VideoReader - Create object to read video files - MATLAB (original) (raw)

Create object to read video files

Description

Use a VideoReader object to read files containing video data. The object contains information about the video file and enables you to read data from the video. You can create a VideoReader object using theVideoReader function, query information about the video using the object properties, and then read the video using object functions.

For more information, see Supported Video and Audio File Formats.

Creation

Syntax

Description

`v` = VideoReader([filename](#d126e1981145)) creates object v to read video data from the file namedfilename.

example

`v` = VideoReader([filename](#d126e1981145),`Name,Value`) sets the properties CurrentTime, Tag, andUserData using name-value arguments. For example,VideoReader('myfile.mp4','CurrentTime',1.2) starts reading 1.2 seconds into the video. You can specify multiple name-value arguments. Enclose each property name in single quotes followed by the corresponding value.

example

Input Arguments

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File name, specified as a character vector or string scalar.

Depending on the location of your file, filename can take one of these forms.

For more information, see Supported Video and Audio File Formats.

Data Types: char | string

Properties

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The VideoReader object has properties that contain information about the video file. Properties are read-only, exceptCurrentTime, Tag, andUserData. You can view or modify the value of a property after creating the object. For example, this command finds the value of theDuration property of the VideoReader object,v.

This property is read-only.

Bits per pixel of the video data, specified as a numeric scalar.

Data Types: double

Timestamp of the video frame to read, specified as a numeric scalar. The timestamp is specified in seconds from the start of the video file. The value of CurrentTime can be between zero and the duration of the video.

On some platforms, when you create a VideoReader object, the 'CurrentTime' property might contain a value close to, but not exactly, zero. This variation in the value of the'CurrentTime' property is due to differences in how each platform processes and reads videos.

Example: 5.6

Data Types: double

This property is read-only.

Length of the file in seconds, specified as a numeric scalar.

Data Types: double

This property is read-only.

Number of video frames per second, specified as a numeric scalar. For variable-frame rate video, FrameRate is the average frame rate.

Note: For OS X Yosemite (Version 10.10) and later, MPEG-4/H.264 files written using VideoWriter play correctly, but display an inexact frame rate.

Data Types: double

This property is read-only.

Height of the video frame in pixels, specified as a numeric scalar.

Data Types: double

This property is read-only.

File name, specified as a character vector or string scalar.

Data Types: char | string

This property is read-only.

Number of frames in the video stream, specified as a numeric scalar.

Note

For certain length videos, the value of theNumFrames property is not immediately available. To get the NumFrames property, typev.NumFrames in the command line.

Data Types: double

This property is read-only.

Full path to the video file associated with the reader object, specified as a character vector or string scalar.

Data Types: char | string

Generic text, specified as a character vector or string scalar.

Example: 'Experiment 109'

Data Types: char | string

User-defined data, specified as a value of any data type.

This property is read-only.

MATLAB representation of the video format, specified as a character vector or string scalar.

File types, except for Motion JPEG 2000 files, have one of theseVideoFormat values.

Motion JPEG 2000 files, have one of the followingVideoFormat values.

Data Types: char | string

This property is read-only.

Width of the video frame in pixels, specified as a numeric scalar.

Data Types: double

Object Functions

Examples

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Create a VideoReader object for the sample video file xylophone_video.mp4.

v = VideoReader("xylophone_video.mp4");

Read all the frames from the video, one frame at a time.

while hasFrame(v) frame = readFrame(v); end

Display information about the last frame returned by readFrame.

Name Size Bytes Class Attributes

frame 240x320x3 230400 uint8

Clear the VideoReader object.

Create a VideoReader object for the sample video file xylophone_video.mp4.

v = VideoReader("xylophone_video.mp4");

Specify to start reading frames at 2.5 seconds from the beginning of the video.

Create an axes object to display the frame. Then continue to read and display video frames until no more frames are available to read.

currAxes = axes; while hasFrame(v) vidFrame = readFrame(v); image(vidFrame,"Parent",currAxes) currAxes.Visible = "off"; pause(1/v.FrameRate) end

Clear the VideoReader object.

Create a VideoReader object for the sample video file xylophone_video.mp4.

v = VideoReader("xylophone_video.mp4");

Read only the first video frame.

Read only the last video frame.

Read frames 5 through 10.

earlyFrames = read(v,[5 10]);

Read from the 50th frame to the end of the video file.

lateFrames = read(v,[50 Inf]);

Display the size and type information of the video frame variables.

Name Size Bytes Class Attributes

earlyFrames 240x320x3x6 1382400 uint8
firstFrame 240x320x3 230400 uint8
lastFrame 240x320x3 230400 uint8
lateFrames 240x320x3x92 21196800 uint8

Clear the VideoReader object.

Read a frame from a video by specifying a frame index, and then read the remaining frames of the video one frame at a time.

Create a VideoReader object and display the value of the CurrentTime property. A zero value for the CurrentTime property indicates that no frames have been read from the video.

vidObj = VideoReader("xylophone_video.mp4"); vidObj.CurrentTime

Read the 20th frame from the video by specifying the frame index. Then, display the value of the CurrentTime property. The read method automatically updates the CurrentTime property to reflect that the 20th frame has been read.

frame20 = read(vidObj,20); vidObj.CurrentTime

At this point, a call to the readFrame function would return the 21st frame.

Read the remaining frames of the video using the readFrame method. The readFrame method returns the frame corresponding to the time in the CurrentTime property. For instance, this code reads and displays the frames starting at the 21st frame and continues until there are no more frames to read.

while(hasFrame(vidObj)) frame = readFrame(vidObj); imshow(frame) title(sprintf("Current Time = %.3f sec",vidObj.CurrentTime)) pause(2/vidObj.FrameRate) end

Clear the VideoReader object.

Limitations

• For some MP4 files, the NumFrames property can return different values in Windows®, Mac, and Linux® platforms. This variation results from differences in the underlying platform-specific APIs.
• For some AVI, MOV, or MP4 files on Windows, using the readFrame function to read all of the frames in the file can result in a different number of frames from the value returned by theNumFrames property of the VideoReader object.

Tips

• On Windows platforms, you cannot modify or delete an AVI file that is referenced by a VideoReader object in your workspace. To removeVideoReader objects from your workspace, use the clear function.
• The macOS platform no longer supports certain older video file formats. To read such files using VideoReader:
  • Open the video file using the QuickTime player. If the player detects the file to be of an older format, then it automatically converts the file to a newer format.
  • Save the newly converted video file.
  • Use VideoReader to read this newly converted video file.

Extended Capabilities

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Usage notes and limitations:

Code generation for VideoReader supports most formats, syntaxes, methods, and functions with the following limitations.

• Video Format Support:
  • If filename is a compile-time constant, then code generation supports all the formats supported in MATLAB. For more information on video formats that MATLAB supports, see Supported Video and Audio File Formats.
  • If filename is not a compile-time constant, then code generation supports only video files with data that can be decoded to uint8 datatype. Supported video formats include: .MP4,.MOV, and .AVI.
• Object Construction:
  • For MEX targets, partial path to the video file is supported.
  • For RTW targets, you must provide full or relative path to the video file.
• Methods and Functions :
  • read andreadFrame — Code generation does not support the optional positional argument native.
  • VideoReader.getFileFormats — Code generation does not support this method.
  • saveobj andloadobj — Code generation does not support these functions.
  • Property Inspector — Code generation does not support this tool.
  • Code generation does not supportVideoReader object display.
• Platform Dependencies — If the generated code forVideoReader on one specific machine does not work on another machine, then:
  • Ensure that the suitable codecs for your video are available on the target machine.
  • Add test code to check if the video object created on the target machine is valid. Test code can include checking if the video object has valid height or width. For example:
    videoObj = VideoReader(filename);
    if isnan(videoObj.Height)
    fprintf('Failed to create video object.\n');
    return
    end
• Generate Code That Uses Row-Major Layout — Generate Code That Uses Row-Major Array Layout (MATLAB Coder).
• Array Size Restrictions — For code generation, the maximum number of elements of an array is constrained by the code generator and the target hardware. For more information, see Array Size Restrictions for Code Generation (MATLAB Coder).
• Video File Location Restrictions — Code generation does not support reading files from remote location.

Usage notes and limitations:

With MATLAB Coder™ Support Package for NVIDIA® Jetson™ and NVIDIA DRIVE™ Platforms, you can generate CUDA® code for the MATLABVideoReader object to read files containing video data on the NVIDIA target hardware.

To learn how to generate CUDA code for reading video files on the NVIDIA target by using the VideoReader function, see Read Video Files on NVIDIA Hardware (MATLAB Coder).

• The generated code uses the GStreamer library API to read the video files. You must install the GStreamer library (v1.0 or higher) on the NVIDIA target platform.
• For code generation, only the file (container) formats and codecs that are compatible with GStreamer are supported.
• For code generation, the VideoReader function requires the full path to the video file on the target hardware.
• Methods and Functions :
  • read andreadFrame — Code generation does not support the optional positional argument native.
  • VideoReader.getFileFormats — Code generation does not support this method.

Usage notes and limitations:

This function does not support thread-based environments on Windows platforms when reading data in MPEG-1 format (.mpg), Windows Media Video format (.wmv), or other formats that use Microsoft® DirectShow® for decoding.

For more information, see Run MATLAB Functions in Thread-Based Environment.

Version History

Introduced in R2010b

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You can run VideoReader in the background using MATLABbackgroundPool.

Pixel value differences might exist between JPEG 2000 images in R2021b and previous releases of MATLAB.

The VideoReader object supports interchangeable access to video frames using frame index or time. Therefore, you can use read andreadFrame interchangeably. Previously, you could use only one type of access at a time. Attempting to read frames interchangeably usingread and readFrame resulted in an error.

For large video files, the generated code for the VideoReader object with a row-major layout option shows improved performance. For example, thetimingTest function shows about a 4x speed-up on aH.264 video file with a resolution of1280x720.

function [t, data] = timingTest(fileName) vidObj = VideoReader(fileName); data = cell(20,1); tic; for cnt = 1:20 data{cnt} = readFrame(vidObj); end t = toc; end

Generate code for the timingTest function with the row-major flag. The codegen command creates a functiontimingTest_mex with the C and C++ generated code.

codegen timingTest-args{coder.typeof('', [1 inf])}-rowmajor

For a H.264 video file with a resolution of1280x720, the execution times are:

R2019a: 4.04s

R2019b: 0.95s

The code was timed on a Windows 10, Intel® Xeon® CPU W-2133 @ 3.6 GHz test system by calling the functiontimingTest_mex. The higher the video file resolution (measured by frame size), the greater the performance improvement becomes.

The NumberOfFrames property is not recommended. Use theNumFrames property instead. There are no plans to remove the NumberOfFrames property.

You can no longer create an array of VideoReader objects. Update your code to remove arrays of VideoReader objects.