Answer 1

[Image Analyst]

For a gray scale image imhist() will give 256 bins. hist() only gives 10 by default. But you did not use hist() to take the histogram of the image - the badly-named "I". You used it to take a histogram of the histogram counts, x - a pretty weird/strange thing to do.

[Ely Raz]

I understand that I need to use the imhist to generate the gray scale image histogram. When I want to generate it via hist it is completely different, I do not know why and therefore how can I generate an histogram that looks the same like imhist output (even without using the imhist)?

[Image Analyst]

You were doing it wrong, like I said. hist() knows nothing about what the x-values (gray levels) should be - it just hists what is there and is totally ignorant that a uint8 variable goes from 0 to 255, or at least can, but doesn't for the image you took. Here, try this and maybe you'll understand:

close all;
grayImage = imread('pout.tif');
subplot(2,1,1);
[pixelCounts, grayLevels] = imhist(grayImage);
bar(grayLevels, pixelCounts, 'BarWidth', 1);
imhist(grayImage)
subplot(2,1,2);
counts = hist(double(grayImage(:)), 256);
% IMPORTANT WARNING: counts(1) is not for gray level 0!!!
bar(counts, 'BarWidth', 1);
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);

[Miguel Yanez]

Hi, I have not tried your script yet but I wonder if, Can it be used to create a Dicom Image Histogram?

[Image Analyst]

Yes. It does not care what format was used to store the image on disk, it just looks at an array of numbers, which is what you get when you read a disk file into MATLAB.

[Nagabhushan SN]

Hi @ImageAnalyst, you've mentioned that "IMPORTANT WARNING: counts(1) is not for gray level 0!!!" So do you mean that hist doesn't give histogram in order of intensity values? So, histogram plots by imhist(grayImage) and hist(double(grayImage(:)), 256) will be different?