PNG libraries

[Stewart Gordon]

There's been some talk about image format libraries.  And the odd specific 
PNG library.

I've thought a bit about specific PNG libraries lately, considering such 
things as the extensibility of the format, which I'm thinking of making use 
of.

Libpng is the reference implementation of PNG.  The C-based API seems on the 
cumbersome side.  I've a recollection of hearing that libpng has an OO C++ 
interface, but if there is an official one then I can't seem to find it.  I 
have, however, found png++, which is a layer over libpng but seems to be at 
an early stage of development.

http://savannah.nongnu.org/projects/pngpp

The other one I've noticed is Lodepng, which was written in C++ but has been 
ported to D.  It supports a small handful of ancillary chunks, but still 
doesn't provide access to any chunks it doesn't understand or even preserve 
them between loading and saving.

I think D could benefit from a full-featured PNG editing library.  I can see 
a few possible approaches:

(a) Get together some bindings for libpng, and write an OO layer in D on top 
of it.  OOTB, libpng uses setjmp for error handling - we'd need to change 
this.

(b) Improve one of the libraries such as Lodepng.  I don't know to what 
extent that would work.

(c) Write a PNG library from scratch, designed from the start to support 
editing rather than merely loading and saving the image data itself.  I've 
got some design ideas already.  We could still use existing compression and 
CRC implementations.

FWIW I've written a program of just under 200 lines that generates a PNG. 
The encoding logic itself is less than 100 lines (with the help of 
std.zlib), but that's only to encode 24-bit RGB.  Obviously a PNG library of 
any sort would need to support the whole range of colour models and depths.

Thoughts?

Stewart.