[cairo] Cairo efficiency with massively long lines

[Paul Davis]

Greetings Cairo folks. A few people here were incredibly helpful with my
last questions about planning the move of Ardour (http://ardour.org) to
a Cairo-based canvas. We've run into another issue with our current use
of GnomeCanvas, and I wanted to check that Cairo would handle it
properly before I waste time trying to find a workaround.

Will Cairo efficiently render a line that has two coordinates at zero
and (say) 2^32-1 or 2^64-1 (on the x-axis) ?

The current GnomeCanvas code generates a "micro-tile array" (UTA; a list
of 32x32 pixel cells) to describe the areas of the image that need to be
re-rendered, and with a line like this, the UTA is obvously huge - it
correctly reflects the fact that the area to be re-rendered is enormous.
As a result, when our canvas has numerous of these objects on it, any
operations that cause a recomputation of what needs to be rendered are
dog slow.

Does Cairo's internal implementation correctly handle this kind of
situation? Presumably, the correct limitation is to not compute a UTA
that extends beyond the current visible bounds of the
image/canvas/pixbuf. I'd appreciate clarification on how Cairo does
this.

many thanks,
--p