> 1) Can it work with popular IDEs, i.e. RubyMine?
It only cares when your Ruby file changes on disk, so it should work with any IDE that can save files. :)
> What graphic formats does it support, beside png?
jpg, gif, bmp, tga, psd, pic, and pnm. :) It does decoding through SDL for bmp files and stb_image for everything else.
> Does it support animations of any kind, or do I have to convert each frame into png and use manually switch over it in tick?
Currently you have to manage this yourself, but we're working on a system to deal with this for you.
> How does it affect performance?
First time you use a graphic, we load it from disk and upload it to a texture on the GPU. Next time you use it, it's already in texture memory and we just draw with it right away.
> Does it have support for texture maps/atlases or do I need to manually cut them myself, too?
You can use a texture atlas like a normal sprite and draw from a subrectangle of it right now. We're not confident that atlases are super important for simple 2D games, so this hasn't been a large priority for us. As an exception to this rule: font glyphs are packed into atlases behind the scenes on first use, because to render a string would be a _lot_ of texture swapping otherwise.
> I don't see it in tutorial. Is it possible to get delta time between ticks (like in Unity), or is it fixed?
It's fixed! The engine runs at 60fps (about 16 milliseconds a frame). We wanted you to not have to spend the effort to do all the math and management for arbitrary frame times.