KivySpawning

This Kivy project demonstrates how to spawn objects such as bullets.

How it works

Kivy draws three elements: the shot, the barrel, and the turret, in that order. The shot is initially inside the centre of the turret. When the player touches the screen, the program calculates a vector from the turret to the touch point and sets a velocity on the shot matching that vector. (The code must also check that the shot is not already in the air or subsequent touches would change its direction.)

The shot updates its position each frame until it leaves the screen. The position of the shot is then reset to the centre of the turret, and the velocity returned to 0, ready for the next shot.

The barrel is rotated to point in the direction of the target touch. This is done using a rotation matrix.

Create images

Download the turret.png image file or create your own. You will need 3 tiles: a turret, a barrel, and a shot.

Create atlas file

Create an atlas file to describe the individual sprites on your image file. Create a file turret.atlas and paste in the following code in an editor. (If you use Spyder, remember to rename the file after Spyder has saved it because it adds a spurious .py to the end of the file name.)

{
    "turret.png": {
        "turret": [0, 0, 32, 32],
        "barrel": [32, 0, 32, 32],
        "shot":   [64, 0, 32, 32]
    }
}

If you created your own image file to a different scale you will have to replace the sprite positions and sizes listed in the file above,

Create your code file

Create a file called main.py and copy the following code into it:

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.image import Image
from kivy.properties import NumericProperty, ObjectProperty
from kivy.clock import Clock
from kivy.vector import Vector

class Shot(Image):
    velocity = Vector(0, 0)
    def update(self, dt):
        position = Vector(self.center) + self.velocity
        if (position.x < 0 or position.x > self.parent.width or position.y < 0 or position.y > self.parent.height):
            self.velocity = Vector(0, 0)
            position = Vector(self.parent.width / 2, 0)
        self.center = position

class SpawningScreen(Widget):
    shot = ObjectProperty(None)
    angle = NumericProperty(0)
    target = Vector(0, 0)
    def update(self, dt):
        angle = (self.target - Vector(self.width / 2, 0)).angle((0, 100))
        self.angle = angle
        self.shot.angle = angle
        self.shot.update(dt)
        
    def on_touch_down(self, touch):
        velocity = self.shot.velocity
        if (velocity.length2() == 0):
            self.target = Vector(touch.x, touch.y)
            v = self.target - Vector(self.width / 2, 0)
            if (v.length2() > 0):
                self.shot.velocity = v.normalize() * 10

class SpawningApp(App):
    def build(self):
        spawning = SpawningScreen()
        Clock.schedule_interval(spawning.update, 1.0/60.0)
        return spawning

if __name__ == '__main__':
    SpawningApp().run()

Create your layout file

Create a file called spawning.kv and copy in the following code:

#:kivy 1.9.0

<SpawningScreen>:
    angle: 0
    shot: turret_shot

    Shot:
        id: turret_shot
        source: 'atlas://turret/shot'
        size: 32, 32
        center: root.width / 2, 0

    Image:
        source: 'atlas://turret/barrel'
        size: 32, 32
        center: root.width / 2, 13
        canvas.before:
            PushMatrix
            Rotate:
                angle: root.angle
                origin: root.width / 2, 0
        canvas.after:
            PopMatrix

    Image:
        source: 'atlas://turret/turret'
        size: 32, 32
        center: root.width / 2, 16

Test your program

You should now be able to run your program by entering python main.py from the command line. (Don't forget to activate kivy first.)

Challenges

• Add in a target sprite
• Detect collision between the target sprite and the projectile
• Animate your projectile and / or turret
• Rotate your projectile (you'll need a bigger sprite to see this effect)
• Allow more than one bullet in the air at a time
• Add code to guide the projectile towards your touch