[Pixman] Very Large PNGs

[Soeren Sandmann]

Frederik Ramm <frederik at remote.org> writes:

> Dear all,
>
>    I posted this to the cairo list but it occurred to me that this
> list is the appropriate place.
>
> I tried to convert an SVG file to a very large PNG with rsvg-convert.
> If the bitmap requested is larger than about 23k x 23k pixels, it is
> returned an invalid surface (where if you query the state it says "out
> of memory"), happily ignores that and continues to "render" which then
> results in no output.
>
> I then delved into the cairo library and found that it uses pixman to
> create the image; and pixman refuses to create an image that takes
> more than INT32_MAX raw bytes. With one pixel using 4 bytes, that
> means that an image with 23.000 x 23.000 pixels is still ok, but if it
> becomes any larger, that was it.

What pixman is trying to avoid is integer overflows. If you pass
an overflowed number to malloc(), you'll get buffer overruns which could
become security issues

Given that size_t on a 32 bit machine is 32 bits, the limit would be 4
GB in any case, but just to be on the safe side, pixman limits it to 2GB
so that the number will fit in a signed integer.

I think it should be safe to use size_t in create_bits() along with new
functions pixman_multiply/add_overflows_size() if you want to fix this.

However, not that pixman is still limited to 16.16 bits of coordinate
space in various places, so you wouldn't gain that much. This 16 bit
limitation is something that we would like to get rid of, however. 

Out of curiosity, exactly how large are the images you are trying to create?

> Which was a bit sad for me since the machine I was running this on
> could easily have created an image ten times as big, if only
> pixman/cairo had supported that.
>
> I briefly toyed with adding 64-bit support to the whole bundle but it
> became quickly apparent that this is more than an afternoon's job. I
> also tried to squeeze at least a little more out of the combo by using
> 24-bit images instead of 32-bit (no difference since 24-bit images are
> internally stored with 32 bit), and even the deprecated 16-bit data
> type (doesn't work since you cannot create a surface for that).

In cairo 1.10, you actually can create a 16 bit image, and it is stored
internally in a 16 bit data type.


Soren