The general concept behind the original Gravity Power was that the background was completely changable from the fighting that occurred on it. As a result, the collision maps between the players and the background had to be pixel-perfect, even to the extent of the background being deformed until the player became stationary (or until something exploded on it) - think of "Worms" here.
It may not look like much but the following screen demonstrates the following:
#1) images can be used to 'cut out' bits of the landscape
#2) sprites can interact with the landscape (the ship was falling +y until a basic pixel collision happened)
#3) the background can be loaded from an image
Behind each image a 2d mask has been made, these are used in collision tests and are also used as the cookie-cut templates.
One thing I've been (prematurely) pondering now is how to optimise the pixel-collision tests. Bear in mind that this game will see literally hundreds of small bullet-like projectiles flying around so I guarantee that slowdown will occur.
The next thing, however, is to implement a very simple sprite system and fire some bullets into the deformable terrain.
In other news, I released the NeHe news feed which is powered from our very own forum software. This is the first major change to NeHe in a year and will allow people to submit OpenGL news to keep the NeHe page fresh. Hurrah.