Question

I’ve been struggling with this for a while.

Presently, I have a grid of 100 by 100 tiles, which belong to a Map.

The Map implements IDrawable. I call Draw() and it draws itself at 0,0 which is fine.

However, I want to expand this to draw essentially a viewport. The player will be drawn on the screen in the middle, and thus I want to display say, 10 tiles in each direction (rather than the entire map).

I’m having trouble thinking up the architecture for this one. I’m in the mindset that things should draw themselves, ie I say player1.Draw() and it draws itself. This would have worked before, where it drew the player at x,y on the screen, but with a viewport it will no longer know where to draw itself.

So should the viewport be told to draw, and examine every object in the game and draw those which are visible? Should the map tiles be objects that are subjected to this? Or should the viewport intelligently draw the map by coupling both together?

I’d love to know how typical scrolling tile games accomplish this.

If it matters, I’m using XNA

Edit to add: Can you do graphics manipulation such as trying the HTML rendering approach, where you tell things to draw, and they return a graphic of themselves, and then the parent places the graphic in the correct location? I’m thinking, if I had 2 viewports side by side for splitscreen, how would I stop them drawing outside the edges?

Answer 1

Possible design:

1. There’s a 2D “world” that contains object instances.
2. “Object instance” is a sprite reference + its coordinates in the world.
3. When you draw scene, you request list of visible objects that exist in given 2D area, THEN you draw them.

With such design world can be very huge.

I’m in the mindset that things should draw themselves, ie I say player1.Draw() and it draws itself.

visible things should draw themselves. Objects outside of viewport are not visible.

, how would I stop them drawing outside the edges?

Not sure about XNA, but OpenGL has “scissors test”/”glViewport” and Direct3D 9 has “SetViewport” method that allows you to use part of the screen/window for rendering. There are also clipplanes and stencil buffer (using stencil for 2D clipping is overkill, though) You could also render to texture then render the texture. There are many ways to deal with this.

So should the viewport be told to draw, and examine every object in the game and draw those which are visible?

For a large world, you shouldn’t examine every object, because it will be slow. You should be able to find visible object without testing every one of them. For that you’ll need some kind of space partitioning – quad trees (because we are in 2D), k-d trees, etc. This way you should be able to handle few thousands (or even hundreds of thousands) of objects, as long as you don’t see them all at once.

Should the map tiles be objects that are subjected to this?

If you keep drawing invisible things, FPS will drop.

and they return a graphic of themselves

For 2D game this may be very slow. Remember KISS principle.