Firing bullets from a rotating gun

Question

My question is connected to this question: rotating gun with restricted movement

What calculations do you need to do to get bullets firing from the gun muzzle?

Answer 1

Here's an example of a firing cannon. You just have to pass the current angle and the rect.center of the cannon to the newly created bullet and then rotate its image and velocity vector in the __init__ method.

import math
import pygame as pg
from pygame.math import Vector2


pg.init()
screen = pg.display.set_mode((640, 480))
FONT = pg.font.Font(None, 24)
BLACK = pg.Color('black')
BG_COLOR = pg.Color('darkseagreen4')


class Bullet(pg.sprite.Sprite):

    def __init__(self, pos, angle):
        super(Bullet, self).__init__()
        self.image = pg.Surface((20, 11), pg.SRCALPHA)
        pg.draw.rect(self.image, pg.Color('grey11'), [0, 0, 13, 11])
        pg.draw.polygon(
            self.image, pg.Color('grey11'), [(13, 0), (20, 5), (13, 10)])
        self.image = pg.transform.rotate(self.image, -angle)
        self.rect = self.image.get_rect(center=pos)
        # To apply an offset to the start position,
        # create another vector and rotate it as well.
        offset = Vector2(80, 0).rotate(angle)
        # Use the offset to change the starting position.
        self.pos = Vector2(pos) + offset
        self.velocity = Vector2(5, 0)
        self.velocity.rotate_ip(angle)

    def update(self):
        self.pos += self.velocity
        self.rect.center = self.pos


def main():
    clock = pg.time.Clock()
    # The cannon image and rect.
    surf = pg.Surface((40, 22), pg.SRCALPHA)
    surf.fill(pg.Color('grey27'))
    pg.draw.rect(surf, pg.Color('grey11'), [30, 6, 10, 10])
    orig_surf = surf
    rect = surf.get_rect(center=(320, 240))
    angle = 0  # Angle of the cannon.
    # Add bullets to this group.
    bullet_group = pg.sprite.Group()

    playing = True
    while playing:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                playing = False
            if event.type == pg.MOUSEBUTTONDOWN:
                if event.button == 1:  # Left button fires bullet.
                    # Fire a bullet from cannon center with current angle.
                    bullet_group.add(Bullet(rect.center, angle))

        bullet_group.update()
        # Find angle to target (mouse pos).
        x, y = Vector2(pg.mouse.get_pos()) - rect.center
        angle = math.degrees(math.atan2(y, x))
        # Rotate the cannon image.
        surf = pg.transform.rotate(orig_surf, -angle)
        rect = surf.get_rect(center=rect.center)

        # Draw
        screen.fill(BG_COLOR)
        bullet_group.draw(screen)
        screen.blit(surf, rect)
        txt = FONT.render('angle {:.1f}'.format(angle), True, BLACK)
        screen.blit(txt, (10, 10))
        pg.draw.line(
            screen, pg.Color(150, 60, 20), rect.center, pg.mouse.get_pos(), 2)
        pg.display.update()

        clock.tick(30)

if __name__ == '__main__':
    main()
    pg.quit()

You can also use math.cos and sin to calculate the offset.

run = math.cos(math.radians(angle)) * 80
rise = math.sin(math.radians(angle)) * 80
offset = run, rise