# Filename: A05_air_track_cursorTethers.py
# Written by: James D. Miller
# Python
import sys, os
import pygame
import datetime
# PyGame Constants
from pygame.locals import *
from pygame.color import THECOLORS
#=====================================================================
# Classes
#=====================================================================
class Client:
def __init__(self, cursorString_color):
self.cursor_location_px = (0,0) # x_px, y_px
self.mouse_button = 1 # 1, 2, or 3
self.buttonIsStillDown = False
self.cursorString_color = cursorString_color
self.selected_car = None
# Define the nature of the cursor strings, one for each mouse button.
self.mouse_strings = {'string1':{'c_drag': 2.0, 'k_Npm': 60.0},
'string2':{'c_drag': 0.2, 'k_Npm': 2.0},
'string3':{'c_drag': 20.0, 'k_Npm': 1000.0}}
def calc_tether_forces_on_cars(self):
# Calculated the string forces on the selected car and add to the aggregate
# that is stored in the car object.
# Only check for a selected car if one isn't already selected. This keeps
# the car from unselecting if cursor is dragged off the car!
if (self.selected_car == None):
if self.buttonIsStillDown:
self.selected_car = air_track.checkForCarAtThisPosition(self.cursor_location_px)
# If a car is selected
else:
if not self.buttonIsStillDown:
# Unselect the car and bomb out of here.
self.selected_car.selected = False
self.selected_car = None
return None
# If button is down, calculate the forces on the car.
else:
# Use dx difference to calculate the hooks law force being applied by the tether line.
# If you release the mouse button after a drag it will fling the car.
# This tether force will diminish as the car gets closer to the mouse point.
dx_m = env.m_from_px( self.cursor_location_px[0]) - self.selected_car.center_m
stringName = "string" + str(self.mouse_button)
self.selected_car.cursorString_spring_force_N += dx_m * self.mouse_strings[stringName]['k_Npm']
self.selected_car.cursorString_carDrag_force_N += (self.selected_car.v_mps *
(-1) * self.mouse_strings[stringName]['c_drag'])
def draw_cursor_string(self):
car_center_xy_px = (env.px_from_m(self.selected_car.center_m), self.selected_car.center_y_px)
pygame.draw.line(game_window.surface, self.cursorString_color, car_center_xy_px, self.cursor_location_px, 1)
class GameWindow:
def __init__(self, screen_tuple_px):
self.width_px = screen_tuple_px[0]
self.height_px = screen_tuple_px[1]
# Create a reference to display's surface object. This object is a pygame "surface".
# Screen dimensions in pixels (tuple)
self.surface = pygame.display.set_mode(screen_tuple_px)
# Define the physics-world boundaries of the window.
self.left_m = 0.0
self.right_m = env.m_from_px(self.width_px)
# Paint screen black.
self.erase_and_update()
def update_caption(self, title):
pygame.display.set_caption(title)
self.caption = title
def erase_and_update(self):
# Useful for shifting between the various demos.
self.surface.fill(THECOLORS["black"])
pygame.display.flip()
class Detroit:
def __init__(self, color=THECOLORS["white"], left_px=10, width_px=26, height_px=98, v_mps=1):
self.color = color
self.height_px = height_px
self.top_px = game_window.height_px - self.height_px
self.width_px = width_px
# Use y midpoint for drawing the cursor line.
self.center_y_px = int(round( float(game_window.height_px - self.height_px) + float(self.height_px)/2.0) )
# For use with cursor-tethers selection.
self.selected = False
self.width_m = env.m_from_px( width_px)
self.halfwidth_m = self.width_m/2.0
self.height_m = env.m_from_px( height_px)
# Initialize the position and velocity of the car. These are affected by the
# physics calcs in the Track.
self.center_m = env.m_from_px(left_px) + self.halfwidth_m
self.v_mps = v_mps
# Aggregate type forces acting on car.
self.cursorString_spring_force_N = 0
self.cursorString_carDrag_force_N = 0
self.density_kgpm2 = 600.0
self.m_kg = self.height_m * self.width_m * self.density_kgpm2
# Increment the car count. This class variable is shared amoung all instances of the
# car class.
air_track.carCount += 1
# Name this car based on this air_track attribute.
self.name = air_track.carCount
print "New car name =", self.name
# Create a rectangle object based on these dimensions
# Left: distance from the left edge of the screen in px.
# Top: distance from the top edge of the screen in px.
self.rect = pygame.Rect(left_px, self.top_px, self.width_px, self.height_px)
def draw_car(self):
# Update the pixel position of the car's rectangle object to match the value
# controlled by the physics calculations.
self.rect.centerx = env.px_from_m( self.center_m)
# Draw the main rectangle.
pygame.draw.rect(game_window.surface, self.color, self.rect)
class AirTrack:
def __init__(self):
# Initialize the list of cars.
self.cars = []
self.carCount = 0
# Coefficients of restitution.
self.coef_rest_base = 0.90 # Useful for reseting things.
self.coef_rest_car = self.coef_rest_base
self.coef_rest_wall = self.coef_rest_base
# Component of gravity along the length of the track.
self.gbase_mps2 = 9.8/20.0 # one 20th of g.
self.g_mps2 = self.gbase_mps2
self.color_transfer = False
self.collision_count = 0
self.fix_wall_stickiness = True # False True
self.fix_car_stickiness = True # False True
def checkForCarAtThisPosition(self, cursor_location_xy):
x_px = cursor_location_xy[0]
y_px = cursor_location_xy[1]
x_m = env.m_from_px(x_px)
for car in self.cars:
if (((x_m > car.center_m - car.halfwidth_m) and (x_m < car.center_m + car.halfwidth_m)) and
(y_px > game_window.height_px - car.height_px)):
car.selected = True
return car
return None
def update_SpeedandPosition(self, car, dt_s):
# Add up all the forces on the car.
car_forces_N = (car.m_kg * self.g_mps2) + (car.cursorString_spring_force_N +
car.cursorString_carDrag_force_N )
# Calculate the acceleration based on the forces and Newton's law.
car_acc_mps2 = car_forces_N / car.m_kg
# Calculate the velocity at the end of this time step.
v_end_mps = car.v_mps + (car_acc_mps2 * dt_s)
# Calculate the average velocity during this timestep.
v_avg_mps = (car.v_mps + v_end_mps)/2.0
# Use the average velocity to calculate the new position of the car.
# Physics note: v_avg*t is equivalent to (v*t + (1/2)*acc*t^2)
car.center_m = car.center_m + (v_avg_mps * dt_s)
# Assign the final velocity to the car.
car.v_mps = v_end_mps
# Reset the aggregate forces.
car.cursorString_spring_force_N = 0
car.cursorString_carDrag_force_N = 0
def check_for_collisions(self):
# Collisions with walls.
# Enumerate so can efficiently check car-car collisions below.
for i, car in enumerate(self.cars):
# Collisions with Left and Right wall.
# If left-edge of the car is less than... OR If right-edge of car is greater than...
if ((car.center_m - car.width_m/2.0) < game_window.left_m) or ((car.center_m + car.width_m/2.0) > game_window.right_m):
self.collision_count += 1
if self.fix_wall_stickiness:
self.correct_wall_penetrations(car)
car.v_mps = -car.v_mps * self.coef_rest_wall
# This makes use of the "enumerate"d for loop above.
# In doing so, it avoids checking the self-self case and avoids checking pairs twice
# like (2 with 3) and (3 with 2).
# Example checks: (1 with 2,3,4,5), (2 with 3,4,5), (3 with 4,5), (4 with 5) etc...
for ocar in self.cars[i+1:]:
# Check for overlap with other rectangle.
if (abs(car.center_m - ocar.center_m) < (car.halfwidth_m + ocar.halfwidth_m)):
self.collision_count += 1
if self.color_transfer == True:
(car.color, ocar.color) = (ocar.color, car.color)
# Prevent sticking to other cars.
if self.fix_car_stickiness:
self.correct_car_penetrations(car, ocar)
# Calculate the new post-collision velocities.
(car.v_mps, ocar.v_mps) = self.car_and_ocar_vel_AFTER_collision( car, ocar)
def car_and_ocar_vel_AFTER_collision(self, car, ocar, CR=None):
# If no override CR is provided, use the car's value.
if (CR == None):
CR = self.coef_rest_car
# Calculate the AFTER velocities.
car_vel_AFTER_mps = ( (CR * ocar.m_kg * (ocar.v_mps - car.v_mps) + car.m_kg*car.v_mps + ocar.m_kg*ocar.v_mps)/
(car.m_kg + ocar.m_kg) )
ocar_vel_AFTER_mps = ( (CR * car.m_kg * (car.v_mps - ocar.v_mps) + car.m_kg*car.v_mps + ocar.m_kg*ocar.v_mps)/
(car.m_kg + ocar.m_kg) )
return (car_vel_AFTER_mps, ocar_vel_AFTER_mps)
def correct_wall_penetrations(self, car):
penetration_left_x_m = game_window.left_m - (car.center_m - car.halfwidth_m)
if penetration_left_x_m > 0:
car.center_m += 2 * penetration_left_x_m
penetration_right_x_m = (car.center_m + car.halfwidth_m) - game_window.right_m
if penetration_right_x_m > 0:
car.center_m -= 2 * penetration_right_x_m
def correct_car_penetrations(self, car, ocar):
relative_spd_mps = abs(car.v_mps - ocar.v_mps)
penetration_m = (car.halfwidth_m + ocar.halfwidth_m) - abs(car.center_m - ocar.center_m)
penetration_time_s = penetration_m / relative_spd_mps
# First, back up the two cars, to their collision point, along their incoming trajectory paths.
# Use BEFORE collision velocities here!
car.center_m -= car.v_mps * penetration_time_s
ocar.center_m -= ocar.v_mps * penetration_time_s
# Calculate the velocities along the normal AFTER the collision. Use a CR (coefficient of restituion)
# of 1 here to better avoid stickiness.
(car_vel_AFTER_mps, ocar_vel_AFTER_mps) = self.car_and_ocar_vel_AFTER_collision( car, ocar, CR=1.0)
# Finally, travel another penetration time worth of distance using these AFTER-collision velocities.
# This will put the cars where they should have been at the time of collision detection.
car.center_m += car_vel_AFTER_mps * penetration_time_s
ocar.center_m += ocar_vel_AFTER_mps * penetration_time_s
def make_some_cars(self, nmode):
# Update the caption at the top of the pygame window frame.
game_window.update_caption("Air Track (basic): Demo #" + str(nmode))
if (nmode == 1):
air_track.g_mps2 = 0
air_track.carCount = 0
self.cars.append( Detroit(color=THECOLORS["red" ], left_px = 240, width_px=26, v_mps= 0.2))
self.cars.append( Detroit(color=THECOLORS["blue"], left_px = 340, width_px=26, v_mps= -0.2))
elif (nmode == 2):
air_track.g_mps2 = air_track.gbase_mps2
air_track.carCount = 0
self.cars.append( Detroit(color=THECOLORS["yellow" ], left_px = 240, width_px=26, v_mps= -0.1))
self.cars.append( Detroit(color=THECOLORS["green"], left_px = 440, width_px=50, v_mps= -0.2))
elif (nmode == 3):
air_track.carCount = 0
air_track.g_mps2 = 0
self.cars.append( Detroit(color=THECOLORS["yellow" ], left_px = 240, width_px=26, v_mps= -0.1))
self.cars.append( Detroit(color=THECOLORS["green"], left_px = 440, width_px=50, v_mps= -0.2))
class Environment:
def __init__(self, length_px, length_m):
self.px_to_m = length_m/float(length_px)
self.m_to_px = (float(length_px)/length_m)
# Add a local (non-network) client to the client dictionary.
self.clients = {'local':Client(THECOLORS["green"])}
# Convert from meters to pixels
def px_from_m(self, dx_m):
return int(round(dx_m * self.m_to_px))
# Convert from pixels to meters
def m_from_px(self, dx_px):
return float(dx_px) * self.px_to_m
def get_local_user_input(self):
# Get all the events since the last call to get().
for event in pygame.event.get():
if (event.type == pygame.QUIT):
return 'quit'
elif (event.type == pygame.KEYDOWN):
if (event.key == K_ESCAPE):
return 'quit'
elif (event.key==K_1):
return 1
elif (event.key==K_2):
return 2
elif (event.key==K_3):
return 3
elif (event.key==K_s):
air_track.fix_wall_stickiness = not air_track.fix_wall_stickiness
air_track.fix_car_stickiness = not air_track.fix_car_stickiness
elif (event.key==K_c):
air_track.color_transfer = not air_track.color_transfer
else:
return "Nothing set up for this key."
elif (event.type == pygame.KEYUP):
pass
elif (event.type == pygame.MOUSEBUTTONDOWN):
self.clients['local'].buttonIsStillDown = True
(button1, button2, button3) = pygame.mouse.get_pressed()
if button1:
self.clients['local'].mouse_button = 1
elif button2:
self.clients['local'].mouse_button = 2
elif button3:
self.clients['local'].mouse_button = 3
else:
self.clients['local'].mouse_button = 0
elif event.type == pygame.MOUSEBUTTONUP:
self.clients['local'].buttonIsStillDown = False
self.clients['local'].mouse_button = 0
if self.clients['local'].buttonIsStillDown:
# If it down, get the cursor position.
self.clients['local'].cursor_location_px = (mouseX, mouseY) = pygame.mouse.get_pos()
#============================================================
# Main procedural functions.
#============================================================
def main():
# A few globals.
global env, game_window, air_track
# Initiate pygame
pygame.init()
# Tuple to define window dimensions
window_size_px = window_width_px, window_height_px = 950, 120
# Instantiate an Environment object for converting back and forth from pixels and meters.
# The also creates the local client.
env = Environment(window_width_px, 1.5)
# Instantiate the window.
game_window = GameWindow(window_size_px)
# Instantiate an air track (this adds an empty car list to the track).
air_track = AirTrack()
# Make some cars (run demo #1).
air_track.make_some_cars(1)
# Instantiate clock to help control the framerate.
myclock = pygame.time.Clock()
# Control the framerate.
framerate_limit = 400
time_s = 0.0
user_done = False
while not user_done:
# Erase everything.
game_window.surface.fill(THECOLORS["black"])
# Get the delta t for one frame (this changes depending on system load).
dt_s = float(myclock.tick(framerate_limit) * 1e-3)
# Check for user initiated stop or demo change.
resetmode = env.get_local_user_input()
if (resetmode in [0,1,2,3,4,5,6,7,8,9]):
print "reset mode =", resetmode
# This should remove all references to the cars and effectively deletes them.
air_track.cars = []
# Now just black everything out and update the screen.
game_window.erase_and_update()
# Build new set of cars based on the reset mode.
air_track.make_some_cars( resetmode)
elif (resetmode == 'quit'):
user_done = True
elif (resetmode != None):
print resetmode
# Calculate client related forces.
for client_name in env.clients:
env.clients[client_name].calc_tether_forces_on_cars()
# Update velocity and x position of each car based on the dt_s for this frame.
for car in air_track.cars:
air_track.update_SpeedandPosition(car, dt_s)
# Check for collisions and apply collision physics to determine resulting
# velocities.
air_track.check_for_collisions()
print "Collision count =", air_track.collision_count, air_track.fix_wall_stickiness, air_track.fix_car_stickiness
# Draw the car at the new position.
for car in air_track.cars:
car.draw_car()
# Draw cursor strings.
for client_name in env.clients:
if (env.clients[client_name].selected_car != None):
env.clients[client_name].draw_cursor_string()
# Update the total time since starting.
time_s += dt_s
# Make this update visible on the screen.
pygame.display.flip()
#============================================================
# Run the main program.
#============================================================
main()