Create a video with imagesc (VideoWriter)

Hello,

I've got a program that looks pretty much like that, and I would like to create a video resulting of all the plots created during the k iteration:

for k=1:10000
    
   .%calculations
   .%calculations
   W=reshape(w,y,x); %Where W is a Matrix
   lsd=imagesc(W);
   drawnow
end

Any clue? Thank you

Answer 1

[Image Analyst]

See my attached demo which creates a surf plot (you could easily adapt it to display an image), then builds a video frame by frame.

[io mb]

Thanks it worked!

The only thing now is that I wish to record several hours of plot, but the 29s of film I get already weight 346Mo.

(I used :

writerObj = VideoWriter('Film.avi','Uncompressed AVI');

)

Is there a way to decrease the size WITHOUT having a really bad film quality ?(I tried without 'Uncompressed AVI' already, but way too pixelized)

[io mb]

actually I just tried this:

numberOfFrames=10000;
allTheFrames = cell(numberOfFrames,1);
vidHeight = 10000;
vidWidth = 10000;
allTheFrames(:) = {zeros(vidHeight, vidWidth, 3, 'uint8')};
allTheColorMaps = cell(numberOfFrames,1);
allTheColorMaps(:) = {zeros(256, 3)};
writerObj = VideoWriter('Film.avi');

open(writerObj);
set(gcf, 'renderer', 'zbuffer');

 y=300;
 x=1000;
 
for k=1:numberOfFrames
    %Calculation
    
   W=reshape(w,y,x);
   lsd=imagesc(W);
   drawnow

thisFrame = getframe(gca);
writeVideo(writerObj, thisFrame);
myMovie(k) = thisFrame;

end
close(writerObj)

but I got the following error message: "Frame must be 344 by 343"

Can you help me with that?

[Walter Roberson]

Remember to pass in 'FrameRate' when you call VideoWriter. The default is 30 fps, and if your data is intended to be faster than that then the resulting movie would be longer duration than you expected.

VideoWriter does not do well with compression. I would recommend post-processing with a transcoder.

getframe() has a variable sized output unless you specify the rectangle at the time you getframe(). Axes vary slightly in size depending on the data, so getframe() will record varying sizes of data.

If you had used the mat2gray() that I suggested then that would not be an issue.

[Image Analyst]

The size of thisFrame seems like it changed. During this lengthy processing time, did you perhaps move or resize the window?

Answer 2

[Walter Roberson]

imagesc(W) displays the given data, W, after changing the color scaling to be from min(W(:)) to max(W(:)) so that the minimum value in W gets mapped to the first color in the color map and the maximum value in W gets mapped to the last color in the color map.

imagesc(W) does not itself put up any color map: whatever color map was in place would be used.

The rescaling that imagesc does from min to max is the same as what you would get if you used

Wscaled = mat2gray(W);

mat2gray() is a pretty obscure name for this functionality, but as of R2017b you can instead use

Wscaled = rescale(W);

Either way, Wscaled would be values in the range 0 to 1.

You can then im2uint8() to get values in the range 0 to 255, and you can then ind2rgb() passing in a color map in order to get out an RGB image that represents the data. After that you can write the RGB as a video frame by using the functionality of videowriter()

[io mb]

Thanks for your answer, unfortunately I only have a student license wich doesn't contain im2uint8() fonction. Is there any way around?

[Walter Roberson]

im2uint8() is part of the Image Processing Toolbox. That is included in the Student Suite, which you probably have.

In any case, for this purpose, you could probably use

ind2rgb( uint8(Wscaled * (256*(1-eps))), YourColormapGoesHere )

[io mb]

Thank you Walter, I rather use your way as it looks shorter to me in terms of code lines. I'm just a beginner with matlab so not everthing you said is innate to me. I think I could really use some help with the code writing itself.

That's what I've got so far following your lead:

video = VideoWriter('newfile.avi', 'Uncompressed AVI');
video.FrameRate = 60;
open(video)

  y=300;
 x=1000;
 
for k=1:10000
    
   .%calculations
   .%calculations
   W=reshape(w,y,x); %Where W is a Matrix
   Wscaled = rescale(W);
   RGB=ind2rgb( uint8(Wscaled * (256*(1-eps))), parula );
   drawnow
   writeVideo(video,RGB)
end
close(video)

Doesn't work well, but I'm a bit lost, I don't know if I used the commands properly or how to modify them...

[Walter Roberson]

By the way I should probably have had a floor() call inside the uint8() call

uint8( floor(Wscaled * (256*(1-eps))) )

[Walter Roberson]

We do not have your full code, and you did not show any error message, so it is difficult for us to guess what "Doesn't work well" means.

Answer 3

[io mb]

I found a way to record W, letting imagesc() in the code :

video = VideoWriter('newfile.avi', 'Uncompressed AVI');

open(video)

for k=1:10000
    
   .%calculations
   .%calculations
   W=reshape(w,y,x); %Where W is a Matrix
   lsd=imagesc(W);
   drawnow
   F(k) = getframe(gcf);

   writeVideo(video,F)
end
close(video)

The problem now is, that for 3 minutes realtime recording I get a 11 minutes video wich loop from k=0 to the new k (it gets every time further).

[lubna Albarghouty]

that's because you're writing the entire 'F' at each iteration.

i.e. should write the following line outside (rather than inside) the 'for' loop :

writeVideo(video,F)