Obtaining Histogram of Image Stored in Cell Array

Hello,

I have a cell array called output. Output contains matrices of size 1024 x 1024, type = double, grayscale. I would like to plot each matrix and its corresponding histogram on a single plot. Here is what I have so far:

for i = 1:size(output,2)
    figure 
    subplot(2,1,1)
    imagesc(output{1,i});
    colormap('gray')
    colorbar;
    title(num2str(dinfo(i).name))

    subplot(2,1,2)
    [pixelCount, grayLevels] = imhist(output{1,i});
    bar(pixelCount);
    title('Histogram of original image');
    xlim([0 grayLevels(end)]); % Scale x axis manually.
    grid on;

end

The plot I get, however, seems to be faulty... I was expecting a distribution of bars.

I am somewhat lost at how to proceed, any help or suggestions would be appreciated!

Thanks :)

Answer 1

[Image Analyst]

Instead of this

[pixelCount, grayLevels] = imhist(output{1,i});
bar(pixelCount);

try this:

histogram(output{1,i}, 256);
grid on;

[itend]

Thank you for your response.

This works!

Answer 2

[Aylin]

This issue might be caused due to the difference between the binLocations output of the imhist command and the x-axis scale for the bar graph. For example:

A = rand(1024);                        % generate some example data
[counts, binLocations] = imhist(A);    % generate histogram bins
bar(counts);                           % plot histogram data into bar graph

After executing the above code, examine the value of the binLocations variable. It would have a range between 0 to 1, while the histogram would have an x-axis between 0 to 255. Therefore, the following line of code:

xlim([0 binLocations(end)]);

would limit the bar graph to a range of 0 to 1, while the actual data is plotted on a range from 0 to 255. If you would like to show the full range of the bar graph, the following may work better:

xlim([0 numel(binLocations)])

However, if you would like to scale the x-axis of the bar graph correctly between 0 and 1, the following might work better:

bar(binLocations, counts);             % provide both x and y inputs to the 'bar' function
xlim([0 binLocations(end)]);           % apply x-axis limits

For more information, please refer to the MATLAB documentation pages for bar , numel , and imhist .

[itend]

Thank you for your response.