91.204.201 Computing IV

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91.204.201 Computing IV

• Chapter Four Highgui Module. High Level GUI And
  Media
• Xinwen Fu

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References

• Reading assignment Chapter 4
• Application Development in Visual Studio
• An online OpenCV Quick Guide with nice examples

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Outline

• 4.1 Adding a Trackbar to our applications!
• 4.2 Video Input with OpenCV and similarity
  measurement
• 4.3 Creating a video with OpenCV

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Add a Trackbar in an OpenCV window by using
createTrackbar

• In the previous tutorials (about linear blending
  and the brightness and contrast adjustments) you
  might have
• noted that we needed to give some input to our
  programs, such as and beta. We accomplished that
  by entering this data using the Terminal
• OpenCV provides some GUI utilities (highgui.h)
• An example of this is a Trackbar

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Example Code

• Lets modify the program made in the tutorial
  Adding (blending) two images using OpenCV.
• We will let the user enter the value by using
  the Trackbar.

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Outline

• 4.1 Adding a Trackbar to our applications!
• 4.2 Video Input with OpenCV and similarity
  measurement
• 4.3 Creating a video with OpenCV

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Goal

• Today it is common to have a digital video
  recording system at your disposal.
• Therefore, you will eventually come to the
  situation that you no longer process a batch of
  images, but video streams. These may be of two
  kinds
• real-time image feed (in the case of a webcam)
• or prerecorded and hard disk drive stored files.
• Luckily OpenCV threats these two in the same
  manner, with the same C class.
• We will learn
• How to open and read video streams
• Two ways for checking image similarity PSNR and
  SSIM

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Example Code

• A program that reads in two video files and
  performs a similarity check between them.
• This is something you could use to check just how
  well a new video compressing algorithms works.
• Let there be a reference (original) video like
  this small Megamind clip and a compressed version
  of it.

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Outline

• 4.1 Adding a Trackbar to our applications!
• 4.2 Video Input with OpenCV and similarity
  measurement
• 4.3 Creating a video with OpenCV

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Goal

• Whenever you work with video feeds you may
  eventually want to save your image processing
  result in a form of a new video file.
• For simple video outputs you can use the OpenCV
  built-in VideoWriter class, designed for this.
• How to create a video file with OpenCV
• What type of video files you can create with
  OpenCV
• How to extract a given color channel from a video

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What is a video file?

• Every video file in itself is a container.
• The type of the container is expressed in the
  files extension (for example avi, mov or mkv).
• This contains multiple elements like video
  feeds, audio feeds or other tracks (such as
  subtitles).
• How these feeds are stored is determined by the
  codec used for each one of them.
• The audio tracks commonly used codecs are mp3 or
  aac.
• For the video files the list is somehow longer
  and includes names such as XVID, DIVX, H264 or
  LAGS (Lagarith Lossless Codec).
• The full list of codecs you may use on a system
  depends on just what one you have installed.

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Video File as Container
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OpenCVs Capability

• OpenCV is mainly a computer vision library, not a
  video stream, codec and write one.
• OPenCV developers tried to keep this part as
  simple as possible.
• Due to this OpenCV for video containers supports
  only the avi extension, its first version.
• A direct limitation of this is that you cannot
  save a video file larger than 2 GB. Furthermore
  you can only create and expand a single video
  track inside the container.
• No audio or other track editing support here.
  Nevertheless, any video codec present on your
  system might work.

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Add Audio and Other Feeds

• If you encounter some of these limitations you
  will need to look into more specialized video
  writing libraries such as FFMpeg or codecs as
  HuffYUV, CorePNG and LCL.
• As an alternative, create the video track with
  OpenCV and expand it with sound tracks or convert
  it to other formats by using video manipulation
  programs such as VirtualDub or AviSynth.

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Create a Video with OpenCV

• To create a video file you just need to create an
  instance of the VideoWriter class.
• You can specify its properties either via
  parameters in the constructor or later on via the
  open function
• The name of the output that contains the
  container type in its extension. At the moment
  only avi is supported. We construct this from the
  input file, add to this the name of the channel
  to use, and finish it off with the container
  extension.
• The frame per second for the output video. Here
  we keep the input videos frame per second by
  using the get function.
• The size of the frames for the output video. Here
  we keep the input videos frame size per second by
  using the get function.
• The final argument is an optional one. By default
  is true and says that the output will be a
  colorful one (so for write you will send three
  channel images). To create a gray scale video
  pass a false parameter here.

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Example Code

• As a simple demonstration Ill just extract one
  of the RGB color channels of an input video file
  into a new video. You can control the flow of the
  application from its console line arguments
• The first argument points to the video file to
  work on
• The second argument may be one of the characters
  R G B. This will specify which of the channels to
  extract.
• The last argument is the character Y (Yes) or N
  (No).If this is no, the codec used for the input
  video file will be the same as for the output.
  Otherwise, a window will pop up and allow you to
  select yourself the codec to use.
• A valid command line would look like
• video-write.exe video/Megamind.avi R Y