ffmpeg WebM zu MP4: schreckliche Videoqualität

Ich versuche, ein WebM-Video von einer Webcam in ein MP4 x264-Format zu konvertieren. Ich habe versucht, diesen ffmpeg-Befehl zu verwenden:

ffmpeg -stats -nostdin -y -flags global_header -i "/home/Foto/incoming-video/2017-05-01-122614.webm" -c:v libx264 -preset slower -crf 20 -c:a aac "/home/Foto/rename-video/webm-test.mp4"

Das aus dem Befehl kommende Protokoll lautet:

ffmpeg version 3.4.2-1~16.04.york0 Copyright (c) 2000-2018 the FFmpeg developers built with gcc 5.4.0 (Ubuntu 5.4.0-6ubuntu1~16.04.6) 20160609 configuration: --prefix=/usr --extra-version='1~16.04.york0' --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --disable-stripping --enable-avresample --enable-avisynth -enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgs --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-librsvg --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex -enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-omx --enabl-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared libavutil 55. 78.100 / 55. 78.100 libavcodec 57.107.100 / 57.107.100 libavformat 57. 83.100 / 57. 83.100 libavdevice 57. 10.100 / 57. 10.100 libavfilter 6.107.100 / 6.107.100 libavresample 3. 7. 0 / 3. 7. 0 libswscale 4. 8.100 / 4. 8.100 libswresample 2. 9.100 / 2. 9.100 libpostproc 54. 7.100 / 54. 7.100 Input #0, matroska,webm, from '/home/Foto/incoming-video/2017-05-01-122614.webm': Metadata: encoder : GStreamer matroskamux version 1.8.3 creation_time : 2017-05-01T10:26:14.000000Z Duration: 00:00:57.57, start: 0.000000, bitrate: 3198 kb/s Stream #0:0(eng): Video: vp8, yuv420p(progressive), 1280x720, SAR 1:1 DAR 16:9, 250 tbr, 1k tbn, 1k tbc (default) Metadata: title : Video Stream #0:1(eng): Audio: vorbis, 44100 Hz, mono, fltp (default) Metadata: title : Audio Stream mapping: Stream #0:0 -> #0:0 (vp8 (native) -> h264 (libx264)) Stream #0:1 -> #0:1 (vorbis (native) -> aac (native)) [libx264 @ 0x55cb6afacd80] using SAR=1/1 [libx264 @ 0x55cb6afacd80] using cpu capabilities: MMX2 SSE2Fast SSSE3 Cache64 [libx264 @ 0x55cb6afacd80] profile High, level 5.1 [libx264 @ 0x55cb6afacd80] 264 - core 152 r2854 e9a5903 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=1 ref=8 deblock=1:0:0 analyse=0x3:0x133 me=umh subme=9 psy=1 psy_rd=1.00:000 mixed_ref=1 me_range=16 chroma_me=1 trellis=2 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=3 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframs=3 b_pyramid=2 b_adapt=2 b_bias=0 direct=3 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=60 rc=crf mbtree=1 crf=20.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00 Output #0, mp4, to '/home/Foto/rename-video/webm-test.mp4': Metadata: encoder : Lavf57.83.100 Stream #0:0(eng): Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=-1--1, 250 fps, 16k tbn, 250 tbc (default) Metadata: title : Video encoder : Lavc57.107.100 libx264 Side data: cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1 Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default) Metadata: title : Audio encoder : Lavc57.107.100 aac More than 1000 frames duplicated More than 10000 frames duplicated frame=14392 fps= 14 q=-1.0 Lsize= 44929kB time=00:00:57.55 bitrate=6394.8kbits/s dup=12683 drop=0 speed=0.0575x video:44230kB audio:489kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.471058% [libx264 @ 0x55cb6afacd80] frame I:61 Avg QP:17.26 size: 98800 [libx264 @ 0x55cb6afacd80] frame P:3627 Avg QP:20.45 size: 10478 [libx264 @ 0x55cb6afacd80] frame B:10704 Avg QP:23.79 size: 118 [libx264 @ 0x55cb6afacd80] consecutive B-frames: 0.8% 0.0% 0.0% 99.1% [libx264 @ 0x55cb6afacd80] mb I I16..4: 6.5% 79.7% 13.8% [libx264 @ 0x55cb6afacd80] mb P I16..4: 0.7% 1.6% 0.1% P16..4: 33.3% 4.7% 5.2% 0.0% 0.0% skip:54.3% [libx264 @ 0x55cb6afacd80] mb B I16..4: 0.0% 0.0% 0.0% B16..8: 3.2% 0.0% 0.0% direct: 0.0% skip:96.8% L0:30.0% L1:69.7% BI: 0.3% [libx264 @ 0x55cb6afacd80] 8x8 transform intra:71.1% inter:60.1% [libx264 @ 0x55cb6afacd80] direct mvs spatial:100.0% temporal:0.0% [libx264 @ 0x55cb6afacd80] coded y,uvDC,uvAC intra: 68.5% 91.9% 70.6% inter: 4.2% 7.6% 1.4% [libx264 @ 0x55cb6afacd80] i16 v,h,dc,p: 20% 17% 33% 30% [libx264 @ 0x55cb6afacd80] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 16% 11% 11% 6% 11% 15% 10% 10% 10% [libx264 @ 0x55cb6afacd80] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 15% 7% 5% 5% 24% 20% 9% 7% 7% [libx264 @ 0x55cb6afacd80] i8c dc,h,v,p: 50% 20% 16% 14% [libx264 @ 0x55cb6afacd80] Weighted P-Frames: Y:1.7% UV:0.9% [libx264 @ 0x55cb6afacd80] ref P L0: 68.7% 15.8% 8.2% 1.4% 1.9% 2.3% 1.3% 0.3% 0.0% [libx264 @ 0x55cb6afacd80] ref B L0: 92.5% 6.3% 0.7% 0.3% 0.1% 0.1% 0.0% [libx264 @ 0x55cb6afacd80] ref B L1: 95.0% 5.0% [libx264 @ 0x55cb6afacd80] kb/s:6293.87 [aac @ 0x55cb6afc6bc0] Qavg: 233.656 

Ich habe während der Videokomprimierung die Warnmeldung "Mehr als 10000 Frames dupliziert" bemerkt. Am Ende habe ich eine MP4-Datei mit einer schrecklichen Qualität erhalten, ein ruckelndes Video mit vielen Boxen, die sich sehr vom Original unterscheiden (was ein sehr flüssiges Video mit einer diskreten Bildqualität war). Darüber hinaus ist die Größe der Ausgabedatei doppelt so groß wie die ursprüngliche (44 MB statt 22), daher verstehe ich nicht den Grund für diesen Mangel an Qualität.

Kannst du mir helfen?