fast way to update image objects in Matlab figure

Question

I need to display updated images as fast as possible in a Matlab figure. Each image is processed and then displayed. However, the speed of displaying of color images is pretty slow. For example, when I run the following code

videoObj = VideoReader('sample.avi');
nFrames = videoObj.NumberOfFrames;
h = videoObj.Height;
w = videoObj.Width;
mov(1:nFrames) = struct('cdata', zeros(h, w, 3, 'uint8'), 'colormap', []);
for k = 1 : nFrames
    mov(k).cdata = read(interObj, k);
end

tic
for i=1:nFrames
    frame = mov(i).cdata;
    image(frame);
    drawnow;
end
secPerFrame = toc/nFrames

it takes secPerFrame = 0.012 seconds to update each frame. Where each frame is a 640x480 pixel RGB image. So, if I want to process a video stream at 30 frames per second, this leaves "only" 0.033 - 0.012 = 0.021 seconds for actual image processing after subtracting the overhead related to image display.

Is there a faster way to update image objects in Matlab?

Answer 1

If all of the images have same size, you can use the image command once, and then update the CData property of the axes.

videoObj = VideoReader('sample.avi');
nFrames = videoObj.NumberOfFrames;
mov(1:nFrames) = ...
    struct('cdata', zeros(vidHeight, vidWidth, 3, 'uint8'), 'colormap', []);
for k = 1 : nFrames
    mov(k).cdata = read(interObj, k);
end

tic
    image(mov(1).cdata);
    imageHandle = get(gca,'Children');
for i=1:number
    frame = mov(i).cdata;
    set(imageHandle ,'CData',frame);
    drawnow;
end
secPerFrame = toc/number

This will still be slow, since Matlab figures are not optimized for showing videos. If the application allows to do pre-processing, save it as a movie file to the disk, and call external program to show it.