Microsoft XNA Game Studio 3.0 : Adding Bread to Your Game (part 3) - Strange Bounce Behavior, Strange Edge Behavior

5. Making the Cheese Bounce off the Bat

Now that you can detect when the cheese and the bread collide, you need to make the cheese "bounce" off the bat. Because the bread is horizontal, it makes sense to bounce the cheese up and down the screen so that whenever the cheese hits the bat, it reverses its movement in the Y direction. The code to achieve this is very simple; you do the same thing with the YSpeed as you do when the cheese hits the top or bottom of the game region:

if (cheese.SpriteRectangle.Intersects(bread.SpriteRectangle))
{
    cheese.YSpeed = cheese.YSpeed * -1;
}

This code can be placed at the end of the Update method to cause the cheese to bounce off the bat.

6. Strange Bounce Behavior

When you run the game, you find that it works well, and you can guide the cheese around the screen successfully. However, you make the mistake of letting your younger brother have a go, and he’s soon complaining that there’s a bug in your game. Sometimes the cheese gets "stuck" on the bread. You ask him to show you what happens, and it turns out that he’s right. It seems to happen when the bread is moving when it hits the cheese. The cheese travels along the bread, vibrating up and down as it moves. After some thought, you work out what’s causing the problem. Figure 3 shows what’s happening.

When the cheese rectangle and the bread rectangle intersect, the program reverses the direction of movement of the cheese. Normally, this means that the next time the position of the cheese is updated, it moves away from the bread, and the rectangles no longer intersect. However, if the cheese is moving down and the bread is moving up when they collide, the cheese goes so far "in" to the bread that, even after the cheese has been updated, the bread and cheese rectangles still intersect. If this is the case, the Update method reverses the vertical direction of movement of the cheese, causing the cheese to move back into the bread. This continues as the cheese moves along the bread, following the path shown in Figure 12-3, until it finally escapes off the end. There are a number of ways you can solve this problem:

The important thing to remember is that because you own the game universe, including what you say the game is supposed to do, you can change the rules to suit what your program does. The Great Programmer doesn’t have this freedom; usually she’s paid a large sum of money to create a solution that does what the customer wants. However, quite a few games have turned out the way they are because of the way the programmer made them work or because of a bug that turned out to make the game more fun. In this case, you decide to use the third approach and tell your younger brother that the game is meant to work like that, and he has found a secret feature.

7. Strange Edge Behavior

Your younger brother is now very pleased with himself and with you. He is pleased with you for making a game that rewards clever play and pleased with himself for finding this new trick in the game. However, this doesn’t last long because he soon comes back and tells you that he’s found a proper bug in the game. He can make the cheese go right off the screen and not come back. You ask him to show you, and sure enough, if he uses the bread to chase the cheese right to the top of the screen, he can send the cheese right off the screen. This is definitely a bug, and you can’t pass it off as a feature.

7.1. Debugging a Running Program

One of the great things about XNA Game Studio is that you can stop the game and take a look at what’s happening. Once you’ve persuaded your younger brother to make the problem happen, you can put a breakpoint into the program and stop it so that you can look at the values of the variables. You can do this even as the program is running, either on the Xbox, Zune or Windows PC

You can put a breakpoint in the Update method by clicking next to the line at which you want it to stop. XNA Game Studio indicates that a breakpoint has been set by highlighting the line, as shown in Figure 4.

The next time the program reaches this statement, it stops, and XNA Game Studio enters debugging mode. You can then look at the values of the variables to see what’s going wrong. When you take a look at the values in the cheese sprite, you find that the X coordinate value is fine, but the Y coordinate is –50, which is very wrong. The cheese Y coordinate should never get as low as this because the direction of the cheese movement should reverse when it reaches an edge. You take another look at the code that does this, and it looks sensible:

if (cheese.Y <= minDisplayY)
{
    cheese.YSpeed = cheese.YSpeed * -1;
}

If the cheese Y value becomes less than the minimum it’s allowed to have, the direction of movement is reversed to bring it back onto the screen. The program does this by multiplying the speed of the cheese by –1, which made perfect sense when you wrote it. You take a look at the cheese YSpeed and find that for the size of the screen you are using it has been calculated as 4. This means that next time the cheese is updated, the Y position of the cheese will be changed to –46 (which is still much lower than it’s supposed to be). The result is that the same condition triggers again, reversing the direction of the YSpeed and sending the Y position of the cheese back to –50. So the cheese remains forever off the screen, dancing backward and forward just out of view. The problem happens because the bread collision testing is performed after the cheese has been made to bounce when it hits the edge of the screen, so if the cheese repeatedly bounces off the bread when it’s on the edge of the screen, it can be made to vanish like this.

There are a number of ways you can fix this bug. You can stop the bread from going too close to the edges so that it can’t harass the cheese like this, or you can fix the bouncing problem of the cheese. You can’t really say that this behavior is a feature, although you could create a completely different game where the aim was to push all the objects off the screen, perhaps something called "Herd the Cheese" or "Sweep the Table." However, you decide to fix the problem.

The problem lies with the use of multiplication by –1 to change the direction of movement. If the next update brings the cheese back into the required range, then all is well, but if by some mischance it doesn’t, you get the dancing behavior that you’ve just uncovered.

The best way to fix this is to set the direction of movement of the cheese explicitly to the one in which you need it to go. Rather than bouncing, where you simply reverse the sign of the speed value, you should say, "If the cheese Y position is less than the limit, then make the movement positive so that this always brings the cheese back onto the screen." Even if the cheese Y position remains less than the limit next time, the movement will still be correct and result in the cheese heading in the right direction.

This turns out to be easy. You can use a method called Abs, which is provided by .NET. The Abs method is held in the Math class and returns the absolute value or magnitude of a number. The absolute value of a number is simply its value, if the number is zero or positive, or the opposite of its value if the number is negative. For example, the absolute value of –4 is 4. The Math class provides a number of static methods (which are always available) for use in your programs. The Math class is in the System namespace, so you can use it without having to add any using directives to your program. The code to deal with the Y position of the cheese ends up looking like this:

if (cheese.Y + cheese.SpriteRectangle.Height >= maxDisplayY)
{
    cheese.YSpeed = Math.Abs(cheese.YSpeed) * -1;
}

if (cheese.Y <= minDisplayY)
{
    cheese.YSpeed = Math.Abs(cheese.YSpeed) ;
}

If the cheese is too high, you make it move downward. If the cheese is too low, you make it move upward. Now there’s no way the cheese can get stuck off the screen.

Unfortunately it is still possible to move the bat off the screen, To solve this, you have to add code to limit the movement of the bread.