使用FFmpeg合成WAV与PNG时输出视频不完整的问题

核心问题 #

1. concat demuxer的duration指令限制：duration仅对前一个文件生效，最后一张图的duration 7不会被FFmpeg识别，导致第6张图仅显示1帧（约0.04秒），视频实际时长仅为前5张的30秒。
2. -shortest参数截断音频：该参数强制输出时长与最短输入流一致，视频的30秒时长直接导致音频被同步截断。

解决方案 #

方案1：修正concat文件并调整命令 #

步骤1：修改concat列表文件

在最后添加一个空文件占位行（无需实际文件，仅用于触发最后一个duration生效）：

file '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide1.png'
duration 6
file '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide2.png'
duration 6
file '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide3.png'
duration 6
file '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide4.png'
duration 6
file '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide5.png'
duration 6
file '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide6.png'
duration 7
file '/dev/null'  # 占位用，触发最后一个duration生效

步骤2：调整FFmpeg命令

移除-shortest参数以保留完整音频，同时移除-strict experimental（FFmpeg 5.x中AAC编码器已稳定）：

'/opt/homebrew/Cellar/ffmpeg/5.1.2_6/bin/ffmpeg' -y -f concat -safe 0 -i '/private/var/folders/bb/m2b0ry595ys7bfs1r397lnf40000gp/T/Rtmp8gnKo7/ari_input_NQIg95oycVnb.txt' -i '/private/var/folders/bb/m2b0ry595ys7bfs1r397lnf40000gp/T/Rtmp8gnKo7/ari_audio_ngNm5vr2vFta.wav' -c:v libx264 -c:a aac -ac 2 -fps_mode auto -pix_fmt yuv420p -vf fps=5,"scale=trunc(iw/2)*2:trunc(ih/2)*2" -max_muxing_queue_size 9999 -threads 2 '/var/folders/bb/m2b0ry595ys7bfs1r397lnf40000gp/T//Rtmp8gnKo7/file46521ffd4ef8.mp4'

方案2：使用concat滤镜（无需中间文件，推荐） #

直接通过滤镜链处理每张图片的时长并拼接，彻底避免concat demuxer的限制：

'/opt/homebrew/Cellar/ffmpeg/5.1.2_6/bin/ffmpeg' -y \
-loop 1 -i '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide1.png' \
-loop 1 -i '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide2.png' \
-loop 1 -i '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide3.png' \
-loop 1 -i '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide4.png' \
-loop 1 -i '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide5.png' \
-loop 1 -i '/Users/howardbaek/Documents/sandbox/loqui/png_files/slide6.png' \
-i '/private/var/folders/bb/m2b0ry595ys7bfs1r397lnf40000gp/T/Rtmp8gnKo7/ari_audio_ngNm5vr2vFta.wav' \
-filter_complex "\
[0:v]trim=duration=6,setpts=PTS-STARTPTS[v0];\
[1:v]trim=duration=6,setpts=PTS-STARTPTS[v1];\
[2:v]trim=duration=6,setpts=PTS-STARTPTS[v2];\
[3:v]trim=duration=6,setpts=PTS-STARTPTS[v3];\
[4:v]trim=duration=6,setpts=PTS-STARTPTS[v4];\
[5:v]trim=duration=7,setpts=PTS-STARTPTS[v5];\
[v0][v1][v2][v3][v4][v5]concat=n=6:v=1:a=0[v];\
[v]fps=5,scale=trunc(iw/2)*2:trunc(ih/2)*2[vout]" \
-map "[vout]" -map 6:a \
-c:v libx264 -c:a aac -ac 2 -pix_fmt yuv420p \
-max_muxing_queue_size 9999 -threads 2 '/var/folders/bb/m2b0ry595ys7bfs1r397lnf40000gp/T//Rtmp8gnKo7/file46521ffd4ef8.mp4'

该命令会循环播放每张图片并按指定时长截取，拼接后自动调整帧率和分辨率，同时完整保留音频流。

内容的提问来源于stack exchange，提问作者Howard Baik