1Creating a video with OpenCV {#tutorial_video_write}
2============================
3
4Goal
5----
6
7Whenever you work with video feeds you may eventually want to save your image processing result in a
8form of a new video file. For simple video outputs you can use the OpenCV built-in @ref cv::VideoWriter
9class, designed for this.
10
11-   How to create a video file with OpenCV
12-   What type of video files you can create with OpenCV
13-   How to extract a given color channel from a video
14
15As a simple demonstration I'll just extract one of the BGR color channels of an input video file
16into a new video. You can control the flow of the application from its console line arguments:
17
18-   The first argument points to the video file to work on
19-   The second argument may be one of the characters: R G B. This will specify which of the channels
20    to extract.
21-   The last argument is the character Y (Yes) or N (No). If this is no, the codec used for the
22    input video file will be the same as for the output. Otherwise, a window will pop up and allow
23    you to select yourself the codec to use.
24
25For example, a valid command line would look like:
26@code{.bash}
27video-write.exe video/Megamind.avi R Y
28@endcode
29The source code
30---------------
31
32You may also find the source code and these video file in the
33`samples/cpp/tutorial_code/highgui/video-write/` folder of the OpenCV source library or [download it
34from here ](https://github.com/Itseez/opencv/tree/master/samples/cpp/tutorial_code/HighGUI/video-write/video-write.cpp).
35
36@include cpp/tutorial_code/HighGUI/video-write/video-write.cpp
37
38The structure of a video
39------------------------
40
41For start, you should have an idea of just how a video file looks. Every video file in itself is a
42container. The type of the container is expressed in the files extension (for example *avi*, *mov*
43or *mkv*). This contains multiple elements like: video feeds, audio feeds or other tracks (like for
44example subtitles). How these feeds are stored is determined by the codec used for each one of them.
45In case of the audio tracks commonly used codecs are *mp3* or *aac*. For the video files the list is
46somehow longer and includes names such as *XVID*, *DIVX*, *H264* or *LAGS* (*Lagarith Lossless
47Codec*). The full list of codecs you may use on a system depends on just what one you have
48installed.
49
50![](images/videoFileStructure.png)
51
52As you can see things can get really complicated with videos. However, OpenCV is mainly a computer
53vision library, not a video stream, codec and write one. Therefore, the developers tried to keep
54this part as simple as possible. Due to this OpenCV for video containers supports only the *avi*
55extension, its first version. A direct limitation of this is that you cannot save a video file
56larger than 2 GB. Furthermore you can only create and expand a single video track inside the
57container. No audio or other track editing support here. Nevertheless, any video codec present on
58your system might work. If you encounter some of these limitations you will need to look into more
59specialized video writing libraries such as *FFMpeg* or codecs as *HuffYUV*, *CorePNG* and *LCL*. As
60an alternative, create the video track with OpenCV and expand it with sound tracks or convert it to
61other formats by using video manipulation programs such as *VirtualDub* or *AviSynth*.
62
63The *VideoWriter* class
64-----------------------
65
66The content written here builds on the assumption you
67already read the @ref tutorial_video_input_psnr_ssim tutorial and you know how to read video files. To create a
68video file you just need to create an instance of the @ref cv::VideoWriter class. You can specify
69its properties either via parameters in the constructor or later on via the @ref cv::VideoWriter::open function.
70Either way, the parameters are the same: 1. The name of the output that contains the container type
71in its extension. At the moment only *avi* is supported. We construct this from the input file, add
72to this the name of the channel to use, and finish it off with the container extension.
73@code{.cpp}
74const string source      = argv[1];            // the source file name
75string::size_type pAt = source.find_last_of('.');   // Find extension point
76const string NAME = source.substr(0, pAt) + argv[2][0] + ".avi";   // Form the new name with container
77@endcode
78-#  The codec to use for the video track. Now all the video codecs have a unique short name of
79    maximum four characters. Hence, the *XVID*, *DIVX* or *H264* names. This is called a four
80    character code. You may also ask this from an input video by using its *get* function. Because
81    the *get* function is a general function it always returns double values. A double value is
82    stored on 64 bits. Four characters are four bytes, meaning 32 bits. These four characters are
83    coded in the lower 32 bits of the *double*. A simple way to throw away the upper 32 bits would
84    be to just convert this value to *int*:
85    @code{.cpp}
86    VideoCapture inputVideo(source);                                // Open input
87    int ex = static_cast<int>(inputVideo.get(CAP_PROP_FOURCC));     // Get Codec Type- Int form
88    @endcode
89    OpenCV internally works with this integer type and expect this as its second parameter. Now to
90    convert from the integer form to string we may use two methods: a bitwise operator and a union
91    method. The first one extracting from an int the characters looks like (an "and" operation, some
92    shifting and adding a 0 at the end to close the string):
93    @code{.cpp}
94    char EXT[] = {ex & 0XFF , (ex & 0XFF00) >> 8,(ex & 0XFF0000) >> 16,(ex & 0XFF000000) >> 24, 0};
95    @endcode
96    You can do the same thing with the *union* as:
97    @code{.cpp}
98    union { int v; char c[5];} uEx ;
99    uEx.v = ex;                              // From Int to char via union
100    uEx.c[4]='\0';
101    @endcode
102    The advantage of this is that the conversion is done automatically after assigning, while for
103    the bitwise operator you need to do the operations whenever you change the codec type. In case
104    you know the codecs four character code beforehand, you can use the *CV_FOURCC* macro to build
105    the integer:
106    @code{.cpp}
107    CV_FOURCC('P','I','M,'1') // this is an MPEG1 codec from the characters to integer
108    @endcode
109    If you pass for this argument minus one than a window will pop up at runtime that contains all
110    the codec installed on your system and ask you to select the one to use:
111
112    ![](images/videoCompressSelect.png)
113
114-#  The frame per second for the output video. Again, here I keep the input videos frame per second
115    by using the *get* function.
116-#  The size of the frames for the output video. Here too I keep the input videos frame size per
117    second by using the *get* function.
118-#  The final argument is an optional one. By default is true and says that the output will be a
119    colorful one (so for write you will send three channel images). To create a gray scale video
120    pass a false parameter here.
121
122Here it is, how I use it in the sample:
123@code{.cpp}
124VideoWriter outputVideo;
125Size S = Size((int) inputVideo.get(CAP_PROP_FRAME_WIDTH),    //Acquire input size
126              (int) inputVideo.get(CAP_PROP_FRAME_HEIGHT));
127outputVideo.open(NAME , ex, inputVideo.get(CAP_PROP_FPS),S, true);
128@endcode
129Afterwards, you use the @ref cv::VideoWriter::isOpened() function to find out if the open operation succeeded or
130not. The video file automatically closes when the *VideoWriter* object is destroyed. After you open
131the object with success you can send the frames of the video in a sequential order by using the
132@ref cv::VideoWriter::write function of the class. Alternatively, you can use its overloaded operator \<\< :
133@code{.cpp}
134outputVideo.write(res);  //or
135outputVideo << res;
136@endcode
137Extracting a color channel from an BGR image means to set to zero the BGR values of the other
138channels. You can either do this with image scanning operations or by using the split and merge
139operations. You first split the channels up into different images, set the other channels to zero
140images of the same size and type and finally merge them back:
141@code{.cpp}
142split(src, spl);                 // process - extract only the correct channel
143for( int i =0; i < 3; ++i)
144   if (i != channel)
145      spl[i] = Mat::zeros(S, spl[0].type());
146merge(spl, res);
147@endcode
148Put all this together and you'll get the upper source code, whose runtime result will show something
149around the idea:
150
151![](images/resultOutputWideoWrite.png)
152
153You may observe a runtime instance of this on the [YouTube
154here](https://www.youtube.com/watch?v=jpBwHxsl1_0).
155
156\htmlonly
157<div align="center">
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160\endhtmlonly
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