Question regarding strange outputs from histogram plot

Question

Teshan Rezel 2021-4-30

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/817830-question-regarding-strange-outputs-from-histogram-plot

评论： Benjamin Großmann 2021-4-30

Hi folks,

I have an image which is mostly black, with specks of purple. When I plot the RGB histogram for the image, I get an sharp line in the blue region but zero elsewhere, which I don't think is correct.

Can you please advise me on where I've gone wrong?

Thanks

R3=imhist(img3(:,:,1));
G3=imhist(img3(:,:,2));
B3=imhist(img3(:,:,3));
[pk31, locs31] = max(R3);
[pk32, locs32] = max(G3);
[pk33, locs33] = max(B3);
str31 = strcat('\leftarrow', num2str(locs31));
str32 = strcat('\leftarrow', num2str(locs32));
str33 = strcat('\leftarrow', num2str(locs33));
figure
hold on, 
plot(R3,'r'), 
text(locs31, pk31, str31),
plot(G3,'g')
text(locs32, pk32, str32),
plot(B3,'b'), 
text(locs33, pk33, str33),
legend(' Red channel','Green channel','Blue channel','Location','best');
hold off,title('Mask 2 - RGB Histograms');
xlim([0 20])
ylim([0 5000])

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Answer 1

Benjamin Großmann 2021-4-30

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/817830-question-regarding-strange-outputs-from-histogram-plot#answer_688575

编辑：Benjamin Großmann 2021-4-30

在 MATLAB Online 中打开

If you do not provide any code, it is hard to tell what went wrong here. If I use your image, everything looks fine:

imageURL = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/602390/image.jpeg';
img = imread(imageURL);
tiledlayout(3,1)
bins = [0:255];
nexttile()
histogram(img(:,:,1), bins)
title('R')
nexttile()
histogram(img(:,:,2), bins)
title('G')
nexttile()
histogram(img(:,:,3), bins)
title('B')

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Teshan Rezel 2021-4-30

Hi @Benjamin Großmann thanks for this! The reason I used imhist instead of histogram is because I wasn't aware of how to plot histogram() outputs on one graph, which I need to do for comparative purposes. Is there a way around this?

Benjamin Großmann 2021-4-30

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Hey @Teshan Rezel,

the command "hold on" that you used, works on the axes and can also be used for histograms. It sets the axes property "NextPlot" to "add" instead of "replace". I prefer to set the property instead of "hold on"

imageURL = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/602390/image.jpeg';
img = imread(imageURL);
bins = [0:255];
fig = figure();
ax = axes('Parent', fig, 'NextPlot', 'Add');
histogram(img(:,:,1), bins, 'DisplayName', 'Red Channel')
histogram(img(:,:,2), bins, 'DisplayName', 'Green Channel')
histogram(img(:,:,3), bins, 'DisplayName', 'Blue Channel')
legend(ax, 'show')