ABR (Average Bit Rate)轉好後的檔案的確降低 bitrate，但轉換後的檔案卻多出了 B-frame，如此可能會造成某些串流播放器播放時的困難。
ffmpeg -i input -c:v libx264 -b:v 256k output.mp4
以下記錄此問題的解決過程：
1. 只要轉檔時設定為 Baseline profile，便不會有 b-frame
摘錄 https://sites.google.com/site/linuxencoding/x264-ffmpeg-mapping
- --bframes
-bf
B-frames are a core element of H.264 and are more efficient in H.264 than any previous standard. Some specific targets, such as HD-DVD and Blu-Ray, have limitations on the number of consecutive B-frames. Most, however, do not; as a result, there is rarely any negative effect to setting this to the maximum (16) since x264 will, if B-adapt is used, automatically choose the best number of B-frames anyways. This parameter simply serves to limit the max number of B-frames. Note that Baseline Profile, such as that used by iPods, does not support B-frames. Recommended default: 16
原本作法
ffmpeg -i input -c:v libx264 -b:v 1000k output.mp4
此時 output.mp4 的 Encode Settings 如下:
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=15 / scenecut=40 / intra_refresh=0 / rc_lookahead=40 / rc=abr / mbtree=1 / bitrate=256 / ratetol=1.0 / qcomp=0.60 / qpmin=0 / qpmax=69 / qpstep=4 / ip_ratio=1.40 / aq=1:1.00
更改如下
ffmpeg -i 7h800-2.mp4 -c:v libx264 -b:v 256k -profile:v baseline output.mp4
此時 output.mp4 的 Encode Settings 如下:
cabac=0 / ref=3 / deblock=1:0:0 / analyse=0x1:0x111 / me=hex / subme=7 / psy=1 / psy_rd=1.00:0.00 / mixed_ref=1 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=0 / 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=0 / weightp=0 / keyint=250 / keyint_min=15 / scenecut=40 / intra_refresh=0 / rc_lookahead=40 / rc=abr / mbtree=1 / bitrate=128 / ratetol=1.0 / qcomp=0.60 / qpmin=0 / qpmax=69 / qpstep=4 / ip_ratio=1.40 / aq=1:1.00
若檔案有aac，則需要加上 -strict -2
ffmpeg -i 7h800-2.mp4 -c:v libx264 -strict -2 -b:v 256k -profile:v baseline output.mp4
使用命令如下 ffprobe -show_streams -count_frames -pretty filename， 檢視輸出結果中的 "has_b_frames"
其他參考資料：
