1WCAP Tools 2 3WCAP is the video capture format used by Weston (Weston CAPture). 4It's a simple, lossless format, that encodes the difference between 5frames as run-length encoded rectangles. It's a variable framerate 6format, that only records new frames along with a timestamp when 7something actually changes. 8 9Recording in Weston is started by pressing MOD+R and stopped by 10pressing MOD+R again. Currently this leaves a capture.wcap file in 11the cwd of the weston process. The file format is documented below 12and Weston comes with the wcap-decode tool to convert the wcap file 13into something more usable: 14 15 - Extract single or all frames as individual png files. This will 16 produce a lossless screenshot, which is useful if you're trying to 17 screenshot a brief glitch or something like that that's hard to 18 capture with the screenshot tool. 19 20 wcap-decode takes a number of options and a wcap file as its 21 arguments. Without anything else, it will show the screen size and 22 number of frames in the file. Pass --frame=<frame> to extract a 23 single frame or pass --all to extract all frames as png files: 24 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 30 31 - Decode and the wcap file and dump it as a YUV4MPEG2 stream on 32 stdout. This format is compatible with most video encoders and can 33 be piped directly into a command line encoder such as vpxenc (part 34 of libvpx, encodes to a webm file) or theora_encode (part of 35 libtheora, encodes to a ogg theora file). 36 37 Using vpxenc to encode a webm file would look something like this: 38 39 [krh@minato weston]$ wcap-decode --yuv4mpeg2 ../capture.wcap | 40 vpxenc --target-bitrate=1024 --best -t 4 -o foo.webm - 41 42 where we select target bitrate, pass -t 4 to let vpxenc use 43 multiple threads. To encode to Ogg Theora a command line like this 44 works: 45 46 [krh@minato weston]$ wcap-decode ../capture.wcap --yuv4mpeg2 | 47 theora_encode - -o cap.ogv 48 49 50WCAP File format 51 52The file format has a small header and then just consists of the 53individual frames. The header is 54 55 uint32_t magic 56 uint32_t format 57 uint32_t width 58 uint32_t height 59 60all CPU endian 32 bit words. The magic number is 61 62 #define WCAP_HEADER_MAGIC 0x57434150 63 64and makes it easy to recognize a wcap file and verify that it's the 65right endian. There are four supported pixel formats: 66 67 #define WCAP_FORMAT_XRGB8888 0x34325258 68 #define WCAP_FORMAT_XBGR8888 0x34324258 69 #define WCAP_FORMAT_RGBX8888 0x34325852 70 #define WCAP_FORMAT_BGRX8888 0x34325842 71 72Each frame has a header: 73 74 uint32_t msecs 75 uint32_t nrects 76 77which specifies a timestamp in ms and the number of rectangles that 78changed since previous frame. The timestamps are typically just a raw 79system timestamp and the first frame doesn't start from 0ms. 80 81A frame consists of a list of rectangles, each of which represents the 82component-wise difference between the previous frame and the current 83using a run-length encoding. The initial frame is decoded against a 84previous frame of all 0x00000000 pixels. Each rectangle starts out 85with 86 87 int32_t x1 88 int32_t y1 89 int32_t x2 90 int32_t y2 91 92followed by (x2 - x1) * (y2 - y1) pixels, run-length encoded. The 93run-length encoding uses the 'X' channel in the pixel format to encode 94the length of the run. That is for WCAP_FORMAT_XRGB8888, for example, 95the length of the run is in the upper 8 bits. For X values 0-0xdf, 96the 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 98the next 1024 pixels differ by RGB(0x00, 0x01, 0x00) from the previous 99pixels. 100
