[cairo] Large image support?

[Ian Britten]

Hi all,
I was wondering what support, if any, Cairo might have for working
with large images?

I have two major (but distinct, I think) cases I need to address,
and am trying to figure out what I can leverage through Cairo, and
what I'll end up having to handle myself:

1) Rendering large images
    We can have lots of raster data to render, both in terms of
    large images, and/or many images to render.  Naively loading
    all the source pixels into Cairo image surface(s) and then
    asking Cairo to downsample to a particular size on the output
    surface will likely exceed the available memory.

2) Creating large images
    We have workflows where users need to generate very large
    high-res images (eg: 50000x50000 pixels).  Again in this case,
    simply trying to allocate a Cairo image surface that size will
    likely exceed the available memory.

I realize there's probably no simple silver-bullet solution to
these problems, and we'll have to do a lot on our end.  However,
I'd like to take advantage of whatever Cairo offers wherever
possible, rather than reinventing-the-wheel.  I still need/want
vector rendering (in addition to images), PDF output, and all
other stuff Cairo offers too...

For example, as a wacky idea for #1 - If all of Cairo could
access source pixels of an image via some sort of interface that
I could replace/extend (eg: image->getPixel(i,j)), then I could
maybe avoid loading the entire image at once, and just return
pixels as requested.  It'd be slow, but maybe would work for
large images.  Anyways, that's just an example/idea...

Similarly, if anyone out there is doing stuff like this, and has
any insight to offer, I'm open to any suggestions!
Many thanks for any info, pointers, etc!
Ian