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  # How it works

  Asciinema project is built of 2 complementary pieces:

  * a command-line based terminal session recorder, `asciinema`,
  * a website with an API and a player at asciinema.org

  When you run `asciinema rec` in your terminal the recording starts, capturing
  all output that is being printed to your terminal while you're issuing the
  shell commands. When the recording finishes (by hitting <ctrl-d> or
  typing `exit`) then the captured output is uploaded to asciinema.org website
  and prepared for playback on the web.

  Here's a brief overview of how both these parts work.

  ## Recording

  You probably know `ssh`, `screen` or `script` command. Actually, Asciinema
  was inspired by `script` (and `scriptreplay`) commands. What you may not know
  is they all use the same UNIX system capability: [a
  pseudo-terminal](http://en.wikipedia.org/wiki/Pseudo_terminal).

  > A pseudo terminal is a pair of pseudo-devices, one of which, the slave,
  > emulates a real text terminal device, the other of which, the master,
  > provides the means by which a terminal emulator process controls the slave.

  Here's how terminal emulator interfaces with a user and a shell:

  > The role of the terminal emulator process is to interact with the user; to
  > feed text input to the master pseudo-device for use by the shell (which is
  > connected to the slave pseudo-device) and to read text output from the
  > master pseudo-device and show it to the user.

  In other words, pseudo-terminals give programs the ability to act as a
  middlemen between the user, the display and the shell. It allows for
  transparent capture of user input (keyboard) and terminal output (display).
  `screen` command utilizes it for capturing special keyboard shortcuts like
  <ctrl-a> and altering the output in order to display window
  numbers/names and other messages.

  Asciinema recorder does its job by utilizing pseudo-terminal for capturing
  all the output that goes to a terminal and saving it in memory (together with
  timing information). The captured output includes all the text and invisible
  escape/control sequences in a raw, unaltered form. When the recording session
  finishes it uploads the output to asciinema.org. That's all about "recording"
  part.

  For the implementation details check out [recorder source
  code](https://github.com/sickill/asciinema).

  ## Playback

  When asciinema.org accepts the upload of the captured output it saves it in a
  file. Now, as the output is a raw, unaltered stream of text and control
  sequences it can't be just played by incrementally printing text in proper
  intervals. It requires interpretation of [ANSI escape code
  sequences](http://en.wikipedia.org/wiki/ANSI_escape_code) in order to
  correctly display color changes, cursor movement and printing text at proper
  places on the screen.

  Escape sequence interpretation was initially handled by Asciinema's own VT100
  terminal emulation layer written in Javascript but was later replaced with
  [libtsm](http://www.freedesktop.org/wiki/Software/kmscon/libtsm/) based
  interpreter.  libtsm, "terminal-emulator state machine", is a wonderful, rock
  solid library created by David Herrmann that is meant to be used by terminal
  emulator authors and others in need of an escape sequence interpreter.

  asciinema.org pre-processes the captured stream with libtsm based converter and
  saves the result in a JSON file that contains simple representation of screen
  changes for each animation frame (for each line that was changed on the
  screen there is a string to be printed and color attributes for it). The
  player loads the JSON data and simply renders each change at a right time.

  The end result is a smooth animation with all text attributes (bold,
  underline, inverse, ...) and 256 colors perfectly rendered.

  For the implementation details check out [asciinema.org source
  code](https://github.com/sickill/asciinema.org).