[Vala] [newbie] Help editing PNG images with Vala?

[Adam <quark comcast net>]

Hi!

I've been trying to learn Vala, and I've assigned myself a few tasks to 
get started. I'm pretty fluent in C# (.NET), so Vala syntax hasn't been 
much of a problem.

Currently, I'm trying to move beyond some of the basics, and write a 
command line program with the following requirements:
1. The program must edit a png file, modifying each pixel according to 
an internal algorithm.
2. The program must *not* load the entire file into the memory; it 
should only load and edit one row of pixels at a time, and save them 
after each edit. This will allow for editing huge files.
3. The program must be able to run on both Windows and Linux. I'm 
currently stuck in a Windows environment because my main box is having 
problems.

Can anyone help me get started with this? If I use libpng, do I need to 
write a .vapi file for it? Any particular challenges I will run into?

Thanks!