How do I preallocate a matrix to receive structs from getframe?

How do I preallocate a matrix of type struct?

Specifically, I want to preallocate 'M', in the following code, to receive frames from 'getframe'.

    h=figure(1);
    for t=1:T+1
       ... (create a plot)...
       M(t)=getframe(h);
    end
    movie2avi(M, 'filename.avi');

The editor tells me 'M' changes size on every loop iteration, and that I should preallocate for speed (which is important here). I notice in workspace that 'M' is "<1x1001 struct>".

So I tried allocating as

M=zeros(1,T+1),

but Matlab errored on the `getframe` line because I cannot convert from double to sruct.

I also tried allocating as

M=cell(1,T+1);

but I cannot convert from cell to struct, either.

From the documentation of 'struct', it seems that I need to know the structure's fields to preallocate a structure array (is a structure array what I want, btw?). So without knowing the structure of getframe, I cannot preallocate that way, either.

How do I preallocate M, of size 1-by-T+1, to accept structures from getframe'?

Thanks, J

Answer 1

[Walter Roberson]

M = struct('cdata', cell(1,T+1), 'colormap', cell(1,T+1));

[Jeff]

Thanks, this works perfectly. But I'm not sure why. Am I right that it creates a structure with two fields, 'cdata' and 'colormap', and each field is a cell array of T+1 elements?

It seems what I need is an array of T+1 elements where each element is a cell. What am I misunderstanding?

[Walter Roberson]

Please examine the documentation http://www.mathworks.com/help/matlab/ref/struct.html for the subcase where "value" is a cell array.

Answer 2

[Image Analyst]

Good question Jeff! Look at this code snippet from my demo (available upon request) where I extract movie frames, write them to disk, and then recall the individual frames from disk and rebuild the movie:

% Read the frames back in from disk, and convert them to a movie.
% Preallocate recalledMovie, which will be an array of structures.
% First get a cell array with all the frames.
allTheFrames = cell(numberOfFrames,1);
allTheFrames(:) = {zeros(vidHeight, vidWidth, 3, 'uint8')};
% Next get a cell array with all the colormaps.
allTheColorMaps = cell(numberOfFrames,1);
allTheColorMaps(:) = {zeros(256, 3)};
% Now combine these to make the array of structures.
recalledMovie = struct('cdata', allTheFrames, 'colormap', allTheColorMaps)
for frame = 1 : numberOfFrames
  % Construct an output image file name.
  outputBaseFileName = sprintf('Frame %4.4d.png', frame);
  outputFullFileName = fullfile(outputFolder, outputBaseFileName);
  % Read the image in from disk.
  thisFrame = imread(outputFullFileName);
  % Convert the image into a "movie frame" structure.
  recalledMovie(frame) = im2frame(thisFrame);
end

[Jeff]

Thanks for the response. I could not quite get it to work, though. I don't have vidHeight, or vidWidth values. I skipped that line and was told "Frame must be 560 by 420".

I'd like to take a look at your demo. How do I request it?

[Image Analyst]

Jeff, that's simply the number of rows and columns in your image - in your case evidently it's 560 and 420, or vice versa. Here's the full demo:

% Demo macro to extract frames and get frame means from an avi movie
% and save individual frames to separate image files.
% Also computes the mean gray value of the color channels.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures.
clear;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing.
fontSize = 14;

% Change the current folder to the folder of this m-file.
% (The line of code below is from Brett Shoelson of The Mathworks.)
if(~isdeployed)
  cd(fileparts(which(mfilename)));
end

% Open the rhino.avi demo movie that ships with MATLAB.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
movieFullFileName = fullfile(folder, 'rhinos.avi');
% Check to see that it exists.
if ~exist(movieFullFileName, 'file')
  strErrorMessage = sprintf('File not found:\n%s\nYou can choose a new one, or cancel', movieFullFileName);
  response = questdlg(strErrorMessage, 'File not found', 'OK - choose a new movie.', 'Cancel', 'OK - choose a new movie.');
  if strcmpi(response, 'OK - choose a new movie.')
    [baseFileName, folderName, FilterIndex] = uigetfile('*.avi');
    if ~isequal(baseFileName, 0)
      movieFullFileName = fullfile(folderName, baseFileName);
    else
      return;
    end
  else
    return;
  end
end

try
  videoObject = VideoReader(movieFullFileName)
  % Determine how many frames there are.
  numberOfFrames = videoObject.NumberOfFrames;
  vidHeight = videoObject.Height;
  vidWidth = videoObject.Width;
  
  numberOfFramesWritten = 0;
  % Prepare a figure to show the images in the upper half of the screen.
  figure;
  %   screenSize = get(0, 'ScreenSize');
  % Enlarge figure to full screen.
  set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
  
  % Ask user if they want to write the individual frames out to disk.
  promptMessage = sprintf('Do you want to save the individual frames out to individual disk files?');
  button = questdlg(promptMessage, 'Save individual frames?', 'Yes', 'No', 'Yes');
  if strcmp(button, 'Yes')
    writeToDisk = true;
    
    % Extract out the various parts of the filename.
    [folder, baseFileName, extentions] = fileparts(movieFullFileName);
    % Make up a special new output subfolder for all the separate
    % movie frames that we're going to extract and save to disk.
    % (Don't worry - windows can handle forward slashes in the folder name.)
    folder = pwd;   % Make it a subfolder of the folder where this m-file lives.
    outputFolder = sprintf('%s/Movie Frames from %s', folder, baseFileName);
    % Create the folder if it doesn't exist already.
    if ~exist(outputFolder, 'dir')
      mkdir(outputFolder);
    end
  else
    writeToDisk = false;
  end
  
  % Loop through the movie, writing all frames out.
  % Each frame will be in a separate file with unique name.
  meanGrayLevels = zeros(numberOfFrames, 1);
  meanRedLevels = zeros(numberOfFrames, 1);
  meanGreenLevels = zeros(numberOfFrames, 1);
  meanBlueLevels = zeros(numberOfFrames, 1);
  for frame = 1 : numberOfFrames
    % Extract the frame from the movie structure.
    thisFrame = read(videoObject, frame);
    
    % Display it
    hImage = subplot(2, 2, 1);
    image(thisFrame);
    caption = sprintf('Frame %4d of %d.', frame, numberOfFrames);
    title(caption, 'FontSize', fontSize);
    drawnow; % Force it to refresh the window.
    
    % Write the image array to the output file, if requested.
    if writeToDisk
      % Construct an output image file name.
      outputBaseFileName = sprintf('Frame %4.4d.png', frame);
      outputFullFileName = fullfile(outputFolder, outputBaseFileName);
      
      % Stamp the name and frame number onto the image.
      % At this point it's just going into the overlay,
      % not actually getting written into the pixel values.
      text(5, 15, outputBaseFileName, 'FontSize', 20);
      
      % Extract the image with the text "burned into" it.
      frameWithText = getframe(gca);
      % frameWithText.cdata is the image with the text
      % actually written into the pixel values.
      % Write it out to disk.
      imwrite(frameWithText.cdata, outputFullFileName, 'png');
    end
    
    % Calculate the mean gray level.
    grayImage = rgb2gray(thisFrame);
    meanGrayLevels(frame) = mean(grayImage(:));
    
    % Calculate the mean R, G, and B levels.
    meanRedLevels(frame) = mean(mean(thisFrame(:, :, 1)));
    meanGreenLevels(frame) = mean(mean(thisFrame(:, :, 2)));
    meanBlueLevels(frame) = mean(mean(thisFrame(:, :, 3)));
    
    % Plot the mean gray levels.
    hPlot = subplot(2, 2, 2);
    hold off;
    plot(meanGrayLevels, 'k-', 'LineWidth', 2);
    hold on;
    plot(meanRedLevels, 'r-');
    plot(meanGreenLevels, 'g-');
    plot(meanBlueLevels, 'b-');
    grid on;
    
    % Put title back because plot() erases the existing title.
    title('Mean Gray Levels', 'FontSize', fontSize);
    if frame == 1
      xlabel('Frame Number');
      ylabel('Gray Level');
      % Get size data later for preallocation if we read
      % the movie back in from disk.
      [rows columns numberOfColorChannels] = size(thisFrame);
    end
    
    % Update user with the progress.  Display in the command window.
    if writeToDisk
      progressIndication = sprintf('Wrote frame %4d of %d.', frame, numberOfFrames);
    else
      progressIndication = sprintf('Processed frame %4d of %d.', frame, numberOfFrames);
    end
    disp(progressIndication);
    % Increment frame count (should eventually = numberOfFrames
    % unless an error happens).
    numberOfFramesWritten = numberOfFramesWritten + 1;
    
    % Now let's do the differencing
    alpha = 0.5;
    if frame == 1
      Background = thisFrame;
    else
      % Change background slightly at each frame
      %       Background(t+1)=(1-alpha)*I+alpha*Background
      Background = (1-alpha)* thisFrame + alpha * Background;
    end
    % Display the changing/adapting background.
    subplot(2, 2, 3);
    imshow(Background);
    title('Adaptive Background', 'FontSize', fontSize);
    % Calculate a difference between this frame and the background.
    differenceImage = thisFrame - uint8(Background);
    % Threshold with Otsu method.
    grayImage = rgb2gray(differenceImage); % Convert to gray level
    thresholdLevel = graythresh(grayImage); % Get threshold.
    binaryImage = im2bw( grayImage, thresholdLevel); % Do the binarization
    % Plot the binary image.
    subplot(2, 2, 4);
    imshow(binaryImage);
    title('Binarized Difference Image', 'FontSize', fontSize);
  end
  
  % Alert user that we're done.
  if writeToDisk
    finishedMessage = sprintf('Done!  It wrote %d frames to folder\n"%s"', numberOfFramesWritten, outputFolder);
  else
    finishedMessage = sprintf('Done!  It processed %d frames of\n"%s"', numberOfFramesWritten, movieFullFileName);
  end
  disp(finishedMessage); % Write to command window.
  uiwait(msgbox(finishedMessage)); % Also pop up a message box.
  
  % Exit if they didn't write any individual frames out to disk.
  if ~writeToDisk
    return;
  end
  
  % Ask user if they want to read the individual frames from the disk,
  % that they just wrote out, back into a movie and display it.
  promptMessage = sprintf('Do you want to recall the individual frames\nback from disk into a movie?\n(This will take several seconds.)');
  button = questdlg(promptMessage, 'Recall Movie?', 'Yes', 'No', 'Yes');
  if strcmp(button, 'No')
    return;
  end
  
  % Read the frames back in from disk, and convert them to a movie.
  % Preallocate recalledMovie, which will be an array of structures.
  % First get a cell array with all the frames.
  allTheFrames = cell(numberOfFrames,1);
  allTheFrames(:) = {zeros(vidHeight, vidWidth, 3, 'uint8')};
  % Next get a cell array with all the colormaps.
  allTheColorMaps = cell(numberOfFrames,1);
  allTheColorMaps(:) = {zeros(256, 3)};
  % Now combine these to make the array of structures.
  recalledMovie = struct('cdata', allTheFrames, 'colormap', allTheColorMaps)
  for frame = 1 : numberOfFrames
    % Construct an output image file name.
    outputBaseFileName = sprintf('Frame %4.4d.png', frame);
    outputFullFileName = fullfile(outputFolder, outputBaseFileName);
    % Read the image in from disk.
    thisFrame = imread(outputFullFileName);
    % Convert the image into a "movie frame" structure.
    recalledMovie(frame) = im2frame(thisFrame);
  end
  % Get rid of old image and plot.
  delete(hImage);
  delete(hPlot);
  % Create new axes for our movie.
  subplot(1, 3, 2);
  axis off;  % Turn off axes numbers.
  title('Movie recalled from disk', 'FontSize', fontSize);
  % Play the movie in the axes.
  movie(recalledMovie);
  % Note: if you want to display graphics or text in the overlay
  % as the movie plays back then you need to do it like I did at first
  % (at the top of this file where you extract and imshow a frame at a time.)
  msgbox('Done with this demo!');
  
catch ME
  % Some error happened if you get here.
  stError = lasterror;
  strErrorMessage = sprintf('Error extracting movie frames from:\n\n%s\n\nError: %s\n\n)', movieFullFileName, stError.message);
  uiwait(msgbox(strErrorMessage));
end