Question

I’ve been developing a 2d RPG based on the LWJGL alongside with Java 1.6 for 3 months now. My next goal is to write all of the non-game-ish stuff. This includes menus, text input boxes, buttons and things like the inventory and character information screens. As I am a Computer Engineering student, I’m trying to write everything on my own (except, of course, for the OpenGL part of the LWJGL) so that I “test” myself on the writing of a simple 2d game engine.
I know that making such things from scratch requires basically mapping textures to quads (like the buttons), writing stuff on them and testing mouse/keyboard events which trigger other events inside the code.

The doubt I have is: should I use VBO’s (as I’m using for the actual game rendering) or Immediate Mode when rendering such elements? I don’t really know if Immediate Mode would be such a drop on performance. Another point is: do the interface elements have to be updated as fast as the game itself? I don’t think so, because nothing is actually moving… Are actual games made like that?

Answer 1

Immediate Mode is more straightforward for the task, you would not need to take care about caching and controls composition/batching. Performance dropoff is not that big, unless you render a lot of text (thousands of letters) with each glyph in a separate glBegin..glEnd. If you don’t use VBO anywhere else I would recommend trying it out for text output and doing everything else in easier Immediate Mode.

GUI elements might not change as often as game state does, but there’s a catch – you could need to update them each time there’s a cursor interaction (e.g. button gets OnMouseOver event and needs to be rendered with a highlight). These kind of events may happen very frequently, so thats why rendering may be needed at a full speed.