161 lines
8.6 KiB
Markdown
161 lines
8.6 KiB
Markdown
Creating a video with OpenCV {#tutorial_video_write}
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============================
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Goal
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----
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Whenever you work with video feeds you may eventually want to save your image processing result in a
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form of a new video file. For simple video outputs you can use the OpenCV built-in @ref cv::VideoWriter
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class, designed for this.
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- How to create a video file with OpenCV
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- What type of video files you can create with OpenCV
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- How to extract a given color channel from a video
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As a simple demonstration I'll just extract one of the BGR color channels of an input video file
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into a new video. You can control the flow of the application from its console line arguments:
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- The first argument points to the video file to work on
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- The second argument may be one of the characters: R G B. This will specify which of the channels
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to extract.
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- The last argument is the character Y (Yes) or N (No). If this is no, the codec used for the
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input video file will be the same as for the output. Otherwise, a window will pop up and allow
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you to select yourself the codec to use.
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For example, a valid command line would look like:
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@code{.bash}
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video-write.exe video/Megamind.avi R Y
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@endcode
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The source code
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---------------
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You may also find the source code and these video file in the
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`samples/cpp/tutorial_code/highgui/video-write/` folder of the OpenCV source library or [download it
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from here ](https://github.com/Itseez/opencv/tree/master/samples/cpp/tutorial_code/HighGUI/video-write/video-write.cpp).
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@include cpp/tutorial_code/HighGUI/video-write/video-write.cpp
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The structure of a video
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------------------------
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For start, you should have an idea of just how a video file looks. Every video file in itself is a
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container. The type of the container is expressed in the files extension (for example *avi*, *mov*
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or *mkv*). This contains multiple elements like: video feeds, audio feeds or other tracks (like for
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example subtitles). How these feeds are stored is determined by the codec used for each one of them.
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In case of the audio tracks commonly used codecs are *mp3* or *aac*. For the video files the list is
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somehow longer and includes names such as *XVID*, *DIVX*, *H264* or *LAGS* (*Lagarith Lossless
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Codec*). The full list of codecs you may use on a system depends on just what one you have
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installed.
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As you can see things can get really complicated with videos. However, OpenCV is mainly a computer
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vision library, not a video stream, codec and write one. Therefore, the developers tried to keep
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this part as simple as possible. Due to this OpenCV for video containers supports only the *avi*
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extension, its first version. A direct limitation of this is that you cannot save a video file
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larger than 2 GB. Furthermore you can only create and expand a single video track inside the
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container. No audio or other track editing support here. Nevertheless, any video codec present on
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your system might work. If you encounter some of these limitations you will need to look into more
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specialized video writing libraries such as *FFMpeg* or codecs as *HuffYUV*, *CorePNG* and *LCL*. As
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an alternative, create the video track with OpenCV and expand it with sound tracks or convert it to
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other formats by using video manipulation programs such as *VirtualDub* or *AviSynth*.
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The *VideoWriter* class
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-----------------------
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The content written here builds on the assumption you
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already read the @ref tutorial_video_input_psnr_ssim tutorial and you know how to read video files. To create a
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video file you just need to create an instance of the @ref cv::VideoWriter class. You can specify
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its properties either via parameters in the constructor or later on via the @ref cv::VideoWriter::open function.
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Either way, the parameters are the same: 1. The name of the output that contains the container type
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in its extension. At the moment only *avi* is supported. We construct this from the input file, add
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to this the name of the channel to use, and finish it off with the container extension.
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@code{.cpp}
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const string source = argv[1]; // the source file name
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string::size_type pAt = source.find_last_of('.'); // Find extension point
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const string NAME = source.substr(0, pAt) + argv[2][0] + ".avi"; // Form the new name with container
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@endcode
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-# The codec to use for the video track. Now all the video codecs have a unique short name of
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maximum four characters. Hence, the *XVID*, *DIVX* or *H264* names. This is called a four
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character code. You may also ask this from an input video by using its *get* function. Because
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the *get* function is a general function it always returns double values. A double value is
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stored on 64 bits. Four characters are four bytes, meaning 32 bits. These four characters are
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coded in the lower 32 bits of the *double*. A simple way to throw away the upper 32 bits would
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be to just convert this value to *int*:
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@code{.cpp}
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VideoCapture inputVideo(source); // Open input
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int ex = static_cast<int>(inputVideo.get(CAP_PROP_FOURCC)); // Get Codec Type- Int form
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@endcode
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OpenCV internally works with this integer type and expect this as its second parameter. Now to
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convert from the integer form to string we may use two methods: a bitwise operator and a union
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method. The first one extracting from an int the characters looks like (an "and" operation, some
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shifting and adding a 0 at the end to close the string):
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@code{.cpp}
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char EXT[] = {ex & 0XFF , (ex & 0XFF00) >> 8,(ex & 0XFF0000) >> 16,(ex & 0XFF000000) >> 24, 0};
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@endcode
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You can do the same thing with the *union* as:
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@code{.cpp}
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union { int v; char c[5];} uEx ;
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uEx.v = ex; // From Int to char via union
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uEx.c[4]='\0';
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@endcode
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The advantage of this is that the conversion is done automatically after assigning, while for
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the bitwise operator you need to do the operations whenever you change the codec type. In case
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you know the codecs four character code beforehand, you can use the *CV_FOURCC* macro to build
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the integer:
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@code{.cpp}
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CV_FOURCC('P','I','M,'1') // this is an MPEG1 codec from the characters to integer
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@endcode
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If you pass for this argument minus one than a window will pop up at runtime that contains all
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the codec installed on your system and ask you to select the one to use:
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-# The frame per second for the output video. Again, here I keep the input videos frame per second
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by using the *get* function.
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-# The size of the frames for the output video. Here too I keep the input videos frame size per
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second by using the *get* function.
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-# The final argument is an optional one. By default is true and says that the output will be a
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colorful one (so for write you will send three channel images). To create a gray scale video
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pass a false parameter here.
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Here it is, how I use it in the sample:
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@code{.cpp}
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VideoWriter outputVideo;
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Size S = Size((int) inputVideo.get(CAP_PROP_FRAME_WIDTH), //Acquire input size
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(int) inputVideo.get(CAP_PROP_FRAME_HEIGHT));
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outputVideo.open(NAME , ex, inputVideo.get(CAP_PROP_FPS),S, true);
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@endcode
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Afterwards, you use the @ref cv::VideoWriter::isOpened() function to find out if the open operation succeeded or
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not. The video file automatically closes when the *VideoWriter* object is destroyed. After you open
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the object with success you can send the frames of the video in a sequential order by using the
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@ref cv::VideoWriter::write function of the class. Alternatively, you can use its overloaded operator \<\< :
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@code{.cpp}
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outputVideo.write(res); //or
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outputVideo << res;
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@endcode
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Extracting a color channel from an BGR image means to set to zero the BGR values of the other
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channels. You can either do this with image scanning operations or by using the split and merge
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operations. You first split the channels up into different images, set the other channels to zero
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images of the same size and type and finally merge them back:
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@code{.cpp}
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split(src, spl); // process - extract only the correct channel
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for( int i =0; i < 3; ++i)
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if (i != channel)
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spl[i] = Mat::zeros(S, spl[0].type());
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merge(spl, res);
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@endcode
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Put all this together and you'll get the upper source code, whose runtime result will show something
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around the idea:
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You may observe a runtime instance of this on the [YouTube
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here](https://www.youtube.com/watch?v=jpBwHxsl1_0).
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\htmlonly
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<div align="center">
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<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>
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</div>
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\endhtmlonly
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