本節主要講解一下如何使用MATLAB實現視訊轉換成幀圖片，以及幀圖片如何連線成視訊。

• MATLAB將視訊轉換成幀圖片
  我們將該過程分解成一下幾個步驟：

  • 讀取視訊，獲取視訊屬性。
  • 取得視訊的每一幀圖片，並分別儲存。

  對以上幾個需求MATLAB中都有函式可以滿足。

  • 讀取視訊資訊，使用VideoReader函式
  • 讀取每一幀圖片使用read函式，儲存圖片使用imwrite函式

  %%將視訊轉換成幀圖片
  clc;
  clear;
  %% 讀取視訊
  video_path='video_test.avi';    
  video_obj=VideoReader(video_path);   
  
  frame_number=video_obj.
   
  NumberOfFrames;
  
  %% 儲存每一幀圖片到資料夾image
  if ~exist('image','dir')
      mkdir('image');
      disp('successfully create directory image!');
  end
  
  for i=1:frame_number
      image_name=strcat('./image/im_',num2str(i),'.jpg');   
      frame=read(video_obj,i);           
      imwrite(frame,image_name,'jpg');
  end
  
  disp
   
  ('all images are written into directory image')

  下面我們就來詳細解釋一下我們在上述程式中使用的函式。

  • VideoReader
    在MATLAB命令列輸入命令help VideoReader顯示如下：

    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.mp4' with
       % user tag set to 'myreader1'.
       readerobj = VideoReader('xylophone.mp4', 'tag', 'myreader1');
    
       % Read in all video frames.
       vidFrames = read(readerobj);
    
       % 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);
    
    See also audiovideo, VideoReader/get, VideoReader/set, VideoReader/read, mmfileinfo.                
    
    
    Reference page in Help browser
       doc VideoReader

    輸入doc VideoReader能夠看到更詳細的資訊，和VideoReader的屬性和方法。
    
    這裡我調幾個比較重要的屬性講解一下。

    properties	explain
    BitsPerPixel	Bits per pixel of the video data.
    Duration	Total length of the file in seconds.
    FrameRate	Frame rate of the video in frames per second.
    Height	Height of the video frame in pixels.
    Width	Width of the video frame in pixels.
    Name	Name of the file associated with the object.
    NumberOfFrames	Total number of frames in the video stream.Some files store video at a variable frame rate, including many Windows Media Videofiles. For these files, VideoReader cannot determine the number of frames until you read the last frame. When you construct the object, VideoReader returns a warning and does not set the NumberOfFrames property.To count the number of frames in a variable frame rate file,use the read method to read the last frame of the file. For example: vidObj = VideoReader(‘varFrameRateFile.wmv’); lastFrame = read(vidObj, inf); numFrames = vidObj.NumberOfFrames;
    Path	String containing the full path to the file associated with the reader.
    Tag	String that identifies the object. Default: ”
    Type	Class name of the object: ‘VideoReader’.
    UserData	Generic field for data of any class that you want to add to the object.Default: []

• MATLAB將幀圖片轉換成視訊
  幀圖片轉換成視訊是視訊轉換成幀圖片的逆過程。

  framesPath = uigetdir('~/Desktop','please select image path');%影象序列所在路徑，同時要保證影象大小相同
  videoName = './video/Bolt.avi';%表示將要建立的視訊檔案的名字
  fps = 25; %幀率
  startFrame = 1; %從哪一幀開始
  endFrame = 490; %哪一幀結束
  
  if(exist('./video/videoName','file'))
      delete ./video/videoName.avi
  end
  
  %生成視訊的引數設定
  aviobj=VideoWriter(videoName);  %建立一個avi視訊檔案物件，開始時其為空
  aviobj.FrameRate=fps;
  
  open(aviobj);%Open file for writing video data
  %讀入圖片
  for i=startFrame:endFrame
      fileName=sprintf('%d',i);
      fileName=strcat('im_',fileName);
      frames=imread([framesPath '/' fileName,'.jpg']);
      frames=im2frame(frames);  %讀如frames變數的是圖片，這裡要將圖片轉換成幀儲存到變數frames中，這樣才能成功寫入幀到視訊物件中。
      writeVideo(aviobj,frames);
  end
  close(aviobj);% 關閉建立視訊