Concave shapes in Font

Maker Code-128 in Font Concave shapes

With convex and surface strategies, you can build very complex game environments. You can use the convex strategy to build the inner walls of a maze, and the surface strategy to build its outer walls. These techniques are extremely robust. They should be the first techniques you employ to build complex environments using bitmap collision. But there s one situation where these strategies can t help you. Imagine that the lander is flying around the asteroid and discovers that it s hollow inside. The lander enters the hollow area and flies into the very heart of the asteroid. This is a problem for us. The convex collision strategy depends on a vector between the center of the asteroid to the center of the lander. The vector is always pushing the lander away from the asteroid s center. If the lander touches the inner surface of the asteroid s cave, it will be pushed through the cave wall to the outside of the asteroid. Figure 5-11 illustrates what will happen.

Figure 5-11. Concave shapes pose a problem. The distance vector will push the lander away from the center, through the asteroid s wall. Obviously, this means we can t use the distance vector to separate the objects. We need another way to do it. What are our options Here are two contenders which, although promising, tripped over their ski poles in the qualifying round: Figure out whether the lander is inside or outside the asteroid, and reverse the direction of the separation accordingly. This might work, but how can we determine where the outside starts and in the inside ends It s a complex problem, and I haven t found a simple solution to it. Let me know if you think of one! Also, it might work with roundish or squarish shapes, but not complex maze structures with many inner compartments. Use the lander s motion vector. For example, if the lander is moving forward and bumps into the asteroid s wall, we can push it back in a direction opposite to its velocity. This actually works quite well, but it breaks down if the lander gets pushed diagonally into a corner formed by two adjoining pixels. It doesn t know which way to go. It either gets stuck or flits off the screen in a blip. This isn t a robust enough solution. And the winner is Bitmaps are made up of pixels. Pixels are just little squares. Squares have only four sides. That means that when two bitmaps collide, they re only ever touching on one of four sides. If you zoomed in really closely, you would see the flat square side of the first bitmap s pixels touching the flat square side of the other bitmap s pixels.

This simplifies things a lot. It means that we can think of a collision between bitmaps as a collision between squares. That should be an easy problem to solve. All we need to do is find out which of the four sides the collision is occurring on, and push the lander away in the opposite direction. We can t solve this problem with math, because we don t have any mathematical information about the bitmaps. And anyway, who really wants to do more math The solution is actually straightforward, and you know part of it already. The chapter s source files contain a working example. Find the Cave folder and run the SWF. The lander has found an entrance to a hidden cave below the planet s surface. It can fly anywhere inside it. Bump into any surface, and you ll see that the collision is clean and precise, as shown in Figure 5-12.