利用MATLAB将视频的每一帧保存成一幅图像，并自动命名。本文方法简单，容易学习。
首先，读入视频。代码如下：
mov = VideoReader('xxxxxx.avi'); % 将xxxxxx.avi读入MATLAB中，并用名为mov的结构体保存
fnum = mov.NumberOfFrames; % 获取视频帧数
接下来，我们要写一个循环来将视频的每一帧保存成.png格式的图片。代码如下：
% 将第i帧写入到xxx文件夹内"img_000x.png"图片中
imgOrder=0; % 图片按顺序编号
for i = 1:3:fnum % i从1到fnum, step=3
Img=read(mov,i); % 读取第i帧；每次读取一帧可防止内存不足
imwrite(Img,['xxx/img_',sprintf('%04d',imgOrder),'.png']) % 将第i帧写入到xxx文件夹内"img_000j.png"图片, j=imgOrder
imgOrder=imgOrder+1;
end
程序运行结束后，会在xxx文件夹中生成fnum/step张图片，每一张图片对应视频中相应的一帧。
如果需要做一些预处理，如选取感兴趣区域ROI、下采样等，可以通过修改倒数第二句话实现：
imwrite(Img,['xxx/img_',sprintf('%04d',imgOrder),'.png']); % 假设每一帧的原始大小为：1920*1080(宽*高)
--> imwrite(Img(51:950,151:1550,:),['xxx/img_',sprintf('%04d',imgOrder),'.png']); % 选取ROI，保存的图像大小为：1400*900
--> imwrite(Img(51:2:950,151:2:1550,:),['xxx/img_',sprintf('%04d',imgOrder),'.png']); % 选取ROI并下采样，保存的图像大小为：700*450
相关文档：
help VideoReader
VIDEOREADER Create a multimedia reader object.
OBJ = VIDEOREADER(FILENAME) constructs a multimedia reader object, OBJ, that can read in video data from a multimedia file. FILENAME is a string specifying the name of a multimedia file. There are no restrictions on file extensions. By default, MATLAB looks for the file FILENAME on the MATLAB path.
If the object cannot be constructed for any reason (for example, if the file cannot be opened or does not exist, or if the file format is not recognized or supported), then MATLAB throws an error.
OBJ = VIDEOREADER(FILENAME, 'P1', V1, 'P2', V2, ...) constructs a multimedia reader object, assigning values V1, V2, etc. to the specified properties P1, P2, etc.
If an invalid property name or property value is specified, MATLAB throws an error and the object is not created. Note that the property value pairs can be in any format supported by the SET function, e.g. parameter-value string pairs, structures, or parameter-value cell array pairs.
Example:
% Construct a multimedia reader object associated with file 'xylophone.mpg' with
% user tag set to 'myreader1'.
readerobj = VideoReader('xylophone.mpg', 'tag', 'myreader1');
% Read in all video frames.
vidFrames = read(readerobj); % 此句会消耗大量内存，易导致计算机内存不足
% Get the number of frames.
numFrames = get(readerobj, 'numberOfFrames');
% Create a MATLAB movie struct from the video frames.
for k = 1 : numFrames
mov(k).cdata = vidFrames(:,:,:,k);
mov(k).colormap = [];
end
% Create a figure
hf = figure;
% Resize figure based on the video's width and height
set(hf, 'position', [150 150 readerobj.Width readerobj.Height])
% Playback movie once at the video's frame rate
movie(hf, mov, 1, readerobj.FrameRate);
doc VideoReader
Use the VideoReader function with the read method to read video data from a file into the MATLAB workspace.
The file formats that VideoReader supports vary by platform, as follows (with no restrictions on file extensions):
All Platforms:
Motion JPEG 2000 (.mj2)
Windows:
AVI (.avi),
MPEG-1 (.mpg),
Windows Media Video (.wmv, .asf, .asx),
and any format supported by Microsoft DirectShow.
Macintosh:
AVI (.avi),
MPEG-1 (.mpg),
MPEG-4 (.mp4, .m4v),
Apple QuickTime Movie (.mov),
and any format supported by QuickTime as listed on http://www.apple.com/quicktime/player/specs.html.
Linux:
Any format supported by your installed plug-ins for GStreamer 0.10 or above, as listed on http://gstreamer.freedesktop.org/documentation/plugins.html, including AVI (.avi) and Ogg Theora (.ogg).
For more information, see Supported Video File Formats in the MATLAB Data Import and Export documentation
参考文献：
[1] http://jingyan.baidu.com/article/642c9d34e520d9644a46f7b7.html
[2] matlab帮助文档