Comment 5 for bug 32963

This seems to be related to XV_GAMMA handling. There's a lack of documentation at what exactly the six registers do.

  $ sudo apt-get install xvattr

  $ xvattr | awk '/ Name/{printf$2}/Current/{printf"\t%08#x\n",$3}'
  XV_COLORKEY 0x0101fe
  XV_BRIGHTNESS 0xffffffffffffffff
  XV_CONTRAST 0x00007f
  XV_GAMMA0 0x00fe83
  XV_GAMMA1 0x00ff00
  XV_GAMMA2 0x00fe83
  XV_GAMMA3 0x00ff00
  XV_GAMMA4 0x00fe83
  XV_GAMMA5 0x00ff00

The Brightness and Constrast can be changed on-the-fly (while another program has the Xvideo port open) but the GAMMA values cannot. There's actually a comment saying that this is enforced:

  /* Avoid video anomalies, so set gamma registers when overlay is off */

Setting the gamma values with a command like:

  $ xvattr -a XV_GAMMA1 -v $((0x00ff00))

or:

  $ for G in XV_GAMMA{0,2,4} ; do xvattr -a $G -v $((0xff0000)) ; done

causes the display to go mostly RED, GREEN or BLUE. So it's clear that this is some sort of image/mask or multiple channel gamma ramp.

The values are initalised in a very mask-like way:

  ./src/i830_video.c: pPriv->gamma5 = 0xc0c0c0;
  ./src/i830_video.c: pPriv->gamma4 = 0x808080;
  ./src/i830_video.c: pPriv->gamma3 = 0x404040;
  ./src/i830_video.c: pPriv->gamma2 = 0x202020;
  ./src/i830_video.c: pPriv->gamma1 = 0x101010;
  ./src/i830_video.c: pPriv->gamma0 = 0x080808;

I do not understand the '0xc0' values since this covers two bits and the rest only cover 1 bit. It could be that these are a 6-step linear approximation to the desired gamma ramp. To support this there is various code like:

  pPriv->gamma0 = value;
  if (pPriv->gamma1 - pPriv->gamma0 > 0x7d)
    pPriv->gamma1 = pPriv->gamma0 + 0x7d;

However for a masked-value, "randomly" adding 0x7d in just the blue channel is tottally broken.