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Update how-it-works doc section

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Marcin Kulik 2016-01-11 16:49:53 +01:00
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@ -41,38 +41,32 @@ transparent capture of user input (keyboard) and terminal output (display).
<kbd>Ctrl-A</kbd> 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
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.
finishes it uploads the output (in
[asciicast format](https://github.com/asciinema/asciinema/blob/master/doc/asciicast-v1.md))
to asciinema.org. That's all about "recording" part.
For the implementation details check out [recorder source
code](https://github.com/asciinema/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
As the recording is a raw 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 player comes with its own terminal emulator based on
[Paul Williams' parser for ANSI-compatible video terminals](http://vt100.net/emu/dec_ansi_parser).
It covers only the display part of the emulation as this is what the player is
about (input is handled by your terminal+shell at the time of recording anyway)
and its handling of escape sequences is fully compatible with most modern
terminal emulators like xterm, Gnome Terminal, iTerm, mosh etc.
The end result is a smooth animation with all text attributes (bold,
underline, inverse, ...) and 256 colors perfectly rendered.