weston/wcap

WCAP Tools

WCAP is the video capture format used by Weston (Weston CAPture).
It's a simple, lossless format, that encodes the difference between
frames as run-length encoded rectangles.  It's a variable framerate
format, that only records new frames along with a timestamp when
something actually changes.

Recording in Weston is started by pressing MOD+R and stopped by
pressing MOD+R again.  Currently this leaves a capture.wcap file in
the cwd of the weston process.  The file format is documented below
and Weston comes with the wcap-decode tool to convert the wcap file
into something more usable:

 - Extract single or all frames as individual png files.  This will
   produce a lossless screenshot, which is useful if you're trying to
   screenshot a brief glitch or something like that that's hard to
   capture with the screenshot tool.

   wcap-decode takes a number of options and a wcap file as its
   arguments.  Without anything else, it will show the screen size and
   number of frames in the file.  Pass --frame=<frame> to extract a
   single frame or pass --all to extract all frames as png files:

	[krh@minato weston]$ wcap-snapshot capture.wcap 
	wcap file: size 1024x640, 176 frames
	[krh@minato weston]$ wcap-snapshot capture.wcap 20
	wrote wcap-frame-20.png
	wcap file: size 1024x640, 176 frames

 - Decode and the wcap file and dump it as a YUV4MPEG2 stream on
   stdout.  This format is compatible with most video encoders and can
   be piped directly into a command line encoder such as vpxenc (part
   of libvpx, encodes to a webm file) or theora_encode (part of
   libtheora, encodes to a ogg theora file).

   Using vpxenc to encode a webm file would look something like this:

	[krh@minato weston]$ wcap-decode  --yuv4mpeg2 ../capture.wcap |
		vpxenc --target-bitrate=1024 --best -t 4 -o foo.webm  -

   where we select target bitrate, pass -t 4 to let vpxenc use
   multiple threads.  To encode to Ogg Theora a command line like this
   works:

	[krh@minato weston]$ wcap-decode ../capture.wcap  --yuv4mpeg2 |
		theora_encode - -o cap.ogv


WCAP File format

The file format has a small header and then just consists of the
individual frames.  The header is

	uint32_t	magic
	uint32_t	format
	uint32_t	width
	uint32_t	height

all CPU endian 32 bit words.  The magic number is

	#define WCAP_HEADER_MAGIC	0x57434150

and makes it easy to recognize a wcap file and verify that it's the
right endian.  There are four supported pixel formats:

	#define WCAP_FORMAT_XRGB8888	0x34325258
	#define WCAP_FORMAT_XBGR8888	0x34324258
	#define WCAP_FORMAT_RGBX8888	0x34325852
	#define WCAP_FORMAT_BGRX8888	0x34325842

Each frame has a header:

	uint32_t	msecs
	uint32_t	nrects

which specifies a timestamp in ms and the number of rectangles that
changed since previous frame.  The timestamps are typically just a raw
system timestamp and the first frame doesn't start from 0ms.

A frame consists of a list of rectangles, each of which represents the
component-wise difference between the previous frame and the current
using a run-length encoding.  The initial frame is decoded against a
previous frame of all 0x00000000 pixels.  Each rectangle starts out
with

	int32_t		x1
	int32_t		y1
	int32_t		x2
	int32_t		y2

followed by (x2 - x1) * (y2 - y1) pixels, run-length encoded.  The
run-length encoding uses the 'X' channel in the pixel format to encode
the length of the run.  That is for WCAP_FORMAT_XRGB8888, for example,
the length of the run is in the upper 8 bits.  For X values 0-0xdf,
the length is X + 1, for X above or equal to 0xe0, the run length is 1
<< (X - 0xe0 + 7).  That is, a pixel value of 0xe3000100, means that
the next 1024 pixels differ by RGB(0x00, 0x01, 0x00) from the previous
pixels.