Pygame collision detection

Question

So I am trying to create a puzzle platformer game with Python and Pygame, but I'm having a little bit of trouble. How do I make a collision detecter when I am using a blitted image for the main character, not a rect image? I know that rect images have left, right, top and bottom pixel functions (which would be extremely useful for collision detection) but is there anything like that for a blitted image? Or would I just have to create a variable for x and y coordinates + the width/height of the image? I tried that using

import pygame, sys
from pygame.locals import *

WINDOWWIDTH = 400
WINDOWHEIGHT = 300
WHITE = (255, 255, 255)
catImg = pygame.image.load('cat.png')
catx = 0
caty = 0
catRight = catx + 100
catBot = caty + 100

moveRight = False

pygame.init()


FPS = 40 # frames per second setting
fpsClock = pygame.time.Clock()

# set up the window
DISPLAYSURF = pygame.display.set_mode((WINDOWWIDTH, WINDOWHEIGHT), 0, 32)
pygame.display.set_caption('Animation')


while True: # the main game loop
    DISPLAYSURF.fill(WHITE)

    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
        elif event.type == KEYDOWN:
            if event.key in (K_RIGHT, K_w):
                moveRight = True

        elif event.type == KEYUP:
            if event.key in (K_RIGHT, K_w):
                moveRight = False

    if catRight == WINDOWWIDTH:
        moveRight = False
    if moveRight == True:
        catx += 5

    DISPLAYSURF.blit(catImg, (catx, caty))


    pygame.display.update()
    fpsClock.tick(FPS)

But the catImg just kept going right past the end of the window. What am I doing wrong? Thanks in advance.

Answer 1

To prevent the image from going off the right edge you need to compute the maximum value its x coordinate can have, and the make sure that value is never exceeded. So before the loop create a variable with the value in it:

CAT_RIGHT_LIMIT = WINDOWWIDTH - catImg.get_width()

And then in the loop check it:

if catx >= CAT_RIGHT_LIMIT:
    moveRight = False
    catx = CAT_RIGHT_LIMIT
if moveRight == True:
    catx += 5

You can, of course, extend this idea to all the other edges.