本文已参与「新人创作礼」活动， 一起开启掘金创作之路。
ffmpeg官网：ffmpeg.org/ ffmpeg是用来处理音视频图像的最流行的工具，它的功能非常强大
首先介绍一下视频基础服务知识
视频中的视频和音频信息称之为stream，分为看下视频和音频有哪些属性
视频信息
- 常见的视频格式format：mp4、mov、flv、avi、hls等
- 常见的视频编码格式codec：codec_name 如H264、H265、vp8、vp8、av1
- 像素格式：pix_fmt 比如 yuv420p
- 宽、高、时长：width、height、duration
- 码率：bitrate= size*8/duration
- 帧率：fps
音频信息
- 音频编码codec：codec_name 如aac、pcm、mp3
- 音频声道数：channels
- 音频采样：sample_rate
获取视频format
ffprobe -loglevel error -print_format json -show_format input.mp4
{
"format": {
"filename": "ounewse111.mp4",
"nb_streams": 2,
"nb_programs": 0,
"format_name": "mov,mp4,m4a,3gp,3g2,mj2",
"format_long_name": "QuickTime / MOV",
"start_time": "0.000000",
"duration": "9202.370000",
"size": "528909162",
"bit_rate": "459802",
"probe_score": 100,
"tags": {
"major_brand": "isom",
"minor_version": "512",
"compatible_brands": "isomiso2avc1mp41",
"encoder": "Lavf58.29.100"
}
}
}
输出结果格式为json，读取到文本后可以转换成json进行解析
获取视频宽高，读取stream
ffprobe -loglevel error -print_format json -show_format -show_streams input.mp4
{
"streams": [
{
"index": 0,
"codec_name": "h264",
"codec_long_name": "H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10",
"profile": "High",
"codec_type": "video",
"codec_time_base": "1/60",
"codec_tag_string": "avc1",
"codec_tag": "0x31637661",
"width": 854,
"height": 480,
"coded_width": 864,
"coded_height": 480,
"has_b_frames": 2,
"sample_aspect_ratio": "1280:1281",
"display_aspect_ratio": "16:9",
"pix_fmt": "yuv420p",
"level": 31,
"chroma_location": "left",
"refs": 1,
"is_avc": "true",
"nal_length_size": "4",
"r_frame_rate": "30/1",
"avg_frame_rate": "30/1",
"time_base": "1/90000",
"start_pts": 5940,
"start_time": "0.066000",
"duration_ts": 828201000,
"duration": "9202.233333",
"bit_rate": "385468",
"bits_per_raw_sample": "8",
"nb_frames": "276067",
"disposition": {
"default": 1,
"dub": 0,
"original": 0,
"comment": 0,
"lyrics": 0,
"karaoke": 0,
"forced": 0,
"hearing_impaired": 0,
"visual_impaired": 0,
"clean_effects": 0,
"attached_pic": 0,
"timed_thumbnails": 0
},
"tags": {
"language": "und",
"handler_name": "VideoHandler"
}
},
{
"index": 1,
"codec_name": "aac",
"codec_long_name": "AAC (Advanced Audio Coding)",
"profile": "LC",
"codec_type": "audio",
"codec_time_base": "1/44100",
"codec_tag_string": "mp4a",
"codec_tag": "0x6134706d",
"sample_fmt": "fltp",
"sample_rate": "44100",
"channels": 2,
"channel_layout": "stereo",
"bits_per_sample": 0,
"r_frame_rate": "0/0",
"avg_frame_rate": "0/0",
"time_base": "1/44100",
"start_pts": 0,
"start_time": "0.000000",
"duration_ts": 405824512,
"duration": "9202.369887",
"bit_rate": "65325",
"max_bit_rate": "65325",
"nb_frames": "396313",
"disposition": {
"default": 1,
"dub": 0,
"original": 0,
"comment": 0,
"lyrics": 0,
"karaoke": 0,
"forced": 0,
"hearing_impaired": 0,
"visual_impaired": 0,
"clean_effects": 0,
"attached_pic": 0,
"timed_thumbnails": 0
},
"tags": {
"language": "und",
"handler_name": "SoundHandler"
}
}
],
"format": {
"filename": "ounewse111.mp4",
"nb_streams": 2,
"nb_programs": 0,
"format_name": "mov,mp4,m4a,3gp,3g2,mj2",
"format_long_name": "QuickTime / MOV",
"start_time": "0.000000",
"duration": "9202.370000",
"size": "528909162",
"bit_rate": "459802",
"probe_score": 100,
"tags": {
"major_brand": "isom",
"minor_version": "512",
"compatible_brands": "isomiso2avc1mp41",
"encoder": "Lavf58.29.100"
}
}
}
同时读取视频的format和stream，整个视频meta信息已经获取到了，index位流的序号，codec_type=video是视频信息，codec_type=audio是音频信息
说一下视频Stream的SAR/DAR/PAR
sample_aspect_ratio sar像素的采样宽高比，这个值不是固定的1:1
display_aspect_ratio dar视频播放窗口宽高比
par 视频宽和高，像素数量之比，就是看到width和height
视频信息中的width和height，并不一定是视频真实播放的实际大小，需要根据sample_aspect_ratio进行调整
可以得出：DAR=PAR*SAR，dar才是视频播放的真实效果
说一下视频r_frame_rate和avg_frame_rate
avg_frame_rate是实际平均帧率，也就是总帧数/总时长
r_frame_rate是最大的帧率，也就是说这个视频中帧率最高的时间点就是这个值
nb_frames 则为总帧数
这两个值不一样的原因是动态帧率，帧率不是恒定的，可以根据视频画面的运动程度调整，运动程度大或镜头场景切换快的时候帧率就高，镜头场景切换慢的时候帧率就低，这样优点就是既能优化视频的大小，也能保留视频的画质
只读取一个stream
ffprobe -loglevel error -print_format json -select_streams 0 -show_streams input.mp4
select_streams可以选择其中一个，参数是stream的index，一般视频流为0，音频流为1
读取stream中的frame数量
ffprobe -loglevel error -print_format json -count_frames -show_streams input.mp4
{
"streams": [
{
"index": 0,
"codec_name": "h264",
"codec_long_name": "H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10",
"profile": "High",
"codec_type": "video",
"codec_time_base": "1/60",
"codec_tag_string": "avc1",
"codec_tag": "0x31637661",
"width": 854,
"height": 480,
"coded_width": 864,
"coded_height": 480,
"has_b_frames": 2,
"sample_aspect_ratio": "1280:1281",
"display_aspect_ratio": "16:9",
"pix_fmt": "yuv420p",
"level": 31,
"chroma_location": "left",
"refs": 4,
"is_avc": "true",
"nal_length_size": "4",
"r_frame_rate": "30/1",
"avg_frame_rate": "30/1",
"time_base": "1/15360",
"start_pts": 0,
"start_time": "0.000000",
"duration_ts": 460800,
"duration": "30.000000",
"bit_rate": "842240",
"bits_per_raw_sample": "8",
"nb_frames": "900",
"nb_read_frames": "900",
"disposition": {
"default": 1,
"dub": 0,
"original": 0,
"comment": 0,
"lyrics": 0,
"karaoke": 0,
"forced": 0,
"hearing_impaired": 0,
"visual_impaired": 0,
"clean_effects": 0,
"attached_pic": 0,
"timed_thumbnails": 0
},
"tags": {
"language": "und",
"handler_name": "VideoHandler"
}
},
{
"index": 1,
"codec_name": "aac",
"codec_long_name": "AAC (Advanced Audio Coding)",
"profile": "LC",
"codec_type": "audio",
"codec_time_base": "1/44100",
"codec_tag_string": "mp4a",
"codec_tag": "0x6134706d",
"sample_fmt": "fltp",
"sample_rate": "44100",
"channels": 2,
"channel_layout": "stereo",
"bits_per_sample": 0,
"r_frame_rate": "0/0",
"avg_frame_rate": "0/0",
"time_base": "1/44100",
"start_pts": 0,
"start_time": "0.000000",
"duration_ts": 1323000,
"duration": "30.000000",
"bit_rate": "128538",
"max_bit_rate": "128538",
"nb_frames": "1293",
"nb_read_frames": "1292",
"disposition": {
"default": 1,
"dub": 0,
"original": 0,
"comment": 0,
"lyrics": 0,
"karaoke": 0,
"forced": 0,
"hearing_impaired": 0,
"visual_impaired": 0,
"clean_effects": 0,
"attached_pic": 0,
"timed_thumbnails": 0
},
"tags": {
"language": "und",
"handler_name": "SoundHandler"
}
}
]
}
nb_frames是meta信息中的frame数量，可能存在不可解码的
nb_read_frames是实际读取可解码的frame数量
下一章介绍stream下的packet和frame信息
