Beginner. 2d rotation gives unexpected results.

Joshua Landau joshua at landau.ws
Wed Jul 24 23:17:22 CEST 2013


On 23 July 2013 13:34, <enmce at yandex.ru> wrote:

> Hello!
> This is my first post, nice to meet you all!
> I`m biology student from Russia, trying to learn python to perform some
>
> simple simulations.
>
> Here`s my first problem.
> I`m trying to perform some simple 2d vector rotations in pygame, in order
>
> to learn the basics of linear algebra and 2d transformations. So far i
>
> understand matrix multiplication pretty well, and probably all my math is
>
> right. Eventually i`m planning to write Poly class, and use it to rotate
>
> and translate some simple shapes. But when i try and write it in the
>
> program, i get very weird results, like all points of rectangle with
>
> coordinates [0,0],[0,100],[100,0],[100,100] start to go spiral and
>
> eventually shrink to the center. Although even Excel calculations with
>
> this formulas give me right result.
> I use Python 3.3 on Windows Xp.
> What is wrong with my code?
>
> [code]import pygame
> import math as m
>

GAH!

Why on earth would you do such a thing?
Just "import math", there's no need to obfuscate your code.


> black = ( 0, 0, 0)
> white = ( 255, 255, 255)
> green = ( 0, 255, 0)
> red = ( 255, 0, 0)
>

It's probably better to do:

black = pygame.Color("Black")
white = pygame.Color("white")
green = pygame.Color("green")
red   = pygame.Color("red")


> class Poly():
>     pos = [100,100] #x and y coordinates of a point

    rot = m.radians(1) #rotation in degrees
>

*Do not do this*

This is because classes have shared values -- these "pos" and "rot" values
are shared within the class.

>>> class P:
...     n = []
...     def more_n(self):
...         self.n.append(len(self.n))
...
...
...
>>> one_P = P()
>>> two_P = P()
>>>
>>> one_P.more_n()
>>> one_P.more_n()
>>> one_P.more_n()
>>>
>>> two_P.n
[0, 1, 2]


Normally you want to set these at initialisation:

class Poly():
    def __init__(self, pos=None, rot=math.radians(1)):
        self.pos = [100, 100] if pos is None else pos
        self.rot = rot
        super().__init__(self)


>     def draw(self): #draw point
>         pygame.draw.circle(screen,white,self.pos,10,0)
>

Add some spaces, dude.

I was going to say:

> Also, use keyword arguments instead of throwing around "10" and "0" with
no context:
>     def draw(self):
>         pygame.draw.circle(screen, white, self.pos, radius=10, width=0)
> Pygame-ists will know that "width" means border_width, by now. Pygame
isn't known for it's clean design ;).

But pygame, being brilliant (not) decided that it won't let you.

    def rotate(self): # rotation method
>         sin = m.sin(self.rot) #calculationg sin and cos
>         cos = m.cos(self.rot)
>         x_rot = int(self.pos[0]*cos-self.pos[1]*sin) #mulpitplicating

vector to rotation matrix
>         y_rot = int(self.pos[0]*sin+self.pos[1]*cos)
>
>         self.pos[0] = x_rot #set new coordinates to a point
>         self.pos[1] = y_rot
>


A lot of your comments are ridiculous. This one is particularly so:
#mulpitplicating vector to rotation matrix. Don't add comments that talk
about lines. Here is a quick guide for when to use comments:

1) API usage, when docstrings aren't usable

2) When something funny or unexpected occurs in the code, such as:

    # Goes down to 0 (does not include end-point)
    for i in range(foo, -1, -1): ...

3) To explain large-scale methodologies and algorithms

Other than this, are you trying to obfuscate this line? HINT: ADD SPACES ;).


A big problem here (this solves your problem) is your int(...) conversions.
Do *not* store important information less accurately than it deserves. This
is one very important instance. Only convert to int when you need to.

Another thing: "sin = m.sin(self.rot)"??? Really? Write "sin_1deg = ..."
instead, at least.


> a = Poly() #Some simple sample points giving rectangle
> b = Poly()
> c = Poly()
> d = Poly()
>
> b.pos = [0,100]
> c.pos = [100,0]
> d.pos = [0,0]
>

Use:

a = Poly()
b = Poly([0, 100])
c = Poly([100, 0])
d = Poly([0, 0])

pygame.init()
> size = [700,500]
> screen = pygame.display.set_mode(size)
> done = False
> clock = pygame.time.Clock()
> while done == False:
>

"while not done"

Also, just use "while True" and a "break". I know some C-people or what not
think this is "evil" or something, but they're wrong.

Personally, I actually like using:

while "rotating the squares":
    ...

instead of "while True". It's no slower and it's free documentation.


>     for event in pygame.event.get():
>         if event.type == pygame.QUIT:
>             done = True
>
>     a.rotate() #perform rotation
>     b.rotate()
>     c.rotate()
>     d.rotate()
>
>     screen.fill(black)
>
>     a.draw() #draw point
>     b.draw()
>     c.draw()
>     d.draw()
>     pygame.display.flip()
>     clock.tick(30)
>
> pygame.quit()[/code]
>

You don't need pygame.quit(). You *only* want pygame.quit() if you're
quitting Pygame *before* the program is finished.
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