Flash MX 2004 Games : Art to ActionScript

Chapter 18: Sports simulations
Having calculated the nearest player to the ball for both teams, these are stored in the variables
‘a’ and ‘b’. The code is designed so that a player can choose which team to use, then the choice
is stored in the variable ‘playersTeam’. If the opponent closest to the ball has changed after calling
the ‘nearestPlayer’ function then root level variables ‘opponentNo’ and ‘movecount’ are updated,
the function ‘opponentMove’ is executed on the opponent nearest to the ball and the current team
member nearest to the ball is stored in the root level variable ‘closestplayer’.
The opponentMove function
This function is quite complex and needs some careful study. The basic idea is that the player wants
to move towards the ball in such a direction that they can have a direct kick towards the correct
goal. The function starts by creating a vector from the goal to the ball. This vector needs to be
of length 1. To convert an arbitrary vector to a vector of length 1 in the same direction we must
normalize the vector; that is we must divide both the values by the same number, so it is then still
in the same direction, in such a way that if we draw the diagonal vector connecting the start of
the vector to the end the length will be 1. First we find out what the current length is using the
famous Pythagoras theorem. This value is stored in the variable ‘mag’. Now we divide both the
elements in the vector by this value, and hey presto we have a unit length vector. Next we move
along this vector three times the ball’s diameter and create a new target stored in the variables ‘gx’
and ‘gy’ which will be the perfect spot for our player to run and hit the ball to get a terrific goal
kick. Now we create two ramp values ‘a’ and ‘b’. ‘a’ is how much we want our player to run
towards our perfect spot to kick the ball, ‘b’ is how much we want our player to just run at the
ball. If we are within three times the ball’s diameter then we want the player to be more targeted
at hitting the ball the nearer they are. To make sure we show the appropriate images of the player
we must calculate the angle that the player will be running now that we have the information
needed. The player graphics contains 32 separate two-frame run cycles, rotating around a full 360
degrees. We can decide which to show on the basis of this angle. The angle is calculated using
the ‘atan’ function, which takes two sides of a triangle and returns an angle. This angle can be
converted into which frame to show using a division by the constant FRAMETORADIANS.
The player must avoid kicking the ball towards their own goal so if they are positioned in a line
comprising player, ball, own goal then they must run around the outside of the ball. This is done
by adjusting their angle; if this is the case, aiming towards the outside of a circle that is three times
the player’s diameter.
1 function opponentmove(){
2 //Create a vector from the goal to the ball
3 x = _root.Pitch.Ball._x - 260;
4 y = _root.Pitch.Ball._y - 0;
5 //Normalise this vector
6 //mag is the distance from the ball to the goal
7 mag = Math.sqrt(x*x + y*y);
8 x /= mag;
9 y /= mag;
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