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Computing IV
Chapter Four: Highgui Module. High Level
GUI And Media
Xinwen Fu
References
Reading assignment: Chapter 4
 Application Development in Visual Studio
 An online OpenCV Quick Guide with nice
examples

By Dr. Xinwen Fu
CS@UML
2
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

By Dr. Xinwen Fu
CS@UML
3
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
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
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
By Dr. Xinwen Fu
CS@UML
4
Example Code

Let’s 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.
By Dr. Xinwen Fu
CS@UML
5
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

By Dr. Xinwen Fu
CS@UML
6
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
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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
By Dr. Xinwen Fu
CS@UML
7
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.
By Dr. Xinwen Fu
CS@UML
8
By Dr. Xinwen Fu
CS@UML
9
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

By Dr. Xinwen Fu
CS@UML
10
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
By Dr. Xinwen Fu
CS@UML
11
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.
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

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.
By Dr. Xinwen Fu
CS@UML
12
Video File as Container
By Dr. Xinwen Fu
CS@UML
13
OpenCV’s 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.
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
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.
By Dr. Xinwen Fu
CS@UML
14
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.
By Dr. Xinwen Fu
CS@UML
15
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:
1.
2.
3.
4.
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.
By Dr. Xinwen Fu
CS@UML
16
Example Code

As a simple demonstration I’ll 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
By Dr. Xinwen Fu
CS@UML
17