MATLAB: In the Image Processing Toolbox 3.2 (R13), why does IMHIST error out when called with RGB images

3derrorimageImage Processing Toolboximhistr13rgb

The following code will reproduce this problem:
rgb = imread('flowers.tif');
imhist(rgb)
 ??? Error using ==> checkinput (check_attributes)
 Function imhist expected its first input argument, I or X,
 to be two-dimensional.
 Error in ==> D:\Applications\MATLAB6p5\toolbox\images\images\checkinput.m
 On line 37  ==> check_attributes(A, attributes, function_name, variable_name, ...
 Error in ==> D:\Applications\MATLAB6p5\toolbox\images\images\imhist.m (parse_inputs)
 On line 173  ==> checkinput(a, 'double uint8 logical uint16', '2d', mfilename, 'I or X', 1);
 Error in ==> D:\Applications\MATLAB6p5\toolbox\images\images\imhist.m
 On line 49  ==> [a, n, isScaled, top, map] = parse_inputs(varargin{:});

Best Answer

This is the expected behavior of IMHIST. According to the documentation, IMHIST was never meant to accept RGB images. However, in versions of the Image Processing Toolbox prior to 3.2 (R13), IMHIST would follow through execution without erroring out. The results would most likely not be the desired results. IMHIST R13 now errors out so that you will not receive erroneous results.
In order to fix the problem, you will need to read the documentation on IMHIST and understand the image types that it supports. You will basically need to convert your RGB image into a grayscale or indexed image.
For a simple example, you can use the following code as a guide:
% Read in RGB image
rgb = imread('flowers.tif');
% Convert RGB to grayscale
gray = rgb2gray(rgb);
% Now call IMHIST
figure;
imhist(gray)
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