Ultra Fractal 3Apophysis

Flames are a variant of IFS fractals. Apophysis is a tool for designing flame fractals, which you can then paste into UF (since flame formulas have been written for UF). The advantage to pasting them into UF is you can layer them in UF along with other fractal formulas, and in theory render them at larger sizes.

The problem is that because of the way flame fractals are generated, the computer needs to have enough memory for the entire image. With other types of fractals, you can render an image bigger than you have memory for, because only a part of the fractal needs to fit into your computer's memory at any one time. But the flame technique (just because of how flames work) needs to fit it all in memory at once, so if you try to render a really big flame, you need lots of memory. If you have lots of flame layers, this becomes even more difficult.

This is why people say you can't render flames large enough for print. There are some other issues, such as flames looking slightly different on the large render (again, because of how flame fractals are generated), so flame prints aren't easy.

--Damien