On 07/23/2013 08:27 AM, enmce 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.
Hello, and welcome.
However, please don't post identical messages on tutor and python-list.
The response you got there from Peter Otten was right on.
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 instead of rotating right. Although even Excel calculations with this
formulas give me right result.
What is wrong with my code?
[code]import pygame
import math as m
black = ( 0, 0, 0)
white = ( 255, 255, 255)
green = ( 0, 255, 0)
red = ( 255, 0, 0)
class Poly():
pos = [100,100] #x and y coordinates of a point
rot = m.radians(1) #rotation in degrees
def draw(self): #draw point
pygame.draw.circle(screen,white,self.pos,10,0)
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)
By calling int() on those two lines, you have just drastically changed
the point's location. That won't be visible right away, but when you
use that value repeatedly, the error will add up pretty quickly.
In fact, just repeatedly calculating those values will eventually
produce some noticeable errors. Those errors can be eliminated as well,
but it'll be a bit harder.
--
DaveA
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