BezierCurve

This script contains a function, calculate_bezier to calculate a bezier curve and a short demo to display how it can be used. The calculate_bezier function is a direct port of the forward differencing bezier curve algorithm described here.

It depends on the 2d vector class here in the pygame cookbook: 2DVectorClass

Controls for the demo:
Right mouse button - Add a control point
Left mouse button - Move a control point

"""
bezier.py - Calculates a bezier curve from control points. 
 
Depends on the 2d vector class found here: http://www.pygame.org/wiki/2DVectorClass
 
2007 Victor Blomqvist
Released to the Public Domain
"""
import pygame
from pygame.locals import *
 
from vec2d import *
 
gray = (100,100,100)
lightgray = (200,200,200)
red = (255,0,0)
green = (0,255,0)
blue = (0,0,255)
X,Y,Z = 0,1,2
 
def calculate_bezier(p, steps = 30):
    """
    Calculate a bezier curve from 4 control points and return a 
    list of the resulting points.
    
    The function uses the forward differencing algorithm described here: 
    http://www.niksula.cs.hut.fi/~hkankaan/Homepages/bezierfast.html
    """
    
    t = 1.0 / steps
    temp = t*t
    
    f = p[0]
    fd = 3 * (p[1] - p[0]) * t
    fdd_per_2 = 3 * (p[0] - 2 * p[1] + p[2]) * temp
    fddd_per_2 = 3 * (3 * (p[1] - p[2]) + p[3] - p[0]) * temp * t
    
    fddd = 2 * fddd_per_2
    fdd = 2 * fdd_per_2
    fddd_per_6 = fddd_per_2 / 3.0
    
    points = []
    for x in range(steps):
        points.append(f)
        f += fd + fdd_per_2 + fddd_per_6
        fd += fdd + fddd_per_2
        fdd += fddd
        fdd_per_2 += fddd_per_2
    points.append(f)
    return points
    
def main():
    pygame.init()
    screen = pygame.display.set_mode((600, 600))
 
    ### Control points that are later used to calculate the curve
    control_points = [vec2d(100,100), vec2d(150,500), vec2d(450, 500), vec2d(500,150)]
 
    ### The currently selected point
    selected = None
    
    clock = pygame.time.Clock()
    
    running = True
    while running:
        for event in pygame.event.get():
            if event.type in (QUIT, KEYDOWN):
                running = False
            elif event.type == MOUSEBUTTONDOWN and event.button == 1:
                for p in control_points:
                    if abs(p.x - event.pos[X]) < 10 and abs(p.y - event.pos[Y]) < 10 :
                        selected = p
            elif event.type == MOUSEBUTTONDOWN and event.button == 3:
                x,y = pygame.mouse.get_pos()
                control_points.append(vec2d(x,y))
            elif event.type == MOUSEBUTTONUP and event.button == 1:
                selected = None
        
        ### Draw stuff
        screen.fill(gray)
                
        if selected is not None:
            selected.x, selected.y = pygame.mouse.get_pos()
            pygame.draw.circle(screen, green, selected, 10)
        
        ### Draw control points
        for p in control_points:
            pygame.draw.circle(screen, blue, p, 4)
        ### Draw control "lines"
        pygame.draw.lines(screen, lightgray, False, control_points)
        ### Draw bezier curve
        for x in range(0,len(control_points)-1,3):
            b_points = calculate_bezier(control_points[x:x+4])
            pygame.draw.lines(screen, red, False, b_points)
        
        ### Flip screen
        pygame.display.flip()
        clock.tick(100)
        #print clock.get_fps()
    
if __name__ == '__main__':
    main()