*************************************
Slide and Pin Joint Demo Step by Step
*************************************

This is a step by step tutorial explaining the demo demo_slide_and_pinjoint.py 
included in pymunk. You will find a screenshot of it in the list of 
:ref:`examples <examples>`.
It is probably a good idea to have the file near by if I 
miss something in the tutorial or something is unclear.

Before we start
===================

For this tutorial you will need:

* Python (of course)
* pygame (found at www.pygame.org)
* pymunk 

Pygame is required for this tutorial and some of the included demos, but it 
is not required to run just pymunk. Pymunk should work just fine with other 
similar libraries as well, for example you could easily translate this 
tutorial to use pyglet instead.

Pymunk is built on top of the 2d physics library Chipmunk. Chipmunk itself 
is written in C meaning pymunk need to call into the c code. The ctypes 
library helps with this, however if you are on a platform that I haven't been 
able to compile it on you might have to do it yourself. The good news is that 
it is very easy to do!

When you have pymunk installed, try to import it from the python prompt to 
make sure it works and can be imported::

    >>> import pymunk

If you get an error message it usually is because pymunk could not find the 
chipmunk library because it was not compiled (should not happen on windows and 
ubuntu, as pymunk ships with the code compiled for those two). To compile 
chipmunk, do ::

    > python setup.py build_chipmunk
    > python setup.py install

More information on installation can be found here: 
:ref:`Installation <installation>`

If it doesnt work or you have some kind of problem, feel free to write a post 
in the chipmunk forum, contact me directly or add your problem to the issue 
tracker: :ref:`Contact & Support <contact-support>`

An empty simulation 
=======================

Ok, lets start.
Chipmunk (and therefore pymunk) has a couple of central concepts, which is 
explained pretty good in this citation from the Chipmunk docs:

rigid bodies
    A rigid body holds the physical properties of an object. (mass, position, rotation, velocity, etc.) It does not have a shape by itself. If you've done physics with particles before, rigid bodies differ mostly in that they are able to rotate.
collision shapes
    By attaching shapes to bodies, you can define the a body's shape. You can attach many shapes to a single body to define a complex shape, or none if it doesn't require a shape.
constraints/joints
    You can attach joints between two bodies to constrain their behavior.
spaces
    Spaces are the basic simulation unit in Chipmunk. You add bodies, shapes and joints to a space, and then update the space as a whole._

The documentation for chipmunk can be found here: 
http://chipmunk-physics.net/release/ChipmunkLatest-Docs/ 
It is for the c-library but is a good complement to the pymunk documentation.

The API documentation for pymunk can be found here: :doc:`../pymunk`.

Anyway, we are now ready to write some code::

    import sys
    import pygame
    from pygame.locals import *
    from pygame.color import *
    import pymunk #1

    def main():
        pygame.init()
        screen = pygame.display.set_mode((600, 600))
        pygame.display.set_caption("Joints. Just wait and the L will tip over")
        clock = pygame.time.Clock()
        running = True
        
        space = pymunk.Space() #2
        space.gravity = (0.0, -900.0)
        
        while running:
            for event in pygame.event.get():
                if event.type == QUIT:
                    running = False
                elif event.type == KEYDOWN and event.key == K_ESCAPE:
                    running = False
                            
            screen.fill(THECOLORS["white"])
            
            space.step(1/50.0) #3
            
            pygame.display.flip()
            clock.tick(50)
            
    if __name__ == '__main__':
        sys.exit(main())

The code will display a blank window, and will run a physics simulation of an empty space.

#. We need to import pymunk in order to use it...

#. We then create a space and set its gravity to something good. Remember 
that what is important is what looks good on screen, not what the real 
world value is. -900 will make a good looking simulation, but feel free 
to experiment when you have the full code ready.

#. In our game loop we call the step() function on our space. The step 
function steps the simulation one step forward in time. Note: It is best 
to keep the stepsize constant and not adjust it depending on the 
framrate. The physic simulation will work much better with a constant step 
size.

Falling balls
=================

The easiest shape to handle (and draw) is the circle. Therefore our next 
step is to make a ball spawn once in while. In most demos all code is in one 
big pile in the main() function as they are so small and easy, but I will 
extract some methods in this tutorial to make it more easy to follow. First, 
a function to add a ball to a space::

    def add_ball(space):
        mass = 1
        radius = 14
        inertia = pymunk.moment_for_circle(mass, 0, radius) # 1
        body = pymunk.Body(mass, inertia) # 2
        x = random.randint(120,380)
        body.position = x, 550 # 3
        shape = pymunk.Circle(body, radius) # 4
        space.add(body, shape) # 5
        return shape


#. All bodies must have their moment of inertia set. If our object is a 
normal ball we can use the predefined function moment_for_circle to 
calculate it given its mass and radius. However, you could also select a 
value by experimenting with what looks good for your simulation.

#. After we have the inertia we can create the body of the ball.

#. And we set its position

#. And in order for it to collide with things, it needs to have one 
(or many) collision shape(s).  

#. Finally we add the body and shape to the space to include it in our 
simulation.

Now that we can create balls we want to display them::

    def draw_ball(screen, ball):
        p = int(ball.body.position.x), 600-int(ball.body.position.y)
        pygame.draw.circle(screen, THECOLORS["blue"], p, int(ball.radius), 2)


As I have used pygame in this example, we can use the draw.circle function 
to draw the balls. But first we must convert the position of the ball. We 
earlier set the gravity to -900 (that is, it will point down the y axis). 
Pygame thinks 0,0 is at the top left of the screen, with y increasing 
downwards. So we need to make a simple conversion of the y value.

An alternative way to handle the display would have been to use the 
pygame_util.draw_space function that is included in pymunk. However, in this 
tutorial I wanted to show how to draw things yourself as you are likley to 
want that in your own code anyway at some point.

With these two functions and a little code to spawn balls you should see a 
couple of balls falling. Yay!

::

    import sys, random
    import pygame
    from pygame.locals import *
    from pygame.color import *
    import pymunk

    #def add_ball(space):
    #def draw_ball(screen, ball):

    def main():
        pygame.init()
        screen = pygame.display.set_mode((600, 600))
        pygame.display.set_caption("Joints. Just wait and the L will tip over")
        clock = pygame.time.Clock()
        running = True
        
        space = pymunk.Space()
        space.gravity = (0.0, -900.0)
        
        balls = []
        
        ticks_to_next_ball = 10
        while running:
            for event in pygame.event.get():
                if event.type == QUIT:
                    running = False
                elif event.type == KEYDOWN and event.key == K_ESCAPE:
                    running = False
            
            ticks_to_next_ball -= 1
            if ticks_to_next_ball <= 0:
                ticks_to_next_ball = 25
                ball_shape = add_ball(space)
                balls.append(ball_shape)

            screen.fill(THECOLORS["white"])
            
            for ball in balls:
                draw_ball(screen, ball)
            
            space.step(1/50.0)
            
            pygame.display.flip()
            clock.tick(50)
            
    if __name__ == '__main__':
        sys.exit(main())   

A static L
==============
Falling balls are quite boring. We don't see any physics simulation except 
basic gravity, and everyone can do gravity without help from a physics 
library. So lets add something the balls can land on, two static lines 
forming an L. As with the balls we start with a function to add an L to the 
space::

    def add_static_L(space):
        body = pymunk.Body() # 1
        body.position = (300, 300)    
        l1 = pymunk.Segment(body, (-150, 0), (255, 0), 5) # 2
        l2 = pymunk.Segment(body, (-150, 0), (-150, 50), 5)
                
        space.add(l1, l2) # 3
        return l1,l2

#. We create a "static" body. The important step is to never add it to the space. Note how static bodies are created by not passing any arguments to the Body constructor.
#. A line shaped shape is created here.
#. Again, we only add the segments, not the body to the space.

Next we add a function to draw the L shape::

    def draw_lines(screen, lines):
        for line in lines:
            body = line.body
            pv1 = body.position + line.a.rotated(body.angle) # 1
            pv2 = body.position + line.b.rotated(body.angle)
            p1 = to_pygame(pv1) # 2
            p2 = to_pygame(pv2)
            pygame.draw.lines(screen, THECOLORS["lightgray"], False, [p1,p2])

#. In order to get the position with the line rotation we use this 
calculation. line.a is the first endpoint of the line, line.b the second. 
At the moment the lines are static, so we don't really have to do this extra 
calculation, but we will soon make them move and rotate.

#. This is a little function to convert coordinates from pymunk to pygame 
world. Now that we have it we can use it in the draw_ball() function as 
well. We want to flip the y coordinate (-p.y), and then offset it with the 
screen height (+600). It looks like this:

::

    def to_pygame(p):
        """Small hack to convert pymunk to pygame coordinates"""
        return int(p.x), int(-p.y+600)


We add a call to add_static_L() and one to draw_lines() and now we should 
see an inverted L shape in the middle will balls spawning and hitting the 
shape. ::


    import sys, random
    import pygame
    from pygame.locals import *
    from pygame.color import *
    import pymunk as pm
    import math

    #def to_pygame(p):
    #def add_ball(space):
    #def draw_ball(screen, ball):
    #def add_static_l(space):
    #def draw_lines(screen, lines):

    def main():
        pygame.init()
        screen = pygame.display.set_mode((600, 600))
        pygame.display.set_caption("Joints. Just wait and the L will tip over")
        clock = pygame.time.Clock()
        running = True
        
        space = pymunk.Space()
        space.gravity = (0.0, -900.0)
        
        lines = add_static_L(space)
        balls = []
        
        ticks_to_next_ball = 10
        while running:
            for event in pygame.event.get():
                if event.type == QUIT:
                    running = False
                elif event.type == KEYDOWN and event.key == K_ESCAPE:
                    running = False
            
            ticks_to_next_ball -= 1
            if ticks_to_next_ball <= 0:
                ticks_to_next_ball = 25
                ball_shape = add_ball(space)
                balls.append(ball_shape)

            screen.fill(THECOLORS["white"])
            
            for ball in balls:
                draw_ball(screen, ball)
            
            draw_lines(screen, lines)
            
            space.step(1/50.0)
            
            pygame.display.flip()
            clock.tick(50)
            
    if __name__ == '__main__':
        sys.exit(main())
    

Joints (1)
==============

A static L shape is pretty boring. So lets make it a bit more exciting by 
adding two joints, one that it can rotate around, and one that prevents it 
from rotating too much. In this part we only add the rotation joint, and in 
the next we constrain it. As our static L shape won't be static anymore we 
also rename the function to add_L(). ::

    def add_L(space):
        rotation_center_body = pymunk.Body() # 1
        rotation_center_body.position = (300,300)
        
        body = pymunk.Body(10, 10000) # 2
        body.position = (300,300)    
        l1 = pymunk.Segment(body, (-150, 0), (255.0, 0.0), 5.0)
        l2 = pymunk.Segment(body, (-150.0, 0), (-150.0, 50.0), 5.0)
        
        rotation_center_joint = pymunk.PinJoint(body, rotation_center_body, (0,0), (0,0)) # 3    

        space.add(l1, l2, body, rotation_center_joint)
        return l1,l2

#. This is the rotation center body. Its only purpose is to act as a static 
point in the joint so the line can rotate around it. As you see we never add 
any shapes to it.

#. The L shape will now be moving in the world, and therefor it can no 
longer have infinite mass. I have precalculated the inertia to 10000. 
(ok, I just took a number that worked, the important thing is that it looks 
good on screen!).
#. A pin joint allow two objects to pivot about a single point. In our case 
one of the objects will be stuck to the world.

To make it easy to see the point we draw a little red ball in its center ::

    pygame.draw.circle(screen, THECOLORS["red"], (300,300), 5)

In a bigger program you will want to get the rotation_center_body.position 
instead of my little cheat here with (300,300), but it will work for this 
tutorial as the rotation center is static.

Joints (2)
==============

In the previous part we added a pin joint, and now its time to constrain the 
rotating L shape to create a more interesting simulation. In order to do this 
we modify the add_L() function::

    def add_L(space):
        rotation_center_body = pymunk.Body()
        rotation_center_body.position = (300,300)
        
        rotation_limit_body = pymunk.Body() # 1
        rotation_limit_body.position = (200,300)
        
        body = pymunk.Body(10, 10000)
        body.position = (300,300)    
        l1 = pymunk.Segment(body, (-150, 0), (255.0, 0.0), 5.0)
        l2 = pymunk.Segment(body, (-150.0, 0), (-150.0, 50.0), 5.0)
        
        rotation_center_joint = pymunk.PinJoint(body, rotation_center_body, (0,0), (0,0)) 
        joint_limit = 25
        rotation_limit_joint = pymunk.SlideJoint(body, rotation_limit_body, (-100,0), (0,0), 0, joint_limit) # 2

        space.add(l1, l2, body, rotation_center_joint, rotation_limit_joint)
        return l1,l2

#. We add a body..

#. Create a slide joint. It behaves like pin joints but have a minimum and 
maximum distance. The two bodies can slide between the min and max, and in 
our case one of the bodies is static meaning only the body attached with the 
shapes will move.

And to make it a bit more clear, we draw a circle to do symbolize the joint 
with a green circle with its radius set to the joint max::

    pygame.draw.circle(screen, THECOLORS["green"], (200,300), 25, 2)


The end
===========

You might notice that we never delete balls. This will make the simulation 
require more and more memory and use more and more cpu, and this is of 
course not what we want. So in the final step we add some code to remove 
balls from the simulation when they are bellow the screen. ::

    balls_to_remove = []
    for ball in balls:
        if ball.body.position.y < 0: # 1
            balls_to_remove.append(ball) # 2
        draw_ball(screen, ball)
    
    for ball in balls_to_remove:
        space.remove(ball, ball.body) # 3
        balls.remove(ball) # 4

#. As we already have a loop we reuse it.. Check if the body.position is less 
than 0.
#. If that is the case, we add it to our list of balls to remove.
#. To remove an object from the space, we need to remove its shape and its 
body.
#. And then we remove it from our list of balls.

And now, done! You should have an inverted L shape in the middle of the 
screen being filled will balls, tipping over releasing them, tipping back and 
start over. You can check demo_slide_and_pinjoint.py included in pymunk, but 
it doesn't follow this tutorial exactly as I factored out a couple of blocks 
to functions to make it easier to follow in tutorial form. 

If anything is unclear, not working feel free to add a comment in the bottom 
of the page. If you have an idea for another tutorial you want to read, or 
some example code you want to see included in pymunk, please write it 
somewhere (like in the chipmunk forum)

The full code for this tutorial is::

    import sys, random
    import pygame
    from pygame.locals import *
    from pygame.color import *
    import pymunk
    import math

    def to_pygame(p):
        """Small hack to convert pymunk to pygame coordinates"""
        return int(p.x), int(-p.y+600)

    def add_ball(space):
        """Add a ball to the given space at a random position"""
        mass = 1
        radius = 14
        inertia = pymunk.moment_for_circle(mass, 0, radius, (0,0))
        body = pymunk.Body(mass, inertia)
        x = random.randint(120,380)
        body.position = x, 550
        shape = pymunk.Circle(body, radius, (0,0))
        space.add(body, shape)
        return shape

    def draw_ball(screen, ball):
        """Draw a ball shape"""
        p = int(ball.body.position.x), 600-int(ball.body.position.y)
        pygame.draw.circle(screen, THECOLORS["blue"], p, int(ball.radius), 2)

    def add_L(space):
        """Add a inverted L shape with two joints"""
        rotation_center_body = pymunk.Body()
        rotation_center_body.position = (300,300)
        
        rotation_limit_body = pymunk.Body() # 1
        rotation_limit_body.position = (200,300)
        
        body = pymunk.Body(10, 10000)
        body.position = (300,300)    
        l1 = pymunk.Segment(body, (-150, 0), (255.0, 0.0), 5.0)
        l2 = pymunk.Segment(body, (-150.0, 0), (-150.0, 50.0), 5.0)
        
        rotation_center_joint = pymunk.PinJoint(body, rotation_center_body, (0,0), (0,0)) 
        joint_limit = 25
        rotation_limit_joint = pymunk.SlideJoint(body, rotation_limit_body, (-100,0), (0,0), 0, joint_limit) # 3

        space.add(l1, l2, body, rotation_center_joint, rotation_limit_joint)
        return l1,l2

    def draw_lines(screen, lines):
        """Draw the lines"""
        for line in lines:
            body = line.body
            pv1 = body.position + line.a.rotated(body.angle)
            pv2 = body.position + line.b.rotated(body.angle)
            p1 = to_pygame(pv1)
            p2 = to_pygame(pv2)
            pygame.draw.lines(screen, THECOLORS["lightgray"], False, [p1,p2])


    def main():
        pygame.init()
        screen = pygame.display.set_mode((600, 600))
        pygame.display.set_caption("Joints. Just wait and the L will tip over")
        clock = pygame.time.Clock()
        running = True
        
        space = pymunk.Space()
        space.gravity = (0.0, -900.0)
        
        lines = add_L(space)
        balls = []
        
        ticks_to_next_ball = 10
        while running:
            for event in pygame.event.get():
                if event.type == QUIT:
                    running = False
                elif event.type == KEYDOWN and event.key == K_ESCAPE:
                    running = False
            
            ticks_to_next_ball -= 1
            if ticks_to_next_ball <= 0:
                ticks_to_next_ball = 25
                ball_shape = add_ball(space)
                balls.append(ball_shape)

            screen.fill(THECOLORS["white"])
            
            balls_to_remove = []
            for ball in balls:
                if ball.body.position.y < 150:
                    balls_to_remove.append(ball)
                draw_ball(screen, ball)
            
            for ball in balls_to_remove:
                space.remove(ball, ball.body)
                balls.remove(ball)
            
            draw_lines(screen, lines)
            
            pygame.draw.circle(screen, THECOLORS["red"], (300,300), 5)
            pygame.draw.circle(screen, THECOLORS["green"], (200,300), 25, 2)

            space.step(1/50.0)
            
            pygame.display.flip()
            clock.tick(50)
            
    if __name__ == '__main__':
        sys.exit(main())
