Collision Detection in Pygame

Question

I'm making a game where the main character is a cat and has to avoid dogs and eat food. The collision detection works for the dogs and will subtract a bar of health from the cat, but when the cat collides with the food, it does not recognize that a collision has occurred.

Here is the code for the dog collision:

for i in range(len(dogObjs)-1, -1, -1):
                dgObj = dogObjs[i]
                if 'rect' in dgObj and playerObj['rect'].colliderect(dgObj['rect']):
                    # a player/dog collision has occurred

                        if not invulnerableMode:
                            # player is smaller and takes damage
                            invulnerableMode = True
                            invulnerableStartTime = time.time()
                            playerObj['health'] -= 1
                            if playerObj['health'] == 0:
                                gameOverMode = True # turn on "game over mode"
                                gameOverStartTime = time.time()

The above code is the code that works. Now here is the code for the food

for i in range(len(foodObjs)-1, -1, -1):
                fdObj = foodObjs[i]
                if 'rect' in fdObj and playerObj['rect'].colliderect(fdObj['rect']):

                    if fdObj['width'] * fdObj['height'] <= playerObj['size']**2:
                        playerObj['size'] += int( (playerObj['width'] * playerObj['height'])**0.2 ) + 1
                        del foodObjs[i]

                    if playerObj['facing'] == LEFT:
                        playerObj['surface'] = pygame.transform.scale(L_CAT_IMG, (playerObj['size'], playerObj['size']))
                    if playerObj['facing'] == RIGHT:
                        playerObj['surface'] = pygame.transform.scale(R_CAT_IMG, (playerObj['size'], playerObj['size']))

                    if playerObj['size'] > WINSIZE:
                        winMode = True # turn on "win mode"

I would like to make it where the size of the food does not matter, but I am more focused on just making the collision detection work between the cat and the food.

Both of these for loops are inside an if statement. I will show the whole thing below.

if not gameOverMode:
            # actually move the player
            if moveLeft:
                playerObj['x'] -= MOVERATE
            if moveRight:
                playerObj['x'] += MOVERATE
            if moveUp:
                playerObj['y'] -= MOVERATE
            if moveDown:
                playerObj['y'] += MOVERATE

            # check if the player has collided with any dogs
            for i in range(len(dogObjs)-1, -1, -1):
                dgObj = dogObjs[i]
                if 'rect' in dgObj and playerObj['rect'].colliderect(dgObj['rect']):
                    # a player/dog collision has occurred

                        if not invulnerableMode:
                            # player is smaller and takes damage
                            invulnerableMode = True
                            invulnerableStartTime = time.time()
                            playerObj['health'] -= 1
                            if playerObj['health'] == 0:
                                gameOverMode = True # turn on "game over mode"
                                gameOverStartTime = time.time()

            # check if player has collided with food
            for i in range(len(foodObjs)-1, -1, -1):
                fdObj = foodObjs[i]
                if 'rect' in fdObj and playerObj['rect'].colliderect(fdObj['rect']):

                    if fdObj['width'] * fdObj['height'] <= playerObj['size']**2:
                        playerObj['size'] += int( (playerObj['width'] * playerObj['height'])**0.2 ) + 1
                        del foodObjs[i]

                    if playerObj['facing'] == LEFT:
                        playerObj['surface'] = pygame.transform.scale(L_CAT_IMG, (playerObj['size'], playerObj['size']))
                    if playerObj['facing'] == RIGHT:
                        playerObj['surface'] = pygame.transform.scale(R_CAT_IMG, (playerObj['size'], playerObj['size']))

                    if playerObj['size'] > WINSIZE:
                        winMode = True # turn on "win mode"
        else:
            # game is over, show "game over" text
            DISPLAYSURF.blit(gameOverScreen, (0,0))
            if time.time() - gameOverStartTime > GAMEOVERTIME:
                return # end the current game

So I figured out that it is not even entering the food for loop. Could this be due to the scope that it is in? — Taylor
– Taylor, Commented Apr 19, 2017 at 18:25
As an unrelated side-question: Why are you using dictionaries as object data storage instead of a nicer specific class? — user35344
– user35344, Commented Apr 19, 2017 at 18:58
I just wanted to get the code down and working before I compartmentalized into classes. — Taylor
– Taylor, Commented Apr 19, 2017 at 19:48

Taylor · Accepted Answer · 2017-04-19 18:53:39Z

I figured it out. It was not a problem with the code I posted but with some code earlier in the file. My food objects did not have a 'rect' aspect to them, so when I tried to compare the two 'rect's of both the character and the food, nothing happened.

Here is the before code of the food Objects:

for fObj in foodObjs:
            gRect = pygame.Rect( (fObj['x'] - camerax,
                                  fObj['y'] - cameray,
                                  fObj['width'],
                                  fObj['height']) )
            DISPLAYSURF.blit(FOODIMAGES[fObj['foodImage']], gRect)

Here is the fixed and working code:

for fObj in foodObjs:
            fObj['rect'] = pygame.Rect( (fObj['x'] - camerax,
                                  fObj['y'] - cameray,
                                  fObj['width'],
                                  fObj['height']) )
            DISPLAYSURF.blit(FOODIMAGES[fObj['foodImage']], fObj['rect'])

Any idea how to do it with a mesh of any shape? stackoverflow.com/questions/61518457/… — B''H Bi'ezras -- Boruch Hashem
– B''H Bi'ezras -- Boruch Hashem, Commented Apr 30, 2020 at 8:04

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