Getting Animated

Announcements

• Grading of midterm 1 is still ongoing! My apologies!
• Problem set #4 is being posted, due next week Monday at 10 pm
  • You may have the opportunity to work on the problem set #4 and ask questions in class today.
• Polling continues here

Review Question!

def rev_q():
    def act_A(e):
        sq.set_filled(True)
    def act_B(e):
        sq.set_size(
            sq.get_width() - 10,
            sq.get_height() - 10
        )
    gw = GWindow(500, 500)
    sq = GRect(200, 200, 100, 100)
    sq.set_color("blue")
    gw.add(sq)
    gw.add_event_listener("mousedown", act_B)
    gw.add_event_listener("click", act_A)

Growing Circles

Waiting vs Events

• Many would probably try to approach this doing something like as follows:

  def growing_circles():
    gw = GWindow(WIDTH, HEIGHT)
    for i in range(NUM_CIRCLES):
        # Create a new circle
        # Animate the circle to grow it
        # Wait for the animation to complete

• The problem here is that there is no clear way to “wait” for an animation to complete

  • Code you write runs basically instantly or when run by a callback

• Instead need an event callback that takes care of both circle creation (when needed) and growing animations

Using Events Wisely

• Need to keep track of what the program should be doing, and then have the timer callback function handle whatever is needed
• Conceptually, for these circles, might look more like this:

def step():
    if """ there is a circle still growing """
        """then increase its size """
    elif """ a new circle needs to be created """
        """ then create one """
    else:
        timer.stop()

Making those circles grow!

from pgl import GWindow, GOval
import random

GWIDTH = 500
GHEIGHT = 400
N_CIRCLES = 20
MIN_RADIUS = 15
MAX_RADIUS = 100
DELTA_TIME = 10
DELTA_SIZE = 1

def random_color():
    color = "#"
    for i in range(6):
        color += random.choice("0123456789ABCDEF")
    return color

def create_filled_circle(x, y, r, color="black"):
    circ = GOval(x-r, y-r, 2*r, 2*r)
    circ.set_filled(True)
    circ.set_color(color)
    return circ

def growing_circles():
    def start_new_circle():
        r = random.uniform(MIN_RADIUS, MAX_RADIUS)
        x = random.uniform(r, GWIDTH - r)
        y = random.uniform(r, GHEIGHT - r)
        gw.circle = create_filled_circle(
                            x, y, 
                            0, random_color()
                        )
        gw.desired_size = 2 * r
        gw.current_size = 0
        gw.circles_created += 1
        return gw.circle

    def step():
        # Grow a circle if needed
        if gw.current_size < gw.desired_size:
            gw.current_size += DELTA_SIZE
            x = gw.circle.get_x() - DELTA_SIZE / 2
            y = gw.circle.get_y() - DELTA_SIZE / 2
            gw.circle.set_bounds(
                            x, y, 
                            gw.current_size,
                            gw.current_size
                        )
        # or add a circle if you can
        elif gw.circles_created < N_CIRCLES:
            gw.add(start_new_circle())
        # or stop
        else:
            timer.stop()

    gw = GWindow(GWIDTH, GHEIGHT)
    gw.circles_created = 0
    gw.current_size = 0
    gw.desired_size = 0
    timer = gw.set_interval(step, DELTA_TIME)

Simulation

• Our technique of piecing together many small movements to resemble motion is not limited to just making pretty animations!
• Physicists use similar techniques to break complex problems into simple pieces
  • “In this small time interval, the motion is simple”
  • Chain together many time intervals to construct the full motion
• There are many areas where this is the only way to solve a problem, as we can not write down equations to express the result otherwise!

The Two Body Problem

from pgl import GWindow, GOval, GLine
from pgl_tools import create_filled_circle

def two_body():
    def step():
        # Compute forces and accelerations
        dx = planet1.get_x() - planet2.get_x()
        dy = planet1.get_y() - planet2.get_y()
        r3 = (dx ** 2 + dy ** 2) ** (3 / 2)
        ax = 1000 / r3 * dx
        ay = 1000 / r3 * dy

        # Update velocities
        gw.vx1 += -ax
        gw.vy1 += -ay
        gw.vx2 += ax
        gw.vy2 += ay

        # Augment history paths
        path1 = GLine(
            planet1.get_x() + 10,
            planet1.get_y() + 10,
            planet1.get_x() + 10 + gw.vx1,
            planet1.get_y() + 10 + gw.vy1,
        )
        path1.set_color("red")
        path1.set_line_width(3)

        path2 = GLine(
            planet2.get_x() + 10,
            planet2.get_y() + 10,
            planet2.get_x() + 10 + gw.vx2,
            planet2.get_y() + 10 + gw.vy2,
        )
        path2.set_color("cyan")
        path2.set_line_width(3)

        # Move planets
        planet1.move(gw.vx1, gw.vy1)
        planet2.move(gw.vx2, gw.vy2)

        gw.add(path1)
        gw.add(path2)

    gw = GWindow(600, 600)
    # Defining state variables
    gw.vx1, gw.vy1 = 0, 1
    gw.vx2, gw.vy2 = 0, -1

    planet1 = create_filled_circle(200, 200, 10, "red")
    planet2 = create_filled_circle(400, 200, 10, "cyan")

    gw.add(planet1)
    gw.add(planet2)

    gw.set_interval(step, 30)

if __name__ == '__main__':
    two_body()