Using imshow with subplot in for loop

Question

Richard Penrose 2020-12-16

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https://ww2.mathworks.cn/matlabcentral/answers/695190-using-imshow-with-subplot-in-for-loop

回答： Raag 2025-4-27

Dear MATLAB community,

I'm not good with figures, I could not grasp the figure window handling, axes properties etc. I have 2x2 subplot which I use first one with imshow to show current frame from video, second using imagesc to show binary image of the current frame, for 3 and 4th slots I plot the counts of area detected. My problem is at each iteration imshow kinda look like shaking, I couldn't figured out why, but its probably because I don't know how to handle figures in loops effeciently or more correct manner. My code look something like this,

for ii = startingframe:numberofframes
    Frame = read(video,ii);
    [Frame_Binary,rgb] = createMask(Frame,red_min,red_max,green_min,green_max,blue_min,blue_max);
    subplot(2,2,1)
    imshow(Frame)
    title('Original Frame')
    subplot(2,2,2)
    imagesc(Frame_Binary)
    % Detecting Particles
    %
    %
    title('Detected Particles')
    subplot(2,2,[3 4])
    plot(k/video_fps,particles(k),'-r')
    axis([0 inf 0 inf  ])
    xlim([0 NumberOfFrames/video_fps])
    title('Particle Counts')
    xlabel('Seconds')
    ylabel('Particle Count')
end

Could you give me suggestions about how can I solve this issue and maybe other suggestions on how can I improve the implementation?

Thanks in advance!

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Answer 1

Raag 2025-4-27

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Hi Richard,

The "shaking" or flickering observed when displaying video frames and plots in a MATLAB loop can happen because each call to ‘subplot’ , ‘imshow’ , or ‘imagesc’ inside the loop can recreate or resize axes, causing the display to jump or flicker.

To address this, create your figure and axes just once before the loop and then update only the image and plot data inside the loop. This approach keeps the figure layout stable and makes updates much smoother.

You can use the sample code below to organize your code for flicker-free animation.

% Example parameters 
NumberOfFrames = 100; 
video_fps = 10; 
FrameSize = [240, 320, 3]; % Example frame size 
Frame_Binary_Size = [240, 320]; 
 
% Simulate particle counts and frames for demonstration 
particles = randi([0 50], NumberOfFrames, 1); 
videoFrames = uint8(randi([0 255], [FrameSize, NumberOfFrames])); 
binaryFrames = randi([0 1], [Frame_Binary_Size, NumberOfFrames]); 
 
% Create figure and axes once 
figure; 
ax1 = subplot(2,2,1); 
hFrame = imshow(videoFrames(:,:,:,1), 'Parent', ax1); 
title('Original Frame') 
 
ax2 = subplot(2,2,2); 
hBinary = imagesc(binaryFrames(:,:,1), 'Parent', ax2); 
axis(ax2, 'image'); 
title('Detected Particles') 
 
ax3 = subplot(2,2,[3 4]); 
hPlot = plot(ax3, nan, nan, '-r'); 
axis([0 NumberOfFrames/video_fps 0 max(particles)+5]) 
title('Particle Counts') 
xlabel('Seconds') 
ylabel('Particle Count') 
 
for ii = 1:NumberOfFrames 
    % Update images and plot data 
    set(hFrame, 'CData', videoFrames(:,:,:,ii)); 
    set(hBinary, 'CData', binaryFrames(:,:,ii)); 
    set(hPlot, 'XData', (1:ii)/video_fps, 'YData', particles(1:ii)); 
    drawnow limitrate 
end 