drawnow() causing very slow display of images

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Karl 2014-2-18

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/116543-drawnow-causing-very-slow-display-of-images

评论： Yu Liu 2020-5-13

采纳的回答： Image Analyst

在 MATLAB Online 中打开

Hi all,

I've read several other questions on this but didn't find a satisfactory answer. My apologies however, if this is already up.

I am building a GUI to display processed video. I do this using

imshow(image,'Parent',axes)

then

drawnow

for each frame in a loop. It is VERY slow. Approx 0.5 seconds per frame (~12 seconds to display 1 second of video in real time). It also accounts for half my total processing time per frame.

Can I make it faster?

Any help is greatly appreciated!!

Cheers, KB

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Mariusz Wilk 2018-9-9

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You can use:

drawnow limitrate

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采纳的回答

Image Analyst 2014-2-18

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/116543-drawnow-causing-very-slow-display-of-images#answer_124827

I solved that problem with older versions of MATLAB by putting in a "cla" or "cla('reset')" into the code just before the imshow. Evidently with older versions sometimes the images "stack up" and are all in there and you're just piling one on top of another and it gets slower and slower as it runs out of memory. Try that and let me know if it speeds it up.

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Karl 2014-2-20

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Updating CData was the answer I needed (I just had to implement it poperly). MY GUI is now running much faster. Example code pasted below, maybe it will help someone else.

Thanks very much Image Analyst :)

Updating CData vs imshow() example code:

%%Setup video
filename = 'your_video_here.mp4';
vidObj = VideoReader(filename);
%%Display first frame and setup handle to image
frame = read(vidObj, 1);
figure;
h = imshow(frame);
%%Loop through subsequent frames and refresh 'CData'
for k = 1:1000
    tic
    frame = read(vidObj, k);
    set(h,'Cdata',frame);
    drawnow
    toc
end
for k = 1:1000
    tic
    frame = read(vidObj, k);
    imshow(frame)
    drawnow
    toc
end