Terrain minimap in OpenGL?

Question

So I have what is essentially a game... There is terrain in this game. I'd like to be able to create a top-down view minimap so that the "player" can see where they are going. I'm doing some shading etc on the terrain so I'd like that to show up in the minimap as well. It seems like I just need to create a second camera and somehow get that camera's display to show up in a specific box. I'm also thinking something like a mirror would work.

I'm looking for approaches that I could take that would essentially give me the same view I currently have, just top down... Does this seem feasible? Feel free to ask questions... Thanks!

Answer 1

One way to do this is to create an FBO (frame buffer object) with a render buffer attached, render your minimap to it, and then bind the FBO to a texture. You can then map the texture to anything you'd like, generally a quad. You can do this for all sorts of HUD objects. This also means that you don't have to redraw the contents of your HUD/menu objects as often as your main view; update the the associated buffer only as often as you require. You will often want to downsample (in the polygon count sense) the objects/scene you are rendering to the FBO for this case. The functions in the API you'll want to check into are:

• glGenFramebuffersEXT
• glBindFramebufferEXT
• glGenRenderbuffersEXT
• glBindRenderbufferEXT
• glRenderbufferStorageEXT
• glFrambufferRenderbufferEXT
• glFrambufferTexture2DEXT
• glGenerateMipmapEXT

There is a write-up on using FBOs on gamedev.net . Another potential optimization is that if the contents of the minimap are static and you are simply moving a camera over this static view (truly just a map). You can render a portion of the map that is much larger than what you actually want to display to the player and fake a camera by adjusting the texture coordinates of the object it's mapped onto. This only works if your minimap is in orthographic projection.

Answer 2

Well, I don't have an answer to your specific question, but it's common in games to render the world to an image using an orthogonal perspective from above, and use that for the minimap. It would at least be less performance intensive than rendering it on the fly.