FFmpeg installation and tutorial in Ubuntu~~~

1. Installation

Installable via PPA
sudo add-apt-repository ppa:kirillshkrogalev/ffmpeg-next

sudo apt-get update

sudo apt-get install ffmpeg

Check if the installation is successful:

ffmpeg -version

2. Related concepts

bit rate

Bit rate is a parameter that determines the overall quality of audio and video. He decides the number of bits processed per time unit, which is bit rate in English, a parameter describing how many KB are output per second, the unit is Kbps, that is, kbit/s, 8Kbit/s = 1KB/s . When compressing the same video, the larger the video encoding rate, the larger the file size and the better the picture quality.

MP3 generally uses a bit rate of 8~320kbps.

Set the bitrate:

The bit rate determines how many bits are needed to process a 1s encoded stream, set with the -b option. Use -b:a and -b:v to distinguish audio and video

For example: set the overall 1.5Mbit per second

ffmpeg -i file.avi -b 1.5M file.mp4

ffmpeg -i input.avi -b:v 1500K output.mp4

frame number

The number of pictures played per second , in fps (English: Frames Per Second), and the frame rate is the number of frames encoded into the video file per second . The human eye needs at least 15 frames per second to coherent images together. The unit of frame rate is HZ , and the LCD display generally has a flat rate of 60Hz. Higher frame rates result in smoother, more realistic animations. Generally speaking, 30fps is acceptable, but increasing the performance to 60fps can significantly improve the sense of interaction and realism, but generally speaking, it is not easy to notice a significant improvement in fluency beyond 75fps. If the frame rate exceeds the screen refresh rate it just wastes the power of the graphics processing, because the monitor can't update so fast, so the frame rate exceeding the refresh rate is wasted.

In the case of the same video and the same bit rate, the larger the number of frames, the worse the picture quality. Especially moving pictures. Because each picture will share the limited file volume per second, the more pictures, the more limited the content that each picture can express.

When the FPS of the picture reaches 60 frames per second, it can already meet the needs of most applications. Under normal circumstances, if you can ensure that the average FPS of the game screen can reach 30 frames/second, then the picture is basically smooth; if it can reach 50 frames/second, you can basically experience the feeling of flowing water. It's hard for the average person to tell the difference between 60 fps and 100 fps .

Frame rate settings

Use the -r option

ffmpeg -i input -r fps output

E.g:

ffmpeg -i input.avi -r 30 output.mp4

Resolution

The best concept to understand is the size of the screen, in pixels px.

The relationship with the encoding rate: the higher the resolution, the higher the encoding rate is required, because the details of the image are more, the required file size should also increase , otherwise it is not as good as the smaller picture, you will find that the same bit rate, the picture The larger the value, the more obvious the mosaic degree of the image.

Sampling Rate

每秒钟对音频信号的采样次数，采样频率越高声音还原度越高，声音更加自然。单位是赫兹 Hz。音频文件一般使用的采样率是 44100 Hz ，也就是一秒钟采样 44100 次，之所以使用这个数值是因为经过了反复实验，人们发现这个采样精度最合适，低于这个值就会有较明显的损失，而高于这个值人的耳朵已经很难分辨，而且增大了数字音频所占用的空间。我们所使用的CD的采样标准就是44.1k，目前44.1k还是一个最通行的标准。

三、常见用法

主要参数：

-i 设定输入流
-f 设定输出格式
-ss 开始时间
视频参数：
-b 设定视频流量，默认为200Kbit/s
-r 设定帧速率，默认为25
-s 设定画面的宽与高
-aspect 设定画面的比例
-vn 不处理视频
-vcodec 设定视频编解码器，未设定时则使用与输入流相同的编解码器
音频参数：
-ar 设定采样率
-ac 设定声音的Channel数
-acodec 设定声音编解码器，未设定时则使用与输入流相同的编解码器
-an 不处理音频

用法举例

显示视频信息

ffmpeg -i input.avi

1、格式转换

ffmpeg最常用功能就是格式转换，在这里要特别提的是，音、视频文件格式有两个容器格式（如mov、flv)与编码格式（如H.264）

ffmpeg -i input.flv output.mp4

2、尺寸变换

ffmpeg -i input.mp4 -s 640x360 output.mp4

3、剪切视频段

ffmpeg -i input.mp4 -ss 5 -t 10 output.mp4

上面的命令-ss 5指定从输入视频第5秒开始截取，-t 10指明最多截取10秒。但是上面的命令可能会比较慢，更好的命令如下：

ffmpeg -ss 5 -i input.mp4 -t 10 -c:v copy -c:a copy output.mp4

上面的命令把-ss 5放到-i前面，与原来的区别是，这样会先跳转到第5秒在开始解码输入视频，而原来的会从开始解码，只是丢弃掉前5秒的结果。而-c:v copy -c:a copy标示视频与音频的编码不发生改变，而是直接复制，这样会大大提升速度，因为这样就不需要完全解码视频（视频剪切也不需要完全解码）

注意：-vcodec 有一个缩写叫做 -c:v ， -acodec 有一个缩写叫做 -c:a 。

4、改变FPS

FFmpeg可以用于降低或提高视频的帧率，因为信息丢失不可逆法则，提高帧率只会简单地让某些帧的画面多重复一次或多次，所以提高帧率不会提高画质。

ffmpeg -i input.mp4 -r 30 output.mp4

上面的命令，不论原始视频帧率是多少，输出视频都会是30帧每秒。这种情况之下视频的时间轴不会变化，不会有慢动作或快动作的效果。

5、截取图片

视频10秒的地方(-ss 参数)截取一张1920x1080尺寸大小的，格式为jpg的图片 -ss后跟的时间单位为秒

ffmpeg -i input_video.mp4 -y -f image2 -t 0.001 -ss 10 -s 1920x1080 output.jpg

把视频的前30帧转换成一个Gif

ffmpeg -i input_video.mp4 -vframes 30 -y -f gif output.gif

将视频转成 gif

ffmpeg -ss 00:00:00.000 -i input.mp4 -pix_fmt rgb24 -r 10 -s 320x240 -t 00:00:10.000 output.gif

将输入的文件从(-ss)设定的时间开始以10帧频率，输出到320x240大小的 gif 中，时间长度为-t 设定的参数。通过这样转换出来的 gif 一般都比较大，可以使用 ImageMagick 来优化图片的大小。

四、转码时输出信息

frame=   28 fps=0.0 q=0.0 size=       2kB time=00:00:01.49 bitrate=  11.3kbits/s
frame=   30 fps= 17 q=-0.0 size=      13kB time=00:00:01.49 bitrate=  71.1kbits/
frame=   34 fps= 15 q=-0.0 size=      20kB time=00:00:01.66 bitrate=  99.9kbits/
frame=   38 fps= 13 q=-0.0 size=      31kB time=00:00:01.83 bitrate= 138.1kbits/
frame=   42 fps= 12 q=-0.0 size=      40kB time=00:00:02.00 bitrate= 165.1kbits/
frame=   46 fps= 11 q=-0.0 size=      49kB time=00:00:02.17 bitrate= 185.4kbits/
frame=   50 fps= 10 q=-0.0 size=      57kB time=00:00:02.34 bitrate= 199.3kbits/
frame=   54 fps= 10 q=-0.0 size=      63kB time=00:00:02.51 bitrate= 204.9kbits/
frame=   58 fps=9.5 q=-0.0 size=      74kB time=00:00:02.68 bitrate= 226.2kbits/
frame=   62 fps=9.2 q=-0.0 size=      85kB time=00:00:02.68 bitrate= 260.5kbits/
frame=   65 fps=8.8 q=-0.0 size=      92kB time=00:00:02.85 bitrate= 264.9kbits/

FFmpeg 确实不会显示进度条和百分比，不过，它会给你比进度条和百分比还要多的信息。

最左边的 frame= 65 是转码所进行到的帧数，显示 65 就表示现在已经转到了第 65 帧。
第二个 fps=8.8 中的 FPS 就是 Frame per Second ，也就是现在电脑每秒所处理的帧的数量。注意这个数字跟视频的帧率并无关系。
其实我也不知道后面那个 `q=-0.0` 是什么意思。
接下来的 size= 92kB 表示现在已经转换出来的视频的体积，这个数字只会越变越大啊。
第五个 time=00:00:02.85 顾名思义就是时间了，它是已经转换出来的视频的时间。在我看来，它也是一个比百分比进度条更加精准的进度显示。

参考博文：https://blog.csdn.net/dianxin113/article/details/68486359