Video4Linux (or at least the bttv/Bt848 driver) supports setting a clipping bitmap, where a 1 in the bitmap means omit the video, a 0 means show the video. This is apparently an undocumented feature; I found it by reading the source. What is documented is that you can set a list of clipping rectangles, but the driver always turns those into a clipping bitmap anyway. There's no documentation (that I know of) but there is this in linux/videodev.h: #define VIDEO_CLIP_BITMAP -1 /* bitmap is 1024x625, a '1' bit represents a clipped pixel */ #define VIDEO_CLIPMAP_SIZE (128 * 625) You can set the clipping bitmap by setting "clipcount" in struct video_window to VIDEO_CLIP_BITMAP, and then by pointing "clips" to the actual bits. The bitmap is 1024 x 625, or VIDEO_CLIPMAP_SIZE in bytes. The bitmap is little endian; you can use bitmaps you get back from XGetImage on XFree86. Neat feature, but it does have limitations and bug(s). There seems to be a limit on how "narrow" the clipping can be. If you have a scanline in the bitmap with every-other bit on, it will generally not accurately clip to it. There is also a nasty limit on how complex the total clipping shape can be. For example, if you use a half-tone stipple pattern (every-other bit on, staggered lines) for the whole screen Video4Linux will lock up the machine, requiring a reboot. Now my question: does anyone out there have any experience with these problems? Is there a fix for the driver? Even knowing what the actual DMA engine limitations are would be useful. Thanks for any info, Peter K
