C# Top down movement limitation

Question

I'm using C# and Visual Studios 2010 with XNA 4.0+lua, and my teacher has asked us to make a 2D RPG game similar to zelda(top down). I was aiming to use just one image for each map region, instead of using the tile approach, however i'm stuck on the aspect of movement limitation.

My teacher mentioned being able to mask another image on top of the map's current image, using only back and white colors and using those to be able control where the avatar can move(move where black is and restrict movement where white is). He did not, however, instruct us on how to do this, and I have been unable to find any tutorials anywhere.

I'm fairly new to C# in general, so if anyone can point me in the direction of a tutorial that could help me accomplish this, or add their own advice I would really, greatly appreciate it.

Answer 1

You can use the Texture2D's GetData method to access the colour of a pixel on a texture: http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.graphics.texture2d.getdata.aspx

So... using your method, you would load 2 textures, one that contains the colour image you want, and a matching black and white (2 colours, not grey-scale) texture that will be your "collision map" for each region. You can use the GetData method to determine whether a particular (x,y) location is black or white (walkable or not) on the collision map.

You will want to test one pixel on each edge of your character. Good choices for these test pixels would be the centres of each edge, or the corners. Depends on how you want it to look and feel. Testing these 4 pixels every time you move should tell you if you will be walking into a wall. This same set of tests will need to happen for any projectiles or bad guys you create.

One problem you may come across is called "Tunnelling". Tunnelling occurs when an object is moving fast enough to "tunnel" through a wall without noticing the wall. To prevent this, make sure your walls are never really thin, and avoid fast-moving objects. There are ways to properly handle this, but they are more complex and unnecessary for a project of this magnitude. You can research more about tunnelling if you come across this problem, or want to learn more about how to prevent it properly.