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<p class="MsoNormal"><b><span style="mso-ligatures:none">From:</span></b><span style="mso-ligatures:none"> Libav-user <libav-user-bounces@ffmpeg.org>
<b>On Behalf Of </b>Vaughn Treude<br>
<b>Sent:</b> Wednesday, September 27, 2023 3:56 PM<br>
<b>To:</b> libav-user@ffmpeg.org<br>
<b>Subject:</b> [Libav-user] Encountering exception while reading all frames of an MKV video file<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hello all,<o:p></o:p></p>
<p class="MsoNormal">I'm using the FFMPEG libraries within an FFmpeg-Autogen C# wrapper in Windows Framework 4.8 to decode a video from an MKV (Matroska) container.<o:p></o:p></p>
<p class="MsoNormal">I open the file and initialize the stream using the following sequence:<o:p></o:p></p>
<p class="MsoNormal"> ffmpeg.avformat_open_input(&pFormatContext, url, null, null).ThrowExceptionIfError();<o:p></o:p></p>
<p class="MsoNormal"> ffmpeg.avformat_find_stream_info(_pFormatContext, null).ThrowExceptionIfError();<o:p></o:p></p>
<p class="MsoNormal"> AVCodec* codec = null;<o:p></o:p></p>
<p class="MsoNormal"> _streamIndex = ffmpeg<o:p></o:p></p>
<p class="MsoNormal"> .av_find_best_stream(_pFormatContext, AVMediaType.AVMEDIA_TYPE_VIDEO, -1, -1, &codec, 0)<o:p></o:p></p>
<p class="MsoNormal"> .ThrowExceptionIfError();<o:p></o:p></p>
<p class="MsoNormal"> _pCodecContext = ffmpeg.avcodec_alloc_context3(codec);<o:p></o:p></p>
<p class="MsoNormal"> ffmpeg.avcodec_parameters_to_context(_pCodecContext, _pFormatContext->streams[_streamIndex]->codecpar)<o:p></o:p></p>
<p class="MsoNormal"> .ThrowExceptionIfError();<o:p></o:p></p>
<p class="MsoNormal"> ffmpeg.avcodec_open2(_pCodecContext, codec, null).ThrowExceptionIfError();<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">After initializing, I can read each frame of the video stream using av_read_frame. I was assuming that at the end, av_read_frame would return an EOF error code. But it doesn't return any kind of error, so I continue with the process, calling
<span style="font-size:9.5pt;font-family:"Cascadia Mono";color:black">avcodec_send_packet with the packet of frame data I’ve supposedly just read, which</span> gives me an illegal memory access exception.<o:p></o:p></p>
<p class="MsoNormal">My normal approach would be to get the length of the video in frames and not read beyond this number. Unfortunately, I've been unable to find a way to determine the length of an MKV video in frames.<o:p></o:p></p>
<p class="MsoNormal">The nb_frames member of the stream object is never set to anything but zero. Alternately, I could multiply the duration in seconds by the frame rate, but the duration member is always set to a large-valued negative number.<o:p></o:p></p>
<p class="MsoNormal">How can I read all the frames in the file and exit gracefully at the end of the stream?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thank you kindly for any hints you can give me,<o:p></o:p></p>
<p class="MsoNormal">Vaughn T.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">An update:<o:p></o:p></p>
<p class="MsoNormal">I believe the crash was due to my attempt to access the Ffmpeg-Autogen wrapper with multiple threads. That thing’s static! I put a lock on the decoder routine to prevent simultaneous access and the crash went away.<o:p></o:p></p>
<p class="MsoNormal">That said, it would still be helpful to have a better way to determine the number of frames in the video without needing to read it until I encounter an error code.<o:p></o:p></p>
<p class="MsoNormal">If anyone knows of such an approach, please let me know.<o:p></o:p></p>
<p class="MsoNormal">Thanks again,<o:p></o:p></p>
<p class="MsoNormal">Vaughn T.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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