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POSimple 2D gravity with pygame; Value Guidelines?
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copied!<p>I am trying to simulate gravity in a simple 2D window using pygame. It's very simple stuff (a dot rising and falling again) and I understand the mechanics, i.e. speed as a vector and the y portion being consistently diminished by a value g representing gravity during each run through the mainloop and the subsequent update of the position of the dot. <br> <br> It's all working fine, but I am having trouble choosing the correct values to be inserted. It is all trial and error at the moment. <strong>Is there a good rule of thumb on how to determine which numbers to use, to achieve a semi-realistic looking trajectory?</strong></p> <p>I inserted a minimal example below, with the following values important to my question:</p> <pre><code>window = (640, 480) # pixels initial_speed = 20 # pixels per update along the y axis gravity = 0.4 # deduction on initial_speed per update </code></pre> <p>Now, why do these numbers happen to make the illusion work? I tried first to use formulae I learnt in physics class years and years ago, but, with or without unit conversions, the simulation was just not right. Most of the time, I didn't even see the ball, yet the above values, found through trial and error work.</p> <p>Thanks for all your help in advance. If you need more information, just post a comment and I will try to oblige.</p> <p>Here is the minimal example. Note the vector2D library was conveniently borrowed from the pygame website (<a href="http://www.pygame.org/wiki/2DVectorClass" rel="nofollow">follow this link</a>)</p> <pre><code>#!/usr/bin/env python import pygame from pygame.locals import * from vector2D import Vec2d pygame.init() GRAVITY = 0.4 class Dot(pygame.sprite.Sprite): def __init__(self, screen, img_file, init_position, init_direction, speed): pygame.sprite.Sprite.__init__(self) self.screen = screen self.speed = Vec2d(speed) self.base_image = pygame.image.load(img_file).convert_alpha() self.image = self.base_image # A vector specifying the Dot's position on the screen self.pos = Vec2d(init_position) # The direction is a normalized vector self.direction = Vec2d(init_direction).normalized() def blitme(self): """ Blit the Dot onto the screen that was provided in the constructor. """ self.screen.blit(self.image, self.pos) def update(self): self.speed.y -= GRAVITY displacement = Vec2d( self.direction.x * self.speed.x, self.direction.y * self.speed.y ) self.pos += displacement def main(): DIMENSION = SCREEN_WIDTH, SCREEN_HEIGHT = 640, 480 BG_COLOUR = 0,0,0 # Creating the screen window = screen = pygame.display.set_mode( (SCREEN_WIDTH, SCREEN_HEIGHT), 0, 32) screen = pygame.display.get_surface() clock = pygame.time.Clock() dot = Dot(screen, "my/path/to/dot.jpg", (180, SCREEN_HEIGHT), (0, -1), (0, 20)) mainloop = True while mainloop: # Limit frame speed to 50 FPS time_passed = clock.tick(50) for event in pygame.event.get(): if event.type == pygame.QUIT: mainloop = False # Redraw the background screen.fill(BG_COLOUR) dot.update() dot.blitme() pygame.display.flip() if __name__ == '__main__': main() </code></pre>

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