# Cap\_ffmpeg\_impl.hpp changes are not applied to opencv build **URL:** \ **Category:** C++\ **Tags:** ffmpeg, windows, build, videoio\ **Created:** [April 26, 2021, 6:59am UTC](https://forum.opencv.org/t/cap-ffmpeg-impl-hpp-changes-are-not-applied-to-opencv-build/2980 "2021-04-26T06:59:26Z")\ **Posts on this page:** 1\ **Showing post:** 6
**Author:** ![Tykyo](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/tykyo/32/2206_2.png) [@Tykyo](https://forum.opencv.org/u/Tykyo)\ **Post date:** [May 24, 2021, 2:58pm UTC](https://forum.opencv.org/t/cap-ffmpeg-impl-hpp-changes-are-not-applied-to-opencv-build/2980/6 "2021-05-24T14:58:21Z")
Hello zadok, Maybe you already found a solution for your problem. If it’s not the case, I share with you a tutorial I made to build the opencv\_videoio\_ffmpeg\*.dll. It’s made to be use by anybody so don’t be mad if you found it too childish. ## Tutorial to compil you own modified version of opencv\_videoio\_ffmpeg\*.dll **Goal:** We will modify the library to change the pixel format used when writing raw video with the fourcc codec 0x00000000 which stand for BI\_RGB label. There is no fourcc label to use this, you need to specify the hex code when creating the VideoWriter, example: > ``` > cv::VideoWriter outputVideo; > cv::Size S = cv::Size(800, 600); > outputVideo.open( > "out.avi", //output file > 0x00000000, //fourcc codec hex code [or use label VideoWriter::fourcc('I','4','2','0') = 0x30323449 in hex] > 24, //framerate (must be greater than 6 for compatibility with most video player) > S, //the video size > true //boolean value to set if video is with color (which is the case here) > ); > > ``` In the actual repository maintained by the developper of OpenCV, the 0x000000 codec use by default yuv420p pixel, which for me is a mistake (but who am I to judge). _Note: 0x000000 can be replace by 0, of course, but I prefer the hex form since it’s the one used by the fourcc web site. And if you want to change the 0x30323449? Uuhh!!! Yeah, me too._ **Prerequisites:** Before the modification, you will need a few tools: 1. A linux installation on you windows system : Ubuntu for example in the Microsoft Store. 2. Docker desktop —Optional— 3. GitHub Desktop From Ubuntu, you need to configure git if it’s not already done with: > ``` > git config --global user.email "You@example.com" > git config --global user.name "Your Name" > > ``` Before starting to download the source code needed, you need to find the good one for your version of OpenCV. I use the 4.5.0 version released on 11th October 2020 (commit d5fd2f0). The date is important, because you need to download the good branch of opencv\_3rdparty from github (at least the closer one in time). opencv\_3rdparty : [GitHub - opencv/opencv\_3rdparty at ffmpeg/master\_20200908](https://github.com/opencv/opencv_3rdparty/tree/ffmpeg/master_20200908) opencv : [GitHub - opencv/opencv at 4.5.0](https://github.com/opencv/opencv/tree/4.5.0) For opencv\_3rdparty you can download the zip archive of the branch. No need to clone the repository with git. Extract the zip archive somewhere. Inside the extracted directory (opencv\_3rdparty-ffmpeg-master\_20200908 or opencv\_3rdparty for the remaining of this tuto), there is an empty opencv directory. We will clone opencv inside. If you want to git clone opencv\_3rdparty: > ``` > git clone https://github.com/opencv/opencv_3rdparty.git --single-branch --branch ffmpeg/master_20200908 > > ``` For opencv you absolutly need to clone the repository and switch to the good branch. So inside you opencv\_3rdparty directory (not the empty opencv), do from the ubuntu command line ( **not the windows command line** ) > ``` > git clone https://github.com/opencv/opencv.git --single-branch --branch 4.5.0 > > ``` this will clone to opencv\_3rdparty/opencv and switch to the 4.5.0 branch Using --single-branch can reduce the total size by two, mostly for opencv\_3rdparty. Don’t try to build the dll with a different version of opencv than the targeted one, because it doesn’t work. Of course the compilation will be fine, but you will have problem when you use the dll. If you choose the wrong version of opencv\_3rdparty, the compilation will probably failed. **Modification:** This part is up to you, you can modify opencv as you which but for opencv\_videoio\_ffmpeg\*.dll, there is only three files inside opencv\_3rdparty/opencv/modules/videoio/src > ``` > cap_ffmpeg_impl.hpp > cap_ffmpeg_legacy_api.hpp > ffmpeg_codecs.hpp > > ``` In my case, to modify the pixel format for 0x00000000 codec, the file is cap\_ffmpeg\_impl.hpp. The modification is: > ``` > [line 2295] > case CV_CODEC(CODEC_ID_RAWVIDEO): > // RGBA is the only RGB fourcc supported by AVI and MKV format > if(fourcc == CV_FOURCC('R','G','B','A')) > { > codec_pix_fmt = AV_PIX_FMT_RGBA; > } > + else if(fourcc == 0x00000000) > + { > + codec_pix_fmt = AV_PIX_FMT_BGR24; > + } > else > { > switch(input_pix_fmt) > { > case AV_PIX_FMT_GRAY8: > case AV_PIX_FMT_GRAY16LE: > case AV_PIX_FMT_GRAY16BE: > codec_pix_fmt = input_pix_fmt; > break; > default: > codec_pix_fmt = AV_PIX_FMT_YUV420P; > break; > } > } > [line 2314] > > ``` The four lines added (at position 2301 of the file) start with a ‘+’. As you can see, if the fourcc is not RGBA or the input is not GRAY, the pixel format used by default is AV\_PIX\_FMT\_YUV420P. So I added AV\_PIX\_FMT\_BGR24 for the 0x00000000. **Building:** Aaah, the most important part. First thing to do is to commit you change to the local repository of opencv. It’s realy important, if you not commit the changes, their will not apply to the build. The easiest way is to use GitHub Desktop. Or inside the opencv\_3rdparty/opencv directory from ubuntu command line: > ``` > git commit -am "modify cap_ffmpeg_impl.hpp for pix_fmt" > > ``` 1. Start Docker Desktop if it’s not already running or you will get the following error: > The command ‘docker’ could not be found in this WSL 2 distro. > We recommend to activate the WSL integration in Docker Desktop settings. 1. From Ubuntu, inside the opencv\_3rdparty directory : > ``` > ffmpeg/build_via_docker.sh > > ``` That’s all 🙂 The resulting dll can be found in opencv\_3rdparty\ffmpeg, the previous one as been replaced. _Note : during the build, you will see that git package the three .hpp files above, at this stage you can check that inside this archive opencv\_3rdparty\sources\opencv\opencv-videoio-ffmpeg.tar.xz, the files are actually modified. If it’s not the case it’s because you don’t commit you changes._ When you run ffmpeg/build\_via\_docker.sh, if you get ‘: wrong parameter’ or something like that, it’s because you probably clone the opencv\_3rdparty from the windows console. And since it’s windows, the endline is CRLF, not LF. A solution is to run dos2unix on all the file inside opencv\_3rdparty directory. During the build you can see the below warnings: > ``` > /app/opencv_ffmpeg.rc:12: digit exceeds base > /app/opencv_ffmpeg.rc:13: digit exceeds base > > ``` It’s nothing you should care about. I hope it will help. --- _[View the full topic](https://forum.opencv.org/t/cap-ffmpeg-impl-hpp-changes-are-not-applied-to-opencv-build/2980)._