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Giving the Player Ship a Maximum Velocity
If you play with the code in Example 8-8 , you will notice two problems:
1. The ship can go off the sides of the screen and get lost.
2. The ship has no maximum speed.
We'll resolve the first issue when we start to code the complete game, but for now, let's look
at how to apply a maximum velocity to our current movement code. Suppose we give our
player ship a maximum acceleration of two pixels per frame. It's easy to calculate the current
velocity if we are moving in only the four primary directions: up, down, right, left. When we
are moving left or right, the movingY value will always be 0 . If we are moving up or down,
the movingX valuewillalwaysbe 0 .Thecurrentvelocitywearemovingononeaxiswouldbe
easy to compare to the maximum velocity.
But in our game, we are almost always moving in the x and y directions at the same time. To
calculate the current velocity and compare it to a maximum velocity, we must use a bit more
First, let's assume that we will add a maximum velocity variable to our game:
var maxVelocity = 2 ;
Next, we must make sure to calculate and compare the maxVelocity to the current velocity
before wecalculatethenew movingX and movingY values.Wewilldothiswithlocalvariables
used to store the new values for movingX and movingY before they are applied:
var movingXNew = movingX + thrustAcceleration * facingX ;
var movingYNew = movingY + thrustAcceleration * facingY ;
The current velocity of our ship is the square root of movingXNew^2 + movingYNew^2 :
var currentVelocity = Math . sqrt (( movingXNew * movingXNew ) +
( movingYNew * movingYNew ));
If the currentVelocity is less than the maxVelocity , we set the movingX and movingY val-
iif ( currentVelocity < maxVelocity ) {
movingX = movingXNew ;
movingY = movingYNew ;
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