Why plotting colocrbar slows down the performance of MATLAB

Question

Ahmad Gad 2020-12-22

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评论： Ahmad Gad 2021-1-4

Hello everyone. I have a question that I am not able to solve.

I have a cell array 'Temp' that defined the frames of a thermal video. Each cell defines an intensity image of the video. When I plot them using this:

tic
for i = 1:size(Temp,1)
    imagesc(Temp{i});
    drawnow;
end
toc
Elapsed time is 16.539161 seconds.

When I try to include the colorbar to get some unmerical idea about the thermal images, using the following code:

tic
for i = 1:size(Temp,1)
    imagesc(Temp{i});
    colorbar;           % I just asked for the colorbar to be added
    drawnow;
end
toc
Elapsed time is 79.288858 seconds.

Why this big change in the performance of the code when I add the colorbar? Can anyone explain or tell a workaround?

Thanks all and best,

Ahmad

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Subhadeep Koley 2020-12-22

@Ahmad Gad You can call the colorbar function only once outside the for loop. No need call it iteratively.

Although, I'm not sure about your application, but by calling imagesc(Temp{i}) iteratively, you will only see the last frame of your video, as for every iteration the previous imagesc plot will be overwritten with the current one.

Ahmad Gad 2020-12-22

Thanks Subhadeep for the reply.

calling it before the loop doesn't work, due to the reason you mentioned. Do you suggest another function other than imagesc(Temp{i})?

I may have to change this to get things to work fine.

Thanks!

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Answer 1

Walter Roberson 2020-12-22

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https://ww2.mathworks.cn/matlabcentral/answers/699835-why-plotting-colocrbar-slows-down-the-performance-of-matlab#answer_581485

for K = 1 : 60; Temp{K} = rand(320,240); end

tic

fig = figure();

ax = axes(fig);

for i = 1:size(Temp,1)

imagesc(ax, Temp{i});

drawnow;

end

toc

Elapsed time is 0.266523 seconds.

tic

fig = figure();

ax = axes(fig);

for i = 1:size(Temp,1)

imagesc(ax, Temp{i});

colorbar(ax); % I just asked for the colorbar to be added

drawnow;

end

toc

Elapsed time is 0.581383 seconds.

Yup, colorbar added was a lot slower.

tic

fig = figure();

ax = axes(fig);

colorbar(ax); % I just asked for the colorbar to be added

hold(ax, 'on');

h = imagesc(ax, Temp{1});

for i = 2:size(Temp,1)

h.CData = Temp{i};

drawnow;

end

hold(ax, 'off')

toc

Elapsed time is 0.141660 seconds.

But being careful about using handles is faster.

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Ahmad Gad 2021-1-4

Walter, thanks a lot for the reply.

Can you modify this line,

for i = 2:size(Temp,1)

to be,

for i = 2:size(Temp,2)

So, the code can work easily for new readers?

Thanks!

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