[What is scratchSurface ?]

When writing a file type plugin there is scratchSurface in OnSave method. Can anyone tell me what it is and what's for?

[FileType Plugin: Getting Started]

Hello guys,

I've tried the template above, but there are lots of things that I didn’t understand. What is scratchSurface in OnSave event and whyall the pixels there are the same color? What is the ProgressEventHandler and what SaveConfigToken is needed for? I’ve found that there are layers listed in the input Document, so do I have any method to merge them in one, or should I save them separately?

StillPaint.net is not open source I can’t understand how it works inside, so do you have any tutorial on how to write a filetype plugin?

Thanks in advance