[cairo] XCopyArea and Cairo - trying to implement scrolling

Chris Wilson chris at chris-wilson.co.uk
Fri May 15 01:47:20 PDT 2015

On Thu, May 14, 2015 at 08:34:00PM -0700, Richard Billington wrote:
> I'm trying to translate a backend from using Xlib calls to using
> Cairo, where reasonable. The drawing operations I'm doing (text and
> graphics) are ignoring the xlib gcontext, and instead using the
> cairo context. However, when I try to use my cairo implementation of
> an xcopyarea-like function, I get some kind of event generated which
> I don't know how to handle. If I try to use xcopyarea (the xlib
> implementation), it "works" except that the background is always
> black, not the background it ought to be.
> I can go into more specifics, but I'm just wondering if there's any
> standard way of doing this. I found an archived discussion from many
> years ago about this subject, but I can't tell if there were
> patches/reimplementations done to better accommodate this nor if
> there's a standard approach.

Sure, the crux of the issue is that the self-copy case remains undefined
(it more or less boils down to not all hardware truly supports it). The
easiest no-nonsense definition would be that the source is treated as
static from the instance of being bound to the context (that is if it
maps to the cairo_get_target, then we immediately take a copy[-on-write?]).
That is overkill and prevents some optimisations for e.g. converting a
	cairo_set_source_surface(cairo_get_target(), 10, 10);
into a XCopyArea, so finding the right balance is hard (but I think that
is the right definition for the API!).

On the other hand, with the hypothetical cairo-surface API you would be
able to do the transformation into XCopyArea easily (since the definition
there would be an implicit copy-on-write for the source being passed
into the operation, and not at the context).

However, that all has to be compared to

expose-event (or preferrably repaint function after accumulating
	/* use double buffering */
	cairo_rectangle(redraw_x, redraw_y, redraw_w, redraw_h);
	.... bunch of rendering ...


and now in your scroll function you are free to use XCopyArea on the
underlying drawable. For completeness after using XCopyArea, call

Chris Wilson, Intel Open Source Technology Centre

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