使用ffmpeg缩放webm文件时如何保持透明度

我正在使用 ffmpeg 来缩放我的 WEBM 文件，使用以下命令： ffmpeg -i in.webm -c:v libvpx -vf scale=100:100 out.webm 输出具有我预期的正确分辨率，但问题是透明度变成黑色背景。

有人可以给我一个解决方案。

非常感谢。

下面是操作日志：

ffmpeg version 3.4 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 7.2.0 (GCC)
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --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-cuda --enable-cuvid --enable-d3d11va --enable-nvenc --enable-dxva2 --enable-avisynth --enable-libmfx
  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
  libswscale      4.  8.100 /  4.  8.100
  libswresample   2.  9.100 /  2.  9.100
  libpostproc    54.  7.100 / 54.  7.100
Splitting the commandline.
Reading option '-v' ... matched as option 'v' (set logging level) with argument '56'.
Reading option '-i' ... matched as input url with argument 'in.webm'.
Reading option '-c:v' ... matched as option 'c' (codec name) with argument 'libvpx'.
Reading option '-vf' ... matched as option 'vf' (set video filters) with argument 'scale=320:240'.
Reading option 'out.webm' ... matched as output url.
Finished splitting the commandline.
Parsing a group of options: global .
Applying option v (set logging level) with argument 56.
Successfully parsed a group of options.
Parsing a group of options: input url in.webm.
Successfully parsed a group of options.
Opening an input file: in.webm.
[NULL @ 000002387e6322a0] Opening 'in.webm' for reading
[file @ 000002387e632ea0] Setting default whitelist 'file,crypto'
Probing matroska,webm score:100 size:2048
Probing mp3 score:1 size:2048
[matroska,webm @ 000002387e6322a0] Format matroska,webm probed with size=2048 and score=100
st:0 removing common factor 1000000 from timebase
[matroska,webm @ 000002387e6322a0] Before avformat_find_stream_info() pos: 634 bytes read:32768 seeks:0 nb_streams:1
[matroska,webm @ 000002387e6322a0] All info found
[matroska,webm @ 000002387e6322a0] stream 0: start_time: 0.000 duration: -9223372036854776.000
[matroska,webm @ 000002387e6322a0] format: start_time: 0.000 duration: 0.400 bitrate=1432 kb/s
[matroska,webm @ 000002387e6322a0] After avformat_find_stream_info() pos: 34843 bytes read:65536 seeks:0 frames:1
Input #0, matroska,webm, from 'in.webm':
  Metadata:
    ENCODER         : Lavf57.83.100
  Duration: 00:00:00.40, start: 0.000000, bitrate: 1432 kb/s
    Stream #0:0, 1, 1/1000: Video: vp8, 1 reference frame, yuv420p(progressive), 640x480, 0/1, SAR 1:1 DAR 4:3, 10 fps, 10 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      alpha_mode      : 1
      ENCODER         : Lavc57.107.100 libvpx
      DURATION        : 00:00:00.400000000
Successfully opened the file.
Parsing a group of options: output url out.webm.
Applying option c:v (codec name) with argument libvpx.
Applying option vf (set video filters) with argument scale=320:240.
Successfully parsed a group of options.
Opening an output file: out.webm.
[file @ 000002387e658b40] Setting default whitelist 'file,crypto'
Successfully opened the file.
detected 4 logical cores
Stream mapping:
  Stream #0:0 -> #0:0 (vp8 (native) -> vp8 (libvpx))
Press [q] to stop, [?] for help
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
    Last message repeated 4 times
[Parsed_scale_0 @ 000002387e718a60] Setting 'w' to value '320'
[Parsed_scale_0 @ 000002387e718a60] Setting 'h' to value '240'
[Parsed_scale_0 @ 000002387e718a60] Setting 'flags' to value 'bicubic'
[Parsed_scale_0 @ 000002387e718a60] w:320 h:240 flags:'bicubic' interl:0
[graph 0 input from stream 0:0 @ 000002387e743b00] Setting 'video_size' to value '640x480'
[graph 0 input from stream 0:0 @ 000002387e743b00] Setting 'pix_fmt' to value '0'
[graph 0 input from stream 0:0 @ 000002387e743b00] Setting 'time_base' to value '1/1000'
[graph 0 input from stream 0:0 @ 000002387e743b00] Setting 'pixel_aspect' to value '1/1'
[graph 0 input from stream 0:0 @ 000002387e743b00] Setting 'sws_param' to value 'flags=2'
[graph 0 input from stream 0:0 @ 000002387e743b00] Setting 'frame_rate' to value '10/1'
[graph 0 input from stream 0:0 @ 000002387e743b00] w:640 h:480 pixfmt:yuv420p tb:1/1000 fr:10/1 sar:1/1 sws_param:flags=2
[format @ 000002387e7fe1e0] compat: called with args=[yuv420p|yuva420p]
[format @ 000002387e7fe1e0] Setting 'pix_fmts' to value 'yuv420p|yuva420p'
[AVFilterGraph @ 000002387e634e60] query_formats: 4 queried, 3 merged, 0 already done, 0 delayed
[Parsed_scale_0 @ 000002387e718a60] w:640 h:480 fmt:yuv420p sar:1/1 -> w:320 h:240 fmt:yuv420p sar:1/1 flags:0x4
[libvpx @ 000002387e657fe0] v1.6.1
[libvpx @ 000002387e657fe0] --prefix=/Users/kyle/software/libvpx/win64/libvpx-1.6.1-win64 --target=x86_64-win64-gcc
[libvpx @ 000002387e657fe0] vpx_codec_enc_cfg
[libvpx @ 000002387e657fe0] generic settings
  g_usage:                      0
  g_threads:                    0
  g_profile:                    0
  g_w:                          320
  g_h:                          240
  g_bit_depth:                  8
  g_input_bit_depth:            8
  g_timebase:                   {1/30}
  g_error_resilient:            0
  g_pass:                       0
  g_lag_in_frames:              0
[libvpx @ 000002387e657fe0] rate control settings
  rc_dropframe_thresh:          0
  rc_resize_allowed:            0
  rc_resize_up_thresh:          60
  rc_resize_down_thresh:        30
  rc_end_usage:                 0
  rc_twopass_stats_in:          0000000000000000(0)
  rc_target_bitrate:            256
[libvpx @ 000002387e657fe0] quantizer settings
  rc_min_quantizer:             4
  rc_max_quantizer:             63
[libvpx @ 000002387e657fe0] bitrate tolerance
  rc_undershoot_pct:            100
  rc_overshoot_pct:             100
[libvpx @ 000002387e657fe0] decoder buffer model
  rc_buf_sz:                    6000
  rc_buf_initial_sz:            4000
  rc_buf_optimal_sz:            5000
[libvpx @ 000002387e657fe0] 2 pass rate control settings
  rc_2pass_vbr_bias_pct:        50
  rc_2pass_vbr_minsection_pct:  0
  rc_2pass_vbr_maxsection_pct:  400
[libvpx @ 000002387e657fe0] keyframing settings
  kf_mode:                      1
  kf_min_dist:                  0
  kf_max_dist:                  128
[libvpx @ 000002387e657fe0] 
[libvpx @ 000002387e657fe0] vpx_codec_enc_cfg
[libvpx @ 000002387e657fe0] generic settings
  g_usage:                      0
  g_threads:                    0
  g_profile:                    0
  g_w:                          320
  g_h:                          240
  g_bit_depth:                  8
  g_input_bit_depth:            8
  g_timebase:                   {1/10}
  g_error_resilient:            0
  g_pass:                       0
  g_lag_in_frames:              25
[libvpx @ 000002387e657fe0] rate control settings
  rc_dropframe_thresh:          0
  rc_resize_allowed:            0
  rc_resize_up_thresh:          60
  rc_resize_down_thresh:        30
  rc_end_usage:                 0
  rc_twopass_stats_in:          0000000000000000(0)
  rc_target_bitrate:            200
[libvpx @ 000002387e657fe0] quantizer settings
  rc_min_quantizer:             4
  rc_max_quantizer:             63
[libvpx @ 000002387e657fe0] bitrate tolerance
  rc_undershoot_pct:            100
  rc_overshoot_pct:             100
[libvpx @ 000002387e657fe0] decoder buffer model
  rc_buf_sz:                    6000
  rc_buf_initial_sz:            4000
  rc_buf_optimal_sz:            5000
[libvpx @ 000002387e657fe0] 2 pass rate control settings
  rc_2pass_vbr_bias_pct:        50
  rc_2pass_vbr_minsection_pct:  0
  rc_2pass_vbr_maxsection_pct:  400
[libvpx @ 000002387e657fe0] keyframing settings
  kf_mode:                      1
  kf_min_dist:                  0
  kf_max_dist:                  128
[libvpx @ 000002387e657fe0] 
[libvpx @ 000002387e657fe0] vpx_codec_control
[libvpx @ 000002387e657fe0]   VP8E_SET_CPUUSED:             1
[libvpx @ 000002387e657fe0]   VP8E_SET_ARNR_MAXFRAMES:      0
[libvpx @ 000002387e657fe0]   VP8E_SET_ARNR_STRENGTH:       3
[libvpx @ 000002387e657fe0]   VP8E_SET_ARNR_TYPE:           3
[libvpx @ 000002387e657fe0]   VP8E_SET_NOISE_SENSITIVITY:   0
[libvpx @ 000002387e657fe0]   VP8E_SET_TOKEN_PARTITIONS:    0
[libvpx @ 000002387e657fe0]   VP8E_SET_STATIC_THRESHOLD:    0
[libvpx @ 000002387e657fe0] Using deadline: 1000000
Output #0, webm, to 'out.webm':
  Metadata:
    encoder         : Lavf57.83.100
    Stream #0:0, 0, 1/1000: Video: vp8 (libvpx), 1 reference frame, yuv420p, 320x240 [SAR 1:1 DAR 4:3], 0/1, q=-1--1, 200 kb/s, 10 fps, 1k tbn, 10 tbc (default)
    Metadata:
      alpha_mode      : 1
      DURATION        : 00:00:00.400000000
      encoder         : Lavc57.107.100 libvpx
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
Clipping frame in rate conversion by 0.000008
[webm @ 000002387e656880] get_metadata_duration returned: 400000
[webm @ 000002387e656880] Write early duration from metadata = 400
[webm @ 000002387e656880] Writing block at offset 3, size 11223, pts 0, dts 0, duration 100, keyframe 1
[webm @ 000002387e656880] Writing block at offset 11233, size 1288, pts 100, dts 100, duration 100, keyframe 0
[webm @ 000002387e656880] Writing block at offset 12528, size 1504, pts 200, dts 200, duration 100, keyframe 0
[webm @ 000002387e656880] Writing block at offset 14039, size 2481, pts 300, dts 300, duration 100, keyframe 0
[out_0_0 @ 000002387e743d60] EOF on sink link out_0_0:default.
No more output streams to write to, finishing.
[webm @ 000002387e656880] end duration = 400
[webm @ 000002387e656880] stream 0 end duration = 400
frame=    4 fps=0.0 q=0.0 Lsize=      17kB time=00:00:00.30 bitrate= 457.8kbits/s speed=4.45x    
video:16kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 4.413191%
Input file #0 (in.webm):
  Input stream #0:0 (video): 4 packets read (34992 bytes); 4 frames decoded; 
  Total: 4 packets (34992 bytes) demuxed
Output file #0 (out.webm):
  Output stream #0:0 (video): 4 frames encoded; 4 packets muxed (16496 bytes); 
  Total: 4 packets (16496 bytes) muxed
4 frames successfully decoded, 0 decoding errors
[AVIOContext @ 000002387e698c20] Statistics: 14 seeks, 10 writeouts
[AVIOContext @ 000002387cc773e0] Statistics: 71649 bytes read, 0 seeks