[GIMP] Question on transparency

[Václav]

I know that it can sound not-related to Simutrans - but it is. Or at least this should be moved to else topic - but I don't know to which (may be that to Documentiation and tutorials).

I began work on improves of one my ancient building for pak128. My main editor is one else software - but I export from it into PNG and in GIMP I re-assemble that building - because there I can make some final improves (like deleting of unwanted pixels on edges).

I use alfa-transparency to get some textures or at least colours. I exported wall of that building as two parts - when one part was transparent - and other part not. If both parts were exported at once, there are not night lighting pixels - but when those parts are exported separately - and re-assembled in GIMP, then there are some night lighting pixels - unwanted. But at glance result of both ways seems be the same.

It is incomprehensible for me.