Checking Conditions

Usually you'll want to have your animated object react to certain conditions. For example, in the two-axes demo it would be nice to have the object react to reaching the edge of the stage by bouncing off. Other conditions might be a user action, the passing of a certain amount of time, the state of another object you are animating, a random event, or many others.

Checking conditions- source movie

The first step is to express the condition in terms of the variables in your program. In the two-axes demo, you could express that the object has reached the bottom edge of the stage by saying the vertical position of the sprite is greater than the height of the stage, or:

sp.locv > (the stage).rect.height

•How would you express the condition that it reached the top of the stage?
•Right or left of the stage?
•Why is the comparison (>) operator used instead of equality?
  (think when velocity is greater than one)

The next step is to express the desired reaction in terms of the variables in your program. What should the object do when it reaches the bottom edge of the stage? It should start going up instead of down. This means that the vertical velocity should change, which is stored in the yVelocity variable.
      What should it be changed to? If you wanted you could make it take off like a rocket with yVelocity = -20, but since it was going 1 px/frame downward it would make sense if it started going 1 px/frame upward. This means changing yVelocity from 1 to -1 (remember locv gets smaller going upward). The opposite happens when the object reaches the top of the stage: yVelocity changes from -1 to 1. Both of these changes can be accomplished by the statement:

yVelocity = -yVelocity

•How would you express the reaction when the object reaches the right or left edge of the stage?

Finally, the condition and reaction statements are put into an if-then statement:

if sp.locv > (the stage).rect.height or sp.locv < 0 then
  yVelocity = -yVelocity
end if

Every time the object is moved, it has the potential to meet these conditions. Therefore, the conditions should be checked every time the object moves. The usual way to do this is to place the if-then in the same block of code where the object is moved. The full script:

•Why does the object move half-way off the stage before "bouncing"? How would you change the conditional (if-then) to keep that from happening?

•What would happen if you initialized xVelocity and yVelocity to random values between, say, 1 and 10?  i.e. "xVelocity = random(10)"

The type of conditions checked for in this script are called boundary conditions. They keep the value of a variable within certain bounds. In this case the values of the object's position are kept within the boundary of the stage. It is very common to check for boundary conditions.

There are many other ways to make the object react when it reaches the edge of the stage. For example, you could merely limit it to the boundary with

if sp.loch < 0 then sp.loch = 0
  
Or you could make it "wrap" around the stage with

if sp.loch < 0 then sp.loch = (the stage).rect.width

Wrapping the position - source movie

•How would you make it wrap when it reaches the left side, but bounce when it reaches the right?