3 WCAP is the video capture format used by Weston (Weston CAPture).
4 It's a simple, lossless format, that encodes the difference between
5 frames as run-length ecoded rectangles. It's a variable framerate
6 format, that only records new frames along with a timestamp when
7 something actually changes.
9 Recording in Weston is started by pressing MOD+R and stopped by
10 pressing MOD+R again. Currently this leaves a capture.wcap file in
11 the cwd of the weston process. The file format is documented below
12 and Weston comes with two tools to convert the wcap file into
13 something more usable:
15 - wcap-snapshot; a simple tool that will extract a given frame from
16 the capture as a png. This will produce a lossless screenshot,
17 which is useful if you're trying to screenshot a brief glitch or
18 something like that that's hard to capture with the screenshot tool.
20 wcap-snapshot takes a wcap file as its first argument. Without
21 anything else, it will show the screen size and number of frames in
22 the file. With an integer second argument, it will extract that
25 [krh@minato weston]$ wcap-snapshot capture.wcap
26 wcap file: size 1024x640, 176 frames
27 [krh@minato weston]$ wcap-snapshot capture.wcap 20
28 wrote wcap-frame-20.png
29 wcap file: size 1024x640, 176 frames
31 - wcap-decode; this is a copy of the vpxenc tool from the libvpx
32 repository, with wcap input file support added. The tool can
33 encode a wcap file into a webm video (http://www.webmproject.org/).
34 The command line arguments are identical to what the vpxenc tool
35 takes and wcap-decode will print them if run without any arguments.
37 The minimal command line requires a webm output file and a wcap
40 [krh@minato weston]$ wcap-decode -o foo.webm capture.wcap
42 but it's possible to select target bitrate and output framerate and
43 it's typically useful to pass -t 4 to let the tool use multiple
46 [krh@minato weston]$ wcap-decode --target-bitrate=1024 \
47 --best -t 4 -o foo.webm capture.wcap --fps=10/1
52 The file format has a small header and then just consists of the
53 indivial frames. The header is
60 all CPU endian 32 bit words. The magic number is
62 #define WCAP_HEADER_MAGIC 0x57434150
64 and makes it easy to recognize a wcap file and verify that it's the
65 right endian. There are four supported pixel formats:
67 #define WCAP_FORMAT_XRGB8888 0x34325258
68 #define WCAP_FORMAT_XBGR8888 0x34324258
69 #define WCAP_FORMAT_RGBX8888 0x34325852
70 #define WCAP_FORMAT_BGRX8888 0x34325842
72 Each frame has a header:
77 which specifies a timestamp in ms and the number of rectangles that
78 changed since previous frame. The timestamps are typically just a raw
79 system timestamp and the first frame doesn't start from 0ms.
81 A frame consists of a list of rectangles, each of which represents the
82 component-wise different between the previous frame and the current
83 using a run-length encoding. The initial frame is decoded against a
84 previous frame of all 0x00000000 pixels. Each rectangle starts out
92 followed by (x2 - x1) * (y2 - y1) pixels, run-length encoded. The
93 run-length encoding uses the 'X' channel in the pixel format to encode
94 the length of the run. That is for WCAP_FORMAT_XRGB8888, for example,
95 the length of the run is in the upper 8 bits. For X values 0-0xdf,
96 the length is X + 1, for X above or equal to 0xe0, the run length is 1
97 << (X - 0xe0 + 7). That is, a pixel value of 0xe3000100, means that
98 the next 1024 pixels differ by RGB(0x00, 0x01, 0x00) from the previous