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I was looking for a tutorial/book that would teach me how to start to use FFmpeg as a library (a.k.a. libav) and then I found the "How to write a video player in less than 1k lines" tutorial. Unfortunately it was deprecated, so I decided to write this one.

Most of the code in here will be in C but don't worry: you can easily understand and apply it to your preferred language. FFmpeg libav has lots of bindings for many languages like python, go and even if your language doesn't have it, you can still support it through the ffi (here's an example with Lua).

We'll start with a quick lesson about what is video, audio, codec and container and then we'll go to a crash course on how to use FFmpeg command line and finally we'll write code, feel free to skip directly to the section Learn FFmpeg libav the Hard Way.

Some people used to say that the Internet video streaming is the future of the traditional TV, in any case, the FFmpeg is something that is worth studying.

Table of Contents

• Intro
  • video - what you see!
  • audio - what you listen!
  • codec - shrinking data
  • container - a comfy place for audio and video
• FFmpeg - command line
  • FFmpeg command line tool 101
• Common video operations
  • Transcoding
  • Transmuxing
  • Transrating
  • Transsizing
  • Bonus Round: Adaptive Streaming
  • Going beyond
• Learn FFmpeg libav the Hard Way
  • Chapter 0 - The infamous hello world
    • FFmpeg libav architecture
  • Chapter 1 - timing
  • Chapter 2 - remuxing
  • Chapter 3 - transcoding

Intro

video - what you see!

If you have a sequence series of images and change them at a given frequency (let's say 24 images per second), you will create an illusion of movement. In summary this is the very basic idea behind a video: a series of pictures / frames running at a given rate.

<img src="https://upload.wikimedia.org/wikipedia/commons/1/1f/Linnet_kineograph_1886.jpg" title="flip book" height="280"></img>

Zeitgenössische Illustration (1886)

audio - what you listen!

Although a muted video can express a variety of feelings, adding sound to it brings more pleasure to the experience.

Sound is the vibration that propagates as a wave of pressure, through the air or any other transmission medium, such as a gas, liquid or solid.

In a digital audio system, a microphone converts sound to an analog electrical signal, then an analog-to-digital converter (ADC) — typically using pulse-code modulation (PCM) - converts the analog signal into a digital signal.

Source

codec - shrinking data

CODEC is an electronic circuit or software that compresses or decompresses digital audio/video. It converts raw (uncompressed) digital audio/video to a compressed format or vice versa. https://en.wikipedia.org/wiki/Video_codec

But if we chose to pack millions of images in a single file and called it a movie, we might end up with a huge file. Let's do the math:

Suppose we are creating a video with a resolution of 1080 x 1920 (height x width) and that we'll spend 3 bytes per pixel (the minimal point at a screen) to encode the color (or 24 bit color, what gives us 16,777,216 different colors) and this video runs at 24 frames per second and it is 30 minutes long.

toppf = 1080 * 1920 //total_of_pixels_per_frame
cpp = 3 //cost_per_pixel
tis = 30 * 60 //time_in_seconds
fps = 24 //frames_per_second

required_storage = tis * fps * toppf * cpp

This video would require approximately 250.28GB of storage or 1.19 Gbps of bandwidth! That's why we need to use a CODEC.

container - a comfy place for audio and video

A container or wrapper format is a metafile format whose specification describes how different elements of data and metadata coexist in a computer file. https://en.wikipedia.org/wiki/Digital_container_format

A single file that contains all the streams (mostly the audio and video) and it also provides synchronization and general metadata, such as title, resolution and etc.

Usually we can infer the format of a file by looking at its extension: for instance a video.webm is probably a video using the container webm.

FFmpeg - command line

A complete, cross-platform solution to record, convert and stream audio and video.

To work with multimedia we can use the AMAZING tool/library called FFmpeg. Chances are you already know/use it directly or indirectly (do you use Chrome?).

It has a command line program called ffmpeg, a very simple yet powerful binary. For instance, you can convert from mp4 to the container avi just by typing the follow command:

$ ffmpeg -i input.mp4 output.avi

We just made a remuxing here, which is converting from one container to another one. Technically FFmpeg could also be doing a transcoding but we'll talk about that later.

FFmpeg command line tool 101

FFmpeg does have a documentation that does a great job of explaining how it works.

# you can also look for the documentation using the command line

ffmpeg -h full | grep -A 10 -B 10 avoid_negative_ts

To make things short, the FFmpeg command line program expects the following argument format to perform its actions ffmpeg {1} {2} -i {3} {4} {5}, where:

1. global options
2. input file options
3. input url
4. output file options
5. output url

The parts 2, 3, 4 and 5 can be as many as you need. It's easier to understand this argument format in action:

# WARNING: this file is around 300MB
$ wget -O bunny_1080p_60fps.mp4 http://distribution.bbb3d.renderfarming.net/video/mp4/bbb_sunflower_1080p_60fps_normal.mp4

$ ffmpeg \
-y \ # global options
-c:a libfdk_aac \ # input options
-i bunny_1080p_60fps.mp4 \ # input url
-c:v libvpx-vp9 -c:a libvorbis \ # output options
bunny_1080p_60fps_vp9.webm # output url

This command takes an input file mp4 containing two streams (an audio encoded with aac CODEC and a video encoded using h264 CODEC) and convert it to webm, changing its audio and video CODECs too.

We could simplify the command above but then be aware that FFmpeg will adopt or guess the default values for you. For instance when you just type ffmpeg -i input.avi output.mp4 what audio/video CODEC does it use to produce the output.mp4?

Werner Robitza wrote a must read/execute tutorial about encoding and editing with FFmpeg.

Common video operations

While working with audio/video we usually do a set of tasks with the media.

Transcoding

What? the act of converting one of the streams (audio or video) from one CODEC to another one.

Why? sometimes some devices (TVs, smartphones, console and etc) doesn't support X but Y and newer CODECs provide better compression rate.

How? converting an H264 (AVC) video to an H265 (HEVC).

$ ffmpeg \
-i bunny_1080p_60fps.mp4 \
-c:v libx265 \
bunny_1080p_60fps_h265.mp4

Transmuxing

What? the act of converting from one format (container) to another one.

Why? sometimes some devices (TVs, smartphones, console and etc) doesn't support X but Y and sometimes newer containers provide modern required features.

How? converting a mp4 to a ts.

$ ffmpeg \
-i bunny_1080p_60fps.mp4 \
-c copy \ # just saying to ffmpeg to skip encoding
bunny_1080p_60fps.ts

Transrating

What? the act of changing the bit rate, or producing other renditions.

Why? people will try to watch your video in a 2G (edge) connection using a less powerful smartphone or in a fiber Internet connection on their 4K TVs therefore you should offer more than one rendition of the same video with different bit rate.

How? producing a rendition with bit rate between 964K and 3856K.

$ ffmpeg \
-i bunny_1080p_60fps.mp4 \
-minrate 964K -maxrate 3856K -bufsize 2000K \
bunny_1080p_60fps_transrating_964_3856.mp4

Usually we'll be using transrating with transsizing.