Vectorizing nested for loops

Hi,

I am having a few issuing trying to vectorise multiple nested for loops. This application is video and i am looping through each pixel in each line of each frame of all the frames.

for l=1:nFrames
currentVFrame = read(liveVid, l); % Read in next frame
alphaVFrame = zeros(vidHeight, vidWidth, 3, 'uint8'); % create and then reset to 0
for k = 1:3
    for i=1:vidHeight
        for j=1:vidWidth
          if(currentVFrame(i,j,k) == 254)   % 254 = green screen
             alphaVFrame(i,j,k) = 000;      % Set to transparent instead
          else
              alphaVFrame(i,j,k) = currentVFrame(i,j, k); % Keep original file info
          end
        end
    end
end
%imshow(alphaVFrame); % Show a preview of trhe current frame
mov(l).cdata = im2uint8(alphaVFrame); % Write the frame to our mov structure (int)
writeVideo(OutputVideo, alphaVFrame); % write the frame to the video file (ext)
end

I have tried to vectorise it following the example written on this post:

[i,j] = ndgrid(1:vidHeight*vidWidth);
r = reshape(a(i)+a(j), [nFrames, 3, VidHeight, VidWidth]);

But I get an error:

Maximum variable size allowed by the program is exceeded.

Error in ndgrid (line 63)
    varargout{1} = x(:,ones(size(y)));

Can anyone point me in the right direction? I'm using Matlab R2013a.

Answer 1

[Guillaume]

The post you linked does not apply to what you're doing. The simplest thing is to use matrix operations on your frames instead of your three inner loops:

for frame = 1:nframes %why use a meaningless letter instead of a meaningful name?
   currentVFrame = read(liveVid, frame);
   alphaVFrame = currentVFrame;         %copy frame to output
   alphaVFrame(alphaVFrame == 254) = 0; %and replace green screen with transparent?
   %imshow(alphaVFrame); % Show a preview of trhe current frame
   mov(frame).cdata = im2uint8(alphaVFrame); % Write the frame to our mov structure (int)
   writeVideo(OutputVideo, alphaVFrame); % write the frame to the video file (ext)   
end

I'm puzzled by your comment though %254 = green screen, this is only true on the green channel, ( k==2) but you're doing your test on all three colour channels.

[Andrew]

That's great thanks for your quick response Guillaume.

In response to your question, you are right. I do not need to check all three channels when I am only looking at the green content, so in my code above you could just check the green channel, as you rightly said, when k==2.

nFrames is short for Number of Frames, apologies if that was confusing.

[Guillaume]

The code you've posted and my replacement will also set red or blue to 0 when they are 254. If you just want to set pixels to 0 when just green is 254, then:

%to set all three channels to 0:
alphaVFrame(repmat(alphaVFrame(:, :, 2) == 254, [1 1 3])) = 0;

%to just set green to 0:
green = alphaVFrame(:, :, 2);
green(green == 254) = 0;
alphaVFrame(:, :, 2) = green;

My comment about the variable name was about the l which does not mean anything. frame is a much better name. nframes is fine as a variable name.

[Andrew]

Let me just rephrase what you've said to ensure I understand.

To set all the colours (RGB) to 0 when green (and only green) is equal to 254 I would use:

%to set all three channels to 0:
alphaVFrame(repmat(alphaVFrame(:, :, 2) == 254, [1 1 3])) = 0;

To set only the colour that is equal to 254 to 0 (i.e. if blue is equal to 254 then set to 0, but leave green and red alone) I would use:

alphaVFrame(alphaVFrame == 254) = 0;

And finally, to check only green, and set only green to 0 when it's equal to 254 I would use your final snippet of code:

%to just set green to 0:
green = alphaVFrame(:, :, 2);
green(green == 254) = 0;
alphaVFrame(:, :, 2) = green;

[Guillaume]

Yes, exactly.

[Andrew]

Fantastic, thank you for your help Guillaume.