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
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