[cairo] Image fallback in PS backend

[Carl Worth]

On Fri, 21 Apr 2006 08:57:28 -0400 (EDT), Behdad Esfahbod wrote:
>
> Just wondering:  I see there are checks in the PS backend to
> short-circuit clear operations when we have not started a new
> page yet...  One can improve the analyze surface a bit, to allow
> OVER operations with transparency on a new page, even when the
> backend (PS) does not support transparent OVER.  This way we
> should get decent output for the minefield with PS backend, as
> the image is in the background and will be drawn before anything
> else, or that's what I assume :).

It is drawn at the lowest layer, but I don't think it's necessarily
the first element drawn on the page. So the silly little trick I did
for short-circuiting the initial CLEAR won't help here.

Instead, what I've envisioned for this is to extend the
characterization of operations that a backend can provide to the
analysis engine. Currently the information is just a Boolean,
(supported or unsupported). What we want here is to be able to say,
"I can support this operation only if there is nothing under it".

For the finer-than-full-page fallbacks we have planned, the analysis
engine will already be examining the intersections of the affected
regions of image operations. So once that's in place then adding
support for this optimization will be quite easy.

And I can imagine situations where it will be very important. For
example, imagine a 50% translucent full-page watermark and then a
bunch of text on top of it. That's the kind of thing we really would
want to preserve the text as text---and it's exactly this optimization
that will allow it.

-Carl
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