[cairo] Color, Cairo and Cups

[Billy Biggs]

Russell Shaw (rjshaw at netspace.net.au):

> Billy Biggs wrote:
> >Larry Ewing (lewing at gnome.org):
> >
> >>On Wed, 2006-03-08 at 18:13 -0500, Behdad Esfahbod wrote:
> >>
> >>>On Wed, 8 Mar 2006, Larry Ewing wrote:
> >>>
> >>>>Additionally if you ignore gamma (which Cairo does anyway
> >>>>right?) the working space doesn't matter too much for many
> >>>>operations.
> >>>
> >>>It does matter for any operation involving color blending, like
> >>>all operations with non-opaque alpha, and all with antialiasing
> >>>on.
> >>
> >>To clarify I was talking mainly about other RGB spaces here, not wildy
> >>different ones.
> >
> >  The problem is that Cairo currently deals with sRGB (gamma-corrected)
> >values, and compositing in a gamma-corrected colour space is wrong
> >and leads to artifacts.  This problem is known but fixing it is hard
> >(read: slow, and most other software also gets it wrong).
> 
> It's really insane to make the data non-linear for anything other than
> at the final output to a display device, or to linearize the input from
> an input device.

  I'm not sure exactly what you're getting at.

  I would agree that the idea of normalizing to the sRGB chromaticities
only makes sense if you're accepting floating-point linear RGB, assuming
that Larry has linear data.

  In general I think cairo needs to keep using 8-bit per channel sRGB
colours, with their specified gamma curve, since that's a reasonable
default for digital images and user interface colours.

  -Billy