This may be a feature request rather than a question, I really tried looking hard for a way myself.
I'm getting into wasting my free time trying to make a game (May as well be honest right? XD) and want to use TwistedBrush to make the assets. (Right now just working on basic stuff, but also want to make the animation in TB as well) My overall goal is to try and make a game using TB as much as possible.
The Pixilate mosaic 3 filter is quite cool for making some awesome retro-modern style game assets but it always makes black lines in the alpha sapce:
I want to not have that happen.
Ok so I will ask the silly question. Not have what happen? I see the result but I'm just not sure what you were going for. :)
I can appreciate your efforts to make a game. Much before Pixarra I had an even smaller company called Kamyan Software and focused on games and utilities for Windows (3.1). Yes that long ago. Pendulous was the most advanced game I made I suppose...fond memories.
Here is a nice review of it.
I'm making a platformer, so I will be making a lot of levels, however given I wanted to make a more classic style game and forgo HD graphics I wanted to make a richer environment and avoid using static repetitive tile sets, so I plan on making all the basic world elements like the above so that well the basic mock tiles look similar to a classic tile set they are all actually a bit unique.
The Pixilated Mosaic 3 seems like a pretty good starting point, as I can draw a quick rough of the level in a single color and than use the Pixilated Mosaic 3 filter to get prffect blocks with borders... nearly as it mocks up all the transparent space. I've found a couple other techniques were I can get close to the same results, but not nearly as good since I can't get exact pixel blocks like I do with Pixilated Mosaic 3.
You should just be able to mask the transparent areas. There are few ways to do that. Let me know if you need more details.