How to select specific frames (ex. every 10th frame) out of a video for further analysis?

I have an .avi video file with ca. 300,000 frames (7 FPS) and I would like to analyze every nth frame (for example, frame 1, 10, 20, 30, 40, etc.).

I have successfully uploaded the file in clusters of 1000 frames and the analysis (image tracking) works well if I include every video frame. But I cannot seem to get MatLab to loop through every nth frame.

I tried something like this:

FishVid = VideoReader('MP02a.avi')

VidInfo=get(FishVid);
     numberOfFrames = FishVid.NumberOfFrames
     vidHeight = FishVid.Height;
     vidWidth = FishVid.Width;

   i= 1;

   while i<= numberOfFrames
   currentFrame= read(VidInfo,i); 
   combinedString=strcat(int2str(i-1),'.jpg');
   imwrite(i,combinedString);
   i=i+700;
   end

BUT, this 1. saves each frame but doesn't concatenate them for further analysis and 2. in its current form, just gives me blank .jpg images.

Any suggestions on how to do this? Thanks!

Answer 1

[Geoff Hayes]

Rebecca - look closely at the line where you write the image to file

 imwrite(i,combinedString);

The code is writing out the integer i to the file named by combinedString whereas you want to write the frame to file instead

 imwrite(currentFrame,combinedString);

Do this and you should have non-blank images.

If you want to concatenate all images together, then you can continue as you are doing above and just save each frame, the currentFrame variable, to a matrix or cell array.Something like

 % determine which frames are going to be read
 framesToRead = 1:700:numberOfFrames;

 % initialize a matrix for all frames to be read
 allFrames = zeros(vidHeight, vidWidth, 3, length(framesToRead));

 % read in the frames
 for k=1:length(framesToRead)

   frameIdx = framesToRead(k);

   currentFrame   = read(FishVid,frameIdx);
   combinedString = sprintf('%d.jpg',k-1);
   imwrite(currentFrame,combinedString);

   if k==1
       % cast the all frames matrix appropriately
       allFrames = cast(allFrames, class(currentFrame));
   end

   allFrames(:,:,:,k) = currentFrame;
 end

Note how we cast allFrames to be of the same datatype as the video frames (this only occurs the once, for k identical to one).

Try the above and see what happens!

[Rebecca Nagel]

I see my problem - quite simple actually. Thank you very much!