MatLab，如何预先分配帧来制作电影？

Question

Matlab 有以下制作 avi 格式电影的指南。我的目标是能够通过 powerpoint 播放演示文稿中的视频。

nFrames = 20;
% Preallocate movie structure.
mov(1:nFrames) = struct('cdata', [],...
                    'colormap', []);

% Create movie.
Z = peaks; surf(Z); 
axis tight
set(gca,'nextplot','replacechildren');
for k = 1:nFrames 
surf(sin(2*pi*k/20)*Z,Z)
mov(k) = getframe(gcf);
end

% Create AVI file.
movie2avi(mov, 'myPeaks.avi', 'compression', 'None');

我理解这个示例，并且我不应该将压缩加载到 PowerPoint 中。但是我不明白如何使用 struct 正确地预分配内存。

Answer 1

您可以使用avifile来创建电影，甚至可以使用更新的VideoWriter：

avifile

Z = peaks; surf(Z); 
axis tight
set(gca,'nextplot','replacechildren');

vid = avifile('myPeaks.avi', 'fps',15, 'quality',100);
for k = 1:20 
    surf(sin(2*pi*k/20)*Z,Z)
    vid = addframe(vid, getframe(gcf));
end
vid = close(vid);

winopen('myPeaks.avi')

VideoWriter

Z = peaks; surf(Z); 
axis tight
set(gca,'nextplot','replacechildren');

vid = VideoWriter('myPeaks2.avi');
vid.Quality = 100;
vid.FrameRate = 15;
open(vid);
for k = 1:20 
    surf(sin(2*pi*k/20)*Z,Z)
    writeVideo(vid, getframe(gcf));
end
close(vid);

winopen('myPeaks2.avi')

Answer 2

您不需要预先分配。只需初始化mov = []即可。另外，getframe 假定 gcf，因此您可以只使用 mov(k) = getframe()。我同意你想要一个未压缩的视频。 Matlab 附带的编解码器非常有限。如果空间很重要，您可以使用开源工具来压缩视频。

Answer 3

您不必预先分配。它曾经帮助使用 moviein 命令进行预分配，但它不再提供任何性能改进。以下是 MATLAB 的引用：

>> help moviein
moviein Initialize movie frame memory.
moviein is no longer needed as of MATLAB Release 11 (5.3).  
In previous revisions, pre-allocating a movie increased 
performance, but there is no longer a need to pre-allocate 
movies. To create a movie, use something like the 
following example:

  for j=1:n
     plot_command
     M(j) = getframe;
  end
  movie(M)

For code that is compatible with all versions of MATLAB, 
including versions before Release 11 (5.3), use:

  M = moviein(n);
  for j=1:n
     plot_command
     M(:,j) = getframe;
  end
  movie(M)

See also movie, getframe.