我尝试使用 setpts 过滤器从第 100 帧开始将视频速度减慢到一半。这样做是可行的，但从第 100 帧开始，视频又出现了 100 个冻结丢失帧，之后视频又以半速继续播放。是我的命令错误还是 setpts 过滤器有问题？
命令：
ffmpeg -i ball.mp4 -vf "setpts='if(gte(N,100),2,1)'*PTS" ball_slow.mp4
命令行输出：
ffmpeg version N-91592-g87cc7e8d4e Copyright (c) 2000-2018 the FFmpeg developers
built with gcc 8.2.1 (GCC) 20180808
configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth
libavutil 56. 18.102 / 56. 18.102
libavcodec 58. 22.101 / 58. 22.101
libavformat 58. 17.101 / 58. 17.101
libavdevice 58. 4.101 / 58. 4.101
libavfilter 7. 26.100 / 7. 26.100
libswscale 5. 2.100 / 5. 2.100
libswresample 3. 2.100 / 3. 2.100
libpostproc 55. 2.100 / 55. 2.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'ball.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.17.100
Duration: 00:00:10.00, start: 0.066016, bitrate: 412 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720, 409 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
Metadata:
handler_name : VideoHandler
Stream mapping:
Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[libx264 @ 000001ceb370b4c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 000001ceb370b4c0] profile High, level 3.1, 4:2:0, 8-bit
[libx264 @ 000001ceb370b4c0] 264 - core 157 r2932 303c484 - H.264/MPEG-4 AVC codec - Copyleft 2003-2018 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'ball_slow.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.17.101
Stream #0:0(und): Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 1280x720, q=-1--1, 30 fps, 15360 tbn, 30 tbc (default)
Metadata:
handler_name : VideoHandler
encoder : Lavc58.22.101 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
frame= 47 fps=0.0 q=0.0 size= 0kB time=00:00:00.00 bitrate=N/A speed= 0x
frame= 52 fps= 40 q=29.0 size= 0kB time=-00:00:00.03 bitrate=N/A speed=N/A
frame= 97 fps= 53 q=29.0 size= 0kB time=00:00:01.46 bitrate= 0.3kbits/s speed=0.805x
frame= 203 fps= 74 q=29.0 size= 0kB time=00:00:05.00 bitrate= 0.1kbits/s dup=101 drop=0 speed=1.81x
frame= 261 fps= 80 q=29.0 size= 0kB time=00:00:06.93 bitrate= 0.1kbits/s dup=130 drop=0 speed=2.13x
frame= 303 fps= 80 q=29.0 size= 0kB time=00:00:08.33 bitrate= 0.0kbits/s dup=151 drop=0 speed=2.19x
frame= 351 fps= 82 q=29.0 size= 256kB time=00:00:09.93 bitrate= 211.2kbits/s dup=175 drop=0 speed=2.31x
frame= 395 fps= 82 q=29.0 size= 256kB time=00:00:11.40 bitrate= 184.0kbits/s dup=197 drop=0 speed=2.37x
frame= 431 fps= 81 q=29.0 size= 256kB time=00:00:12.60 bitrate= 166.5kbits/s dup=215 drop=0 speed=2.38x
frame= 469 fps= 81 q=29.0 size= 256kB time=00:00:13.86 bitrate= 151.3kbits/s dup=234 drop=0 speed=2.39x
frame= 515 fps= 81 q=29.0 size= 256kB time=00:00:15.40 bitrate= 136.2kbits/s dup=257 drop=0 speed=2.43x
frame= 559 fps= 82 q=29.0 size= 512kB time=00:00:16.86 bitrate= 248.7kbits/s dup=279 drop=0 speed=2.47x
frame= 599 fps= 81 q=-1.0 Lsize= 578kB time=00:00:19.86 bitrate= 238.3kbits/s dup=299 drop=0 speed=2.68x
video:570kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.370572%
[libx264 @ 000001ceb370b4c0] frame I:3 Avg QP:10.25 size: 3598
[libx264 @ 000001ceb370b4c0] frame P:154 Avg QP:20.98 size: 2032
[libx264 @ 000001ceb370b4c0] frame B:442 Avg QP:19.73 size: 587
[libx264 @ 000001ceb370b4c0] consecutive B-frames: 0.7% 2.3% 1.5% 95.5%
[libx264 @ 000001ceb370b4c0] mb I I16..4: 67.9% 28.6% 3.5%
[libx264 @ 000001ceb370b4c0] mb P I16..4: 0.2% 1.1% 0.7% P16..4: 2.0% 1.6% 0.6% 0.0% 0.0% skip:93.8%
[libx264 @ 000001ceb370b4c0] mb B I16..4: 0.0% 0.0% 0.0% B16..8: 3.3% 1.0% 0.2% direct: 0.1% skip:95.4% L0:43.3% L1:48.9% BI: 7.8%
[libx264 @ 000001ceb370b4c0] 8x8 transform intra:41.2% inter:7.4%
[libx264 @ 000001ceb370b4c0] coded y,uvDC,uvAC intra: 12.1% 0.6% 0.0% inter: 0.5% 0.0% 0.0%
[libx264 @ 000001ceb370b4c0] i16 v,h,dc,p: 87% 7% 6% 0%
[libx264 @ 000001ceb370b4c0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 16% 9% 74% 0% 0% 0% 0% 0% 0%
[libx264 @ 000001ceb370b4c0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 23% 18% 39% 4% 4% 3% 4% 3% 2%
[libx264 @ 000001ceb370b4c0] i8c dc,h,v,p: 94% 4% 3% 0%
[libx264 @ 000001ceb370b4c0] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 000001ceb370b4c0] ref P L0: 72.5% 2.9% 18.0% 6.6%
[libx264 @ 000001ceb370b4c0] ref B L0: 65.4% 28.1% 6.5%
[libx264 @ 000001ceb370b4c0] ref B L1: 94.2% 5.8%
[libx264 @ 000001ceb370b4c0] kb/s:233.59
答案1
假设你的时间基准是帧速率的倒数。
根据您的设定表达，
帧 99 将有新的 PTS 99 = 99/30 = 3.30 秒，
帧 100 将有新的 PTS 200 = 200/30 = 6.67 秒；帧 101 将有 6.73 秒。
一帧的 PTS 可以被认为是
timestamp of previous frame + inter-frame interval
对于半速，只需要将帧间间隔加倍即可。
因此，请使用
setpts='if(eq(N,0),0,PREV_OUTPTS+if(gte(N,100),2,1)*(PTS-PREV_INPTS))'