Detecting walls or floors in pygame

Question

I am trying to make bullets bounce of walls, but I can't figure out how to correctly do the collision detection. What I am currently doing is iterating through all the solid blocks and if the bullet hits the bottom, top or sides, its vector is adjusted accordingly. However, sometimes when I shoot, the bullet doesn't bounce, I think it's when I shoot at a border between two blocks.

Here is the update method for my Bullet class:

def update(self, dt):
    if self.can_bounce:
        #if the bullet hasnt bounced find its vector using the mousclick pos and player pos
        speed = -10.
        range = 200
        distance = [self.mouse_x - self.player[0], self.mouse_y - self.player[1]]
        norm = math.sqrt(distance[0] ** 2 + distance[1] ** 2)
        direction = [distance[0] / norm, distance[1 ] / norm]
        bullet_vector = [direction[0] * speed, direction[1] * speed]

        self.dx = bullet_vector[0]
        self.dy = bullet_vector[1]

    #check each block for collision
    for block in self.game.solid_blocks:
        last = self.rect.copy()
        if self.rect.colliderect(block):
            topcheck = self.rect.top < block.rect.bottom and self.rect.top > block.rect.top
            bottomcheck = self.rect.bottom > block.rect.top and self.rect.bottom < block.rect.bottom
            rightcheck = self.rect.right > block.rect.left and self.rect.right < block.rect.right 
            leftcheck = self.rect.left < block.rect.right and self.rect.left > block.rect.left
            each test tests if it hit the top bottom left or right side of the block its colliding with 
            if self.can_bounce:                      
                if topcheck: 
                    self.rect = last
                    self.dy  *= -1 
                    self.can_bounce = False
                    print "top"

                if bottomcheck:
                    self.rect = last
                    self.dy  *= -1 #Bottom check
                    self.can_bounce = False
                    print "bottom"

                if rightcheck:
                    self.rect = last 
                    self.dx  *= -1 #right check
                    self.can_bounce = False
                    print "right"

                if leftcheck:
                    self.rect = last
                    self.dx  *= -1 #left check
                    self.can_bounce = False
                    print "left"

            else:
                # if it has already bounced and colliding again kill it
                self.kill()

    for enemy in self.game.enemies_list:
        if self.rect.colliderect(enemy):
            self.kill()
    #update position
    self.rect.x -= self.dx
    self.rect.y -= self.dy

This definitely isn't the best way to do it but I can't think of another way. If anyone has done this or can help that would be awesome!

Answer 1

If you try and do the X and Y collision at the same time, you will find it troublesome to implement any kind of bouncing; it's more effort to calculate which way the object will need to bounce.

To remedy this, we can split up the X and Y collision calculation, and the velocity reversing calculation. some pseudocode:

move(int x, int y){
    pos.x += x;
    checkHorizontalCollision();
    pos.y += y;
    checkVerticalCollision();
}

the checkXCollision methods simply reverse the velocity and maybe remove some depending on your requirements.

Answer 2

Pygame has a built in and very good collision detection function for sprites. If you have one sprite, and want to tell if it is colliding with members of another group of sprites, you can do this:

collisions = pygame.sprite.spritecollide(<sprite>,<group>,False)     #argument one is the sprite, argument two is the group to test collisions with the sprite for, and the final argument should almost always be False, because when it is True, colliding sprites will automatically get deleted.
for item in collisions:                                              #Spritecollide returns a list of colliding sprites. This will iterate through them.
    if item != <sprite>:                                             #spritecollide will also always return that the sprite is colliding with itself. this will filter that out, because you don't want this information.
        print "collision detected, insert code to run on collision here or return True from a function"

This should work fine.