- it must be compressed with zip format (so a zip compressor/decompressor is needed)
It is part of standard FPC library.
Where ? I only know about zstream, which allows to compress but not in the zip format.
- it uses PNG, which is not reliable on FreePascal
That's bad, but I think in that case fixing PNG support would be
a worthy goal on its own, wouldn't it?
Yes. Some people have already posted about errors.
Or another format to propose ?
There's GIMP's XCF, but AFAIK GIMP developers themselves
say that OpenRaster is preferable as an interchange format.
There is also PSP and PSD formats.
In fact, any of these format contains features that I do not plan to implement, and do not contains features that I have implemented (more blend modes). Maybe there are workarounds for it with OpenRaster by combining filters. Otherwise I should reduce available blend modes.
So I guess I will anyway make a reader/writer with a format that correspond to LazPaint data. But I would like to have maximum interoperability, so the best would be to handle OpenRaster, PSP and PSD too.