Session 10 Adding Gravity to Games

Transcription

1 Session 10 Adding Gravity to Games Authored by Brian Cullen (c) Copyright 2011 Computing At School. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. See for details.

2 Hypotenuse * cosine (Angle) Substitution Cypher Although this competition is called the code breaker competition we haven t talked about codes very much. As this the last session I want to introduce one of the simplest types of codes called a substitution cypher. In this type of cypher you simply replace each letter with another or a number. A simple example would be to replace each A with a 1, B with a 2, C with a 3 and so on. In pairs develop a substitution cypher and write a message using it (with at least 10 words). Make sure that you can decode the message properly and then swap with another group. Can you decode each other s messages? What do you think the weaknesses of this type of cypher are? Cannon Game Today the focus is going to be on getting the cannon ball moving and collision detection. The game will not be complete at that stage and it will be up to you to finish it but I ll mention that again at the end. For now let s get started Making the Cannon Ball Move Getting the cannon ball to move with gravity isn t too complicated but it does require a little bit of trigonometry. In particular you must be understand how you can calculate length of all the sides of a right angles triangle when you only have one angle and the length of the hypotenuse. The diagram below shows the formulae you need to use to do these calculations. Angle Hypotenuse * cosine (Angle) So why do we need to use this formula? To figure out how to move our cannon ball we must be able to calculate how far it moves in terms of X and Y coordinates. Therefore it is necessary for us to get the power and the angle the cannon ball is shot at and use this to calculate how fast it is move in the horizontally (X) and vertically (Y). This should be done in the constructor of the CannonBall class and we then need to store the results into properties of the class. The code to do so is shown below. protected int xspeed = 0; protected int yspeed = 0; public CannonBall (int angle, int power) power = (power * 10) / 25; xspeed = (int)(power*math.cos(math.toradians(angle))); yspeed = (int)(power*math.sin(math.toradians(angle))); The only thing you might not have expected in the constructor is where the value of power is changed. This is done because I felt that having the power at 100 was far too high but you can

3 change it to anything you think works well. Having figured out the cannon balls initial speed we now need to edit the act method to move it. To move it without worrying about gravity we can simply write the following in the act method. setlocation (getx() + xspeed, gety() yspeed); If you test this code the cannon ball should shoot off the screen in a straight line not very useful but a start. To start with we need to define the number we are going to use for gravity and rather than just write it into our formulae we are going to make a constant so we can use it again if necessary. To make a gravity constant add the following property to your CannonBall class. public static final int GRAVITY = 2; As this property is declared as final it means it cannot be changed by the program. Also notice that because this is a constant I have put it in all capitals. So now to take gravity into account in our act method simple add the following line to the end of the act method. This will slowly change the vertical speed of the cannon ball until it plummets off the bottom of the screen. yspeed = yspeed GRAVITY; To finish this class we need to think about when the cannon ball should be removed from the world. We will deal with what happen when it hits a wall or the target later but what if it doesn t hit either? The easiest condition to check for is if the cannon ball goes off the bottom of the screen (which gravity will eventually force it to do) then we should remove it. You should be able to write the code to do that by yourself by now but if you are stuck it is shown below and should be added to the act method in the CannonBall class. if (gety() > cannonworld.getheight()) cannonworld.removeobject(this); Notice how we use the cannonworld.getheight method rather than assuming the world is always going to be 400 pixels high. This means we could change the size of the world without having to change this code a definite advantage! Firing the Cannon Ball Having setup the cannon ball the last step is to fire it. This should be done from the Turret class just like how we fired rockets in the last game we made.

4 if (Greenfoot.isKeyDown( space )) CannonBall ball = new CannonBall (angle, power); cannonworld.addobject(ball, getx(), gety()); One problem I noticed when testing this is that it is almost impossible to fire a single cannon ball at a time and there are two obvious ways of solving this problem. The first is to use a delay similar to the one we used in the rocket game. The other option is to check if there are any other cannon balls in the world and if there are don t shoot again. This method does this by asking the world to give it a list of all the CannonBalls currently in it. It then returns a value of true if the list is empty (meaning that there are no cannon balls currently present). public Boolean nocannonballs () java.util.list balls = cannonworld.getobjects(cannonball.class); return balls.isempty(); We can now change the if statement from before so that we only fire a cannon ball if the space key is pressed AND there are no cannon balls. if (Greenfoot.isKeyDown( space ) && nocannonballs()) // Code to fire the cannon ball is as before Collision Detection As with the other games we can put the collision detection in more than one place and it doesn t really matter. To check if the cannon ball hits the wall we are going to put the collision detection into the act method of the Wall class. In this method we simply want to check if a cannon ball is touching one of the wall blocks and if it is remove it from the world. Actor cannonball = getoneintersectingobject(cannonball.class); if (cannonball!= null ) cannonworld.removeobject(cannonball); To check if the cannon ball has hit the target we will add some code to the act method of the Target class. If they have collided then we simply remove both the target and the cannon ball from the world to show that it has been hit.

5 Actor cannonball = getoneintersectingobject(cannonball.class); if (cannonball!= null ) cannonworld.removeobject(cannonball); cannonworld.removeobject(this); Test the code make sure that it works. If you want you can code to these blocks to use sound or other effects to show that it has been hit. Over to You This game is not complete and there are lots of things you could add. Two examples could be a count of the number of misses it takes for you to hit the target or resetting the world for a second round once you hit the first target. However from here on it is up to you. In general my advice is say/write what you want to do in English first and then worry about how to code it otherwise you may lose sight of what you are trying to do and REMEMBER TO COMMENT YOUR WORK! Happy Coding!

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