Creating a video with OpenCV {#tutorial_video_write}
============================
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 @ref cv::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
As a simple demonstration I'll just extract one of the BGR 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.
For example, a valid command line would look like:
@code{.bash}
video-write.exe video/Megamind.avi R Y
@endcode
The source code
---------------
You may also find the source code and these video file in the
`samples/cpp/tutorial_code/highgui/video-write/` folder of the OpenCV source library or [download it
from here ](https://github.com/Itseez/opencv/tree/master/samples/cpp/tutorial_code/HighGUI/video-write/video-write.cpp).
@include cpp/tutorial_code/HighGUI/video-write/video-write.cpp
The structure of a video
------------------------
For start, you should have an idea of just how a video file looks. 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 (like for
example subtitles). How these feeds are stored is determined by the codec used for each one of them.
In case of 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.
As you can see things can get really complicated with videos. However, OpenCV is mainly a computer
vision library, not a video stream, codec and write one. Therefore, the 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. 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*.
The *VideoWriter* class
-----------------------
The content written here builds on the assumption you
already read the @ref tutorial_video_input_psnr_ssim tutorial and you know how to read video files. To create a
video file you just need to create an instance of the @ref cv::VideoWriter class. You can specify
its properties either via parameters in the constructor or later on via the @ref cv::VideoWriter::open function.
Either way, the parameters are the same: 1. 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.
@code{.cpp}
const string source = argv[1]; // the source file name
string::size_type pAt = source.find_last_of('.'); // Find extension point
const string NAME = source.substr(0, pAt) + argv[2][0] + ".avi"; // Form the new name with container
@endcode
-# The codec to use for the video track. Now all the video codecs have a unique short name of
maximum four characters. Hence, the *XVID*, *DIVX* or *H264* names. This is called a four
character code. You may also ask this from an input video by using its *get* function. Because
the *get* function is a general function it always returns double values. A double value is
stored on 64 bits. Four characters are four bytes, meaning 32 bits. These four characters are
coded in the lower 32 bits of the *double*. A simple way to throw away the upper 32 bits would
be to just convert this value to *int*:
@code{.cpp}
VideoCapture inputVideo(source); // Open input
int ex = static_cast<int>(inputVideo.get(CAP_PROP_FOURCC)); // Get Codec Type- Int form
@endcode
OpenCV internally works with this integer type and expect this as its second parameter. Now to
convert from the integer form to string we may use two methods: a bitwise operator and a union
method. The first one extracting from an int the characters looks like (an "and" operation, some
shifting and adding a 0 at the end to close the string):
@code{.cpp}
char EXT[] = {ex & 0XFF , (ex & 0XFF00) >> 8,(ex & 0XFF0000) >> 16,(ex & 0XFF000000) >> 24, 0};
@endcode
You can do the same thing with the *union* as:
@code{.cpp}
union { int v; char c[5];} uEx ;
uEx.v = ex; // From Int to char via union
uEx.c[4]='\0';
@endcode
The advantage of this is that the conversion is done automatically after assigning, while for
the bitwise operator you need to do the operations whenever you change the codec type. In case
you know the codecs four character code beforehand, you can use the *CV_FOURCC* macro to build
the integer:
@code{.cpp}
CV_FOURCC('P','I','M,'1') // this is an MPEG1 codec from the characters to integer
@endcode
If you pass for this argument minus one than a window will pop up at runtime that contains all
the codec installed on your system and ask you to select the one to use:
-# The frame per second for the output video. Again, here I keep the input videos frame per second
by using the *get* function.
-# The size of the frames for the output video. Here too I 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.
Here it is, how I use it in the sample:
@code{.cpp}
VideoWriter outputVideo;
Size S = Size((int) inputVideo.get(CAP_PROP_FRAME_WIDTH), //Acquire input size
(int) inputVideo.get(CAP_PROP_FRAME_HEIGHT));
outputVideo.open(NAME , ex, inputVideo.get(CAP_PROP_FPS),S, true);
@endcode
Afterwards, you use the @ref cv::VideoWriter::isOpened() function to find out if the open operation succeeded or
not. The video file automatically closes when the *VideoWriter* object is destroyed. After you open
the object with success you can send the frames of the video in a sequential order by using the
@ref cv::VideoWriter::write function of the class. Alternatively, you can use its overloaded operator \<\< :
@code{.cpp}
outputVideo.write(res); //or
outputVideo << res;
@endcode
Extracting a color channel from an BGR image means to set to zero the BGR values of the other
channels. You can either do this with image scanning operations or by using the split and merge
operations. You first split the channels up into different images, set the other channels to zero
images of the same size and type and finally merge them back:
@code{.cpp}
split(src, spl); // process - extract only the correct channel
for( int i =0; i < 3; ++i)
if (i != channel)
spl[i] = Mat::zeros(S, spl[0].type());
merge(spl, res);
@endcode
Put all this together and you'll get the upper source code, whose runtime result will show something
around the idea:
You may observe a runtime instance of this on the [YouTube
here](https://www.youtube.com/watch?v=jpBwHxsl1_0).
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