18

I read an image in matlab using

input = imread ('sample.jpeg');

Then I do 

imhist(input);

It gives this error:

??? Error using ==> iptcheckinput
Function IMHIST expected its first input, I or X, to be two-dimensional.

Error in ==> imhist>parse_inputs at 275
iptcheckinput(a, {'double','uint8','logical','uint16','int16','single'}, ...

Error in ==> imhist at 57
[a, n, isScaled, top, map] = parse_inputs(varargin{:});

After running size(input), I see my input image is of size 300x200x3. I know the third dimension is for color channel, but is there any way to show histogram of this? Thanks.

E_learner
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5 Answers5

29

imhist displays a histogram of a grayscale or binary images. Use rgb2gray on the image, or use imhist(input(:,:,1)) to see one of the channel at a time (red in this example).

Alternatively you can do this:

hist(reshape(input,[],3),1:max(input(:))); 
colormap([1 0 0; 0 1 0; 0 0 1]);

to show the 3 channels simultaneously...

bla
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14

I pefere to plot the histogram for Red, Green and Blue in one plot:

%Split into RGB Channels
Red = image(:,:,1);
Green = image(:,:,2);
Blue = image(:,:,3);

%Get histValues for each channel
[yRed, x] = imhist(Red);
[yGreen, x] = imhist(Green);
[yBlue, x] = imhist(Blue);

%Plot them together in one plot
plot(x, yRed, 'Red', x, yGreen, 'Green', x, yBlue, 'Blue');
Philipp Hofmann
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5

An histogarm plot will have number of pixels for the intensity levels. Yours is an rgb image. So you first need to convert it to an intensity image.

The code here will be:

input = imread ('sample.jpeg');

input=rgb2gray(input);

imhist(input);

imshow(input);

You will be able to get the histogram of the image.

zkanoca
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Manisha
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3
img1=imread('image.jpg');
img1=rgb2gray(img1);
subplot(2,2,1);
imshow(img1);
title('original image');
grayImg=mat2gray(img1);
subplot(2,2,2);
imhist(grayImg);
title('original histogram');

Remember to include mat2gray(); because it converts the matrix A to the intensity image grayImg. The returned matrix grayImg contains values in the range 0.0 (black) to 1.0 (full intensity or white).

Varun Parikh
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0

Histogram is useful to analyze pixel distribution in an image. Histogram plots number of pixel in an image with respect to intensity value. 

img1=imread('image.jpg');
hist(img1);
vignesh Gk
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