en...@yandex.ru wrote: > This is my first post, nice to meet you all!
Welcome! > 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? > 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 The conversion to int introduces a rounding error that accumulates over time. > 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 One way to keep the error low is to keep the float values in self.pos and do the rounding on the fly when you display the point: class Poly(): def __init__(self, color, pos, rot=m.radians(1)): self.color = color self.pos = pos self.rot = rot def draw(self): x, y = self.pos pygame.draw.circle(screen, self.color, [350+int(x), 250+int(y)], 10, 0) def rotate(self): sin = m.sin(self.rot) cos = m.cos(self.rot) x_rot = self.pos[0]*cos-self.pos[1]*sin y_rot = self.pos[0]*sin+self.pos[1]*cos self.pos = [x_rot, y_rot] a = Poly(white, [100, 100]) b = Poly(green, [0, 100]) c = Poly(blue, [100, 0]) d = Poly(red, [0, 0]) -- http://mail.python.org/mailman/listinfo/python-list