readNext

This example shows how to extract and visualize a video stream, stored in an ADTF DAT file. It also shows how to write the video stream into a video file.

Download the sample video DAT file.

downloadURL  = 'https://ssd.mathworks.com/supportfiles/driving/data/sample_can_video_dat.zip';
dataFolder   = fullfile(tempdir, 'adtf-video', filesep); 
options      = weboptions('Timeout', Inf);
zipFileName  = [dataFolder, 'sample_can_video_dat.zip'];
folderExists = exist(dataFolder, 'dir');

% Create a folder in a temporary directory to save the downloaded file.
if ~folderExists  
    mkdir(dataFolder); 
    disp('Downloading sample_can_video_dat.zip (9.74 MB)...') 
    websave(zipFileName, downloadURL, options); 
    
    % Extract contents of the downloaded file.
    disp('Extracting sample_can_video_dat.zip...') 
    unzip(zipFileName, dataFolder); 
end

Downloading sample_can_video_dat.zip (9.74 MB)...

Extracting sample_can_video_dat.zip...

Create the ADTF File Reader object.

datFileName = fullfile(dataFolder,"sample_can_video.dat");
fileReader  = adtfFileReader(datFileName) 

fileReader = 
              DataFileName: "C:\Users\latriwal\AppData\Local\Temp\adtf-video\sample_can_video.dat"
       DescriptionFileName: ""
           PluginDirectory: ""
               StreamCount: 2
                StreamInfo: 

    StreamIndex    StreamName      StreamType      StartTime     EndTime      ItemCount    SubstreamInfo
    ___________    __________    ______________    _________    __________    _________    _____________

         1         {'rawcan'}    {'UNRESOLVED'}      0 sec      14.805 sec       743       {0×1 struct} 
         2         {'video' }    {'adtf/image'}      0 sec      14.799 sec       149       {0×1 struct} 

From the StreamInfo property, note that the index of the video stream is 2. Use the select function of the adtfFileReader object, to select the video stream for reading. The returned adtfStreamReader object has all the information about the selection.

streamReader = select(fileReader,2) 

streamReader = 
  adtfStreamReader with properties:

           DataFileName: "C:\Users\latriwal\AppData\Local\Temp\adtf-video\sample_can_video.dat"
    DescriptionFileName: ""
        PluginDirectory: ""
            StreamIndex: 2
             StartIndex: 1
               EndIndex: 149
     CurrentIndexOffset: 0
              StartTime: [0×0 duration]
                EndTime: [0×0 duration]
              DataCount: 149

Note that the value of CurrentIndexOffset is 0. This signifies that the next readNext call will return the first item.

Preview the first image frame from the stream.

firstFrame = readNext(streamReader);
imshow(firstFrame.Data.Item)

Before creating a video, use the reset function to start reading from the first frame. This resets the value of CurrentIndexOffset to 0.

reset(streamReader);
fprintf("CurrentIndexOffset = %d\n",streamReader.CurrentIndexOffset)

CurrentIndexOffset = 0

Create a VideoWriter object that you can use to write image frames to a video file. Specify a frame rate of 1 frame per second.

videoWriterObj = VideoWriter("example_video.avi"); 
videoWriterObj.FrameRate = 1; 
open(videoWriterObj); 

Using the streamReader object, iterate over the data items in the selection one-by-one. The hasNext function determines if there is an item left to read as we are incrementally reading the file. readNext returns the data item which is basically a structure containing the data and the associated timestamp. In every iteration, extract the image frame and write it to the video file.

while streamReader.hasNext()
    streamData = streamReader.readNext(); 
    imageFrame = streamData.Data.Item; 
    frame      = im2frame(streamData.Data.Item, gray); 
    writeVideo(videoWriterObj, frame); 
end 

Alternatively, you can read all the image frames at once, using the read function, and iterate over it later.

allData = read(streamReader)

allData = struct with fields:
    StreamIndex: 2
           Data: [149×1 struct]

Close the connection with the video file.

close(videoWriterObj);
close all

Visualize the output file example_video.avi using Video Viewer.

implay("example_video.avi")