Picture collisions and bouncing

This activity has the following desired goals:

• Learning to check for picture collisions (A, M).
• Learning to bounce colliding pictures off each other (A, M).
• Using the above ideas to animate multiple objects while: (M, T)
  • keeping them within the stage area.
  • making them bounce off each other.

Step 1

Type in the following code and run it:

clear()
drawStage(cm.black)
val cb = canvasBounds

val pic1 = Picture {
    right(45)
    val n = 6
    repeat(n) {
        forward(50)
        right(360.0 / n)
    }
}
pic1.setPosition(cb.x + 20, 0)
pic1.setFillColor(cm.khaki)
pic1.setPenColor(cm.khaki)
var vel1 = Vector2D(4, 0)

val pic2 = Picture {
    val n = 5
    repeat(n) {
        forward(50)
        right(360.0 / n)
    }
}
pic2.setPosition(cb.x + cb.width - 40 - 20, 0)
pic2.setFillColor(cm.khaki)
pic2.setPenColor(cm.khaki)
var vel2 = -vel1

draw(pic1, pic2)
animate {
    pic1.translate(vel1)
    pic2.translate(vel2)

    if (pic1.collidesWith(pic2)) {
        vel1 = bouncePicOffPic(pic1, vel1, pic2)
        vel2 = -vel1
    }
}

Q1a. Read through the code above and try to understand what it does. Then explain to a friend.

Q1b. What do you think pic1.collidesWith(pic2) does? Is this a command or a function? What are its inputs and outputs?

Q1c. What do you think bouncePicOffPic(pic1, vel1, pic2) does? Is this a command or a function? What are its inputs and outputs?

Explanation

Function descriptions:

• pic.collidesWith(other: Picture) - returns true if pic has collided with other.
• bouncePicOffPic(pic: Picture, vel: Vector2D, obstacle: Picture)- for a picture pic moving with velocity vel - this function returns the velocity after bouncing off obstacle.

Exercise

Write a program that does the following:

• Draws four non-overlapping rectangles in the center region of the stage.
• Moves the rectangles towards the four corners of the stage.
• Keeps the rectangles within the stage borders (by making them bounce off the borders).
• Makes the rectangles bounce off each other (so that whenever any two collide, they bounce off each other).