Testing for pygame sprite collision

Question

I have been going through some pygame tutorials, and I have developed my own code based off of these tutorials. It is as follows:

#!/usr/bin/python

import pygame, sys
from pygame.locals import *

size = width, height = 320, 320
clock = pygame.time.Clock()

xDirection = 0
yDirection = 0
xPosition = 32
yPosition = 256

blockAmount = width/32

pygame.init()
screen = pygame.display.set_mode(size)
screen.fill([0, 155, 255])
pygame.display.set_caption("Mario Test")
background = pygame.Surface(screen.get_size())

mainCharacter = pygame.sprite.Sprite()
mainCharacter.image = pygame.image.load("data/character.png").convert()
mainCharacter.rect = mainCharacter.image.get_rect()
mainCharacter.rect.topleft = [xPosition, yPosition]
screen.blit(mainCharacter.image, mainCharacter.rect)

grass = pygame.sprite.Sprite()
grass.image = pygame.image.load("data/grass.png").convert()
grass.rect = grass.image.get_rect()
for i in range(blockAmount):
    blockX = i * 32
    blockY = 288
    grass.rect.topleft = [blockX, blockY]
    screen.blit(grass.image, grass.rect)

grass.rect.topleft = [64, 256]  
screen.blit(grass.image, grass.rect.topleft )

running = False
jumping = False 
falling = False
standing = True

jumpvel = 22
gravity = -1

while True:

    for event in pygame.event.get():
        if event.type == KEYDOWN and event.key == K_ESCAPE:
            pygame.quit()
            sys.exit()
        elif event.type == QUIT:
            pygame.quit()
            sys.exit()
        if event.type == KEYDOWN:
            if event.key == K_LEFT:
                running = True
                xRun = -5
            elif event.key == K_RIGHT:
                running = True
                xRun = 5
            elif event.key == K_UP or event.key == K_SPACE:
                jumping = True
        elif event.type == KEYUP:
            if event.key == K_LEFT or event.key == K_RIGHT:
                running = False


    if running == True:
        xPosition += xRun

    if mainCharacter.rect.right >= width:
        xPosition = xPosition - 10
        print "hit"
        running = False
    elif mainCharacter.rect.left <= 0:
        xPosition = xPosition + 10
        print "hit"
        running = False

    screen.fill([0, 155, 255])

    for i in range(blockAmount):
        blockX = i * 32
        blockY = 288
        grass.rect.topleft = [blockX, blockY]
        screen.blit(grass.image, grass.rect)

    grass.rect.topleft = [64, 64]   
    screen.blit(grass.image, grass.rect.topleft )


    if jumping:
        yPosition -= jumpvel
        print jumpvel
        jumpvel += gravity
        if jumpvel < -22:
            jumping = False
        if mainCharacter.rect.bottom == grass.rect.top:
            jumping = False

    if not jumping:
        jumpvel = 22


    mainCharacter.rect.topleft = [xPosition, yPosition]
    screen.blit(mainCharacter.image,mainCharacter.rect)

    clock.tick(60)
    pygame.display.update() 

So I have figured out how to make my dude jump, however I have placed a block in the sky and tried to do this:

if mainCharacter.rect.bottom == grass.rect.top:
    jumping = False

to attempt to make the character stop jumping. Is there a built in function to test this or perhaps a way that works in my case. This doesn't work by the way, and I can't seem to figure out how to do it. Any help is greatly appreciated :)

Answer 1

The problem with your code is that your character is moving more than one pixel each step. For example, when moving at maximum velocity, your character moves 22 pixels each step. So if he's 10 pixels above the grass one step, he'll be 12 pixels below the grass on the next step. To fix this, you test to see whether the character is touching or below the grass, like so:

if mainCharacter.rect.bottom <= grass.rect.top:
    jumping = False

Also, Pygame does have a built-in function to test collision detection: rect1.colliderect(rect2) will return True if rect1 and rect2 are colliding, and will return False otherwise. However, it is slightly more costly than the aforementioned code, and it may cause the same problem if your character is moving so fast that he passes through the grass completely.