color calibration and xvideo (xv)

[Graeme Gill]

Alex Deucher wrote:
> On most hardware the LUT only handles the graphics plane.  The overlay
> plane usually has it's own controls.  The are most often exposed as Xv
> attributes.  Use an app like xvattr to adjust them.

The sort of controls provided as xv attributes aren't comparable
to a calibration though, as far as I can tell. Typical controls
are

  XV_BRIGHTNESS (range -512 to 511)
  XV_CONTRAST (range 0 to 8191)
  XV_SATURATION (range 0 to 8191)
  XV_HUE (range 0 to 360)

and on some:

  XV_RED_INTENSITY (range -1000 to 1000)
  XV_GREEN_INTENSITY (range -1000 to 1000)
  XV_BLUE_INTENSITY (range -1000 to 1000)
  XV_GAMMA (range 100 to 1000)
  XV_COLORSPACE (range 0 to 1)

(there may be more I guess, these are what I found with a little Googling)

whereas calibration consists of R, G & B lookup curves that
compensate for response nonlinearity, as well as possibly
setting the white point and brightness.

It's not clear whether the above XV attributes actually have any
direct correspondence to device output response, or are purely
arbitrary, intended only for user subjective adjustment.

Even gamma is unclear, since technically this has to be a real
valued power, not an integer, so it doesn't seem possible to
even convert the calibration curves to the closest corresponding
XV attributes (or can someone point me to a definition of what
video transfer curves result from a given combination
of the above XV attributes ?)

Is the above a reflection of the hardware capability, or is
it a user control synthesis done in the driver software ?

Graeme Gill.