1 Creating a video with OpenCV {#tutorial_video_write} 2 ============================ 3 4 Goal 5 ---- 6 7 Whenever you work with video feeds you may eventually want to save your image processing result in a 8 form of a new video file. For simple video outputs you can use the OpenCV built-in @ref cv::VideoWriter 9 class, 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 15 As a simple demonstration I'll just extract one of the BGR color channels of an input video file 16 into 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 25 For example, a valid command line would look like: 26 @code{.bash} 27 video-write.exe video/Megamind.avi R Y 28 @endcode 29 The source code 30 --------------- 31 32 You 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 34 from 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 38 The structure of a video 39 ------------------------ 40 41 For start, you should have an idea of just how a video file looks. Every video file in itself is a 42 container. The type of the container is expressed in the files extension (for example *avi*, *mov* 43 or *mkv*). This contains multiple elements like: video feeds, audio feeds or other tracks (like for 44 example subtitles). How these feeds are stored is determined by the codec used for each one of them. 45 In case of the audio tracks commonly used codecs are *mp3* or *aac*. For the video files the list is 46 somehow longer and includes names such as *XVID*, *DIVX*, *H264* or *LAGS* (*Lagarith Lossless 47 Codec*). The full list of codecs you may use on a system depends on just what one you have 48 installed. 49 50  51 52 As you can see things can get really complicated with videos. However, OpenCV is mainly a computer 53 vision library, not a video stream, codec and write one. Therefore, the developers tried to keep 54 this part as simple as possible. Due to this OpenCV for video containers supports only the *avi* 55 extension, its first version. A direct limitation of this is that you cannot save a video file 56 larger than 2 GB. Furthermore you can only create and expand a single video track inside the 57 container. No audio or other track editing support here. Nevertheless, any video codec present on 58 your system might work. If you encounter some of these limitations you will need to look into more 59 specialized video writing libraries such as *FFMpeg* or codecs as *HuffYUV*, *CorePNG* and *LCL*. As 60 an alternative, create the video track with OpenCV and expand it with sound tracks or convert it to 61 other formats by using video manipulation programs such as *VirtualDub* or *AviSynth*. 62 63 The *VideoWriter* class 64 ----------------------- 65 66 The content written here builds on the assumption you 67 already read the @ref tutorial_video_input_psnr_ssim tutorial and you know how to read video files. To create a 68 video file you just need to create an instance of the @ref cv::VideoWriter class. You can specify 69 its properties either via parameters in the constructor or later on via the @ref cv::VideoWriter::open function. 70 Either way, the parameters are the same: 1. The name of the output that contains the container type 71 in its extension. At the moment only *avi* is supported. We construct this from the input file, add 72 to this the name of the channel to use, and finish it off with the container extension. 73 @code{.cpp} 74 const string source = argv[1]; // the source file name 75 string::size_type pAt = source.find_last_of('.'); // Find extension point 76 const 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  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. 121 122 Here it is, how I use it in the sample: 123 @code{.cpp} 124 VideoWriter outputVideo; 125 Size S = Size((int) inputVideo.get(CAP_PROP_FRAME_WIDTH), //Acquire input size 126 (int) inputVideo.get(CAP_PROP_FRAME_HEIGHT)); 127 outputVideo.open(NAME , ex, inputVideo.get(CAP_PROP_FPS),S, true); 128 @endcode 129 Afterwards, you use the @ref cv::VideoWriter::isOpened() function to find out if the open operation succeeded or 130 not. The video file automatically closes when the *VideoWriter* object is destroyed. After you open 131 the 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} 134 outputVideo.write(res); //or 135 outputVideo << res; 136 @endcode 137 Extracting a color channel from an BGR image means to set to zero the BGR values of the other 138 channels. You can either do this with image scanning operations or by using the split and merge 139 operations. You first split the channels up into different images, set the other channels to zero 140 images of the same size and type and finally merge them back: 141 @code{.cpp} 142 split(src, spl); // process - extract only the correct channel 143 for( int i =0; i < 3; ++i) 144 if (i != channel) 145 spl[i] = Mat::zeros(S, spl[0].type()); 146 merge(spl, res); 147 @endcode 148 Put all this together and you'll get the upper source code, whose runtime result will show something 149 around the idea: 150 151  152 153 You may observe a runtime instance of this on the [YouTube 154 here](https://www.youtube.com/watch?v=jpBwHxsl1_0). 155 156 \htmlonly 157 <div align="center"> 158 <iframe title="Creating a video with OpenCV" width="560" height="349" src="http://www.youtube.com/embed/jpBwHxsl1_0?rel=0&loop=1" frameborder="0" allowfullscreen align="middle"></iframe> 159 </div> 160 \endhtmlonly 161
