Problem with calculating line intersections

Question

I am trying to display the points of intersecting lines, but the calculated points are between the actual intersectings.

(https://en.wikipedia.org/wiki/Line%E2%80%93line_intersection)

I have checked the formulars multiple times and used another formula for calculating the intersecting points x, y [x3+u*(x4-x3), y3+u*(y4-y3)] instead of [x1+t*(x2-x1), y1+t*(y2-y1)], but that just made the points somewhere really wrong

("d" is not referenced on the wikipedia page and is just the divisor for the formulars of t and u)

Function for calculating the intersection

    def checkcol(self, startObs, endObs):
        x1, y1 = startObs
        x2, y2 = endObs
        x3, y3 = run.lamp
        x4, y4 = self.endpoint

        d = (x1-x2)*(y3-y4)-(y1-y2)*(x3-x4)
        t = ((x1-x3)*(y3-y4)-(y1-y3)*(x3-x4))/d
        u = ((x1-x2)*(y1-y3)-(y1-y2)*(x1-x3))/d
        if 0 < t < 1 and 1 > u > 0:
            pygame.draw.circle(run.screen, pygame.Color('green'), (round(x1+t*(x2-x1)), round(y1+t*(y2-y1))), 3)

The whole code

import pygame
import sys
import math
import random as rd

class Obs(object):
    def __init__(self, startp, endp):
        self.startp = startp
        self.endp = endp

    def drawww(self):
        pygame.draw.line(run.screen, pygame.Color('red'), (self.startp), (self.endp))
        for ray in run.lines:
            ray.checkcol(self.startp, self.endp)


class rays(object):
    def __init__(self, endpoint):
        self.width = 2
        self.endpoint = endpoint

    def draww(self):
        pygame.draw.line(run.screen, pygame.Color('white'), run.lamp, self.endpoint, 2)

    def moveEnd(self, xoff, yoff):
        self.endpoint[0] += xoff
        self.endpoint[1] += yoff

    def checkcol(self, startObs, endObs):
        x1, y1 = startObs
        x2, y2 = endObs
        x3, y3 = run.lamp
        x4, y4 = self.endpoint

        d = (x1-x2)*(y3-y4)-(y1-y2)*(x3-x4)
        t = ((x1-x3)*(y3-y4)-(y1-y3)*(x3-x4))/d
        u = ((x1-x2)*(y1-y3)-(y1-y2)*(x1-x3))/d
        if 0 < t < 1 and 1 > u > 0:
            pygame.draw.circle(run.screen, pygame.Color('green'), (round(x1+t*(x2-x1)), round(y1+t*(y2-y1))), 3)


class Control(object):
    def __init__(self):
        self.winw = 800
        self.winh = 800
        self.screen = pygame.display.set_mode((self.winw, self.winh))
        self.fps = 60
        self.clock = pygame.time.Clock()
        self.lamp = [400, 400]
        self.lampr = 13
        self.lines = []
        self.r = 10
        self.Obs = [Obs((rd.randint(0, self.winw), rd.randint(0, self.winh)),
                        (rd.randint(0, self.winw), rd.randint(0, self.winh))) for i in range(5)]
        self.done = False

    def event_loop(self):
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                self.done = True
            elif event.type == pygame.KEYDOWN:
                if event.key == pygame.K_F5:
                    self.__init__()

        if pygame.mouse.get_pressed() == (1, 0, 0):
            self.lamp = (pygame.mouse.get_pos())
            for line in self.lines:
                line.moveEnd(pygame.mouse.get_rel()[0], pygame.mouse.get_rel()[1])


    def draw(self):
        self.screen.fill((pygame.Color('black')))
        pygame.draw.circle(self.screen, pygame.Color('white'), self.lamp, self.lampr)
        for line in self.lines:
            line.draww()
        for obs in self.Obs:
            obs.drawww()

    def createlines(self):
        self.lines.clear()
        for angle in range(0, 361, 9):
            self.lines.append(rays([self.lamp[0] + 1200 * math.cos(angle), self.lamp[1] + 1200 * math.sin(angle)]))

    def main_loop(self):
        while not self.done:
            self.event_loop()
            self.createlines()
            self.draw()
            pygame.display.update()
            self.clock.tick(self.fps)
            pygame.display.set_caption(f"Draw  FPS: {self.clock.get_fps()}")


if __name__ == '__main__':
    run = Control()
    run.main_loop()
    pygame.quit()
    sys.exit()

Expected the points of intersecting to be at the actual points of intersection.

Answer 1

To find the intersection points of 2 rays or line segments in two-dimensional space, I use vector arithmetic and the following algorithm:

P     ... point on the 1. line
R     ... direction of the 1. line

Q     ... point on the 2. line
S     ... direction of the 2. line

alpha ... angle between Q-P and R
beta  ... angle between R and S

gamma  =  180° - alpha - beta

h  =  | Q - P | * sin(alpha)
u  =  h / sin(beta)

t  = | Q - P | * sin(gamma) / sin(beta)

t  =  dot(Q-P, (S.y, -S.x)) / dot(R, (S.y, -S.x))  =  determinant(mat2(Q-P, S)) / determinant(mat2(R, S))
u  =  dot(Q-P, (R.y, -R.x)) / dot(R, (S.y, -S.x))  =  determinant(mat2(Q-P, R)) / determinant(mat2(R, S))

X  =  P + R * t  =  Q + S * u

See also Line–line intersection

If t == 1, then X = P + R. This can be used to assess whether the intersection is on a line segment.
If a line is defined through the 2 points L1 and L2, it can be defined that P = L1 and R = L2-L1. Therefore the point of intersection (X) lies on the line segment from L1 to L2 if 0 <= t <= 1.
The same relation applies to u and S.

The following function implements the above algorithm using pygame.math.Vector2 objects of the pygame.math module:

def intersect_line_line_vec2(startObs, endObs, origin, endpoint):
    P = pygame.Vector2(startObs)
    R = (endObs - P)
    Q = pygame.Vector2(origin)
    S = (endpoint - Q)
    d = R.dot((S.y, -S.x))
    if d == 0:
        return None
    t = (Q-P).dot((S.y, -S.x)) / d 
    u = (Q-P).dot((R.y, -R.x)) / d
    if 0 <= t <= 1 and 0 <= u <= 1:
        X  =  P + R * t
        return (X.x, X.y)
    return None

The same algorithm without the use of the pygame.math module, less readable but more or less the same:

def intersect_line_line(P0, P1, Q0, Q1):  
    d = (P1[0]-P0[0]) * (Q1[1]-Q0[1]) + (P1[1]-P0[1]) * (Q0[0]-Q1[0]) 
    if d == 0:
        return None
    t = ((Q0[0]-P0[0]) * (Q1[1]-Q0[1]) + (Q0[1]-P0[1]) * (Q0[0]-Q1[0])) / d
    u = ((Q0[0]-P0[0]) * (P1[1]-P0[1]) + (Q0[1]-P0[1]) * (P0[0]-P1[0])) / d
    if 0 <= t <= 1 and 0 <= u <= 1:
        return P1[0] * t + P0[0] * (1-t), P1[1] * t + P0[1] * (1-t)
    return None

---

Applied to your code, this means:

class rays(object):

    # [...]

    def checkcol(self, startObs, endObs):
        P = pygame.Vector2(startObs)
        R = (endObs - P).normalize()
        Q = pygame.Vector2(run.lamp)
        S = (self.endpoint - Q).normalize()
        d = R.dot((S.y, -S.x))
        if R.dot((S.y, -S.x)) == 0:
            return
        t  =  (Q-P).dot((S.y, -S.x)) / d
        u  =  (Q-P).dot((R.y, -R.x)) / d
        if 0 <= t <= 1 and 0 <= u <= 1:
            X = P + R * t
            pygame.draw.circle(run.screen, pygame.Color('green'), (round(X.x), round(X.y)), 3)

---

Minimal example: repl.it/@Rabbid76/PyGame-IntersectLines

import pygame
import math
import random

def intersect_line_line_vec2(startObs, endObs, origin, endpoint):
    P = pygame.Vector2(startObs)
    R = (endObs - P)
    Q = pygame.Vector2(origin)
    S = (endpoint - Q)
    d = R.dot((S.y, -S.x))
    if d == 0:
        return None
    t = (Q-P).dot((S.y, -S.x)) / d 
    u = (Q-P).dot((R.y, -R.x)) / d
    if 0 <= t <= 1 and 0 <= u <= 1:
        X  =  P + R * t
        return (X.x, X.y)
    return None

def intersect_line_line(P0, P1, Q0, Q1):  
    d = (P1[0]-P0[0]) * (Q1[1]-Q0[1]) + (P1[1]-P0[1]) * (Q0[0]-Q1[0]) 
    if d == 0:
        return None
    t = ((Q0[0]-P0[0]) * (Q1[1]-Q0[1]) + (Q0[1]-P0[1]) * (Q0[0]-Q1[0])) / d
    u = ((Q0[0]-P0[0]) * (P1[1]-P0[1]) + (Q0[1]-P0[1]) * (P0[0]-P1[0])) / d
    if 0 <= t <= 1 and 0 <= u <= 1:
        return P1[0] * t + P0[0] * (1-t), P1[1] * t + P0[1] * (1-t)
    return None

def createRays(center):
    return [(center[0] + 1200 * math.cos(angle), center[1] + 1200 * math.sin(angle)) for angle in range(0, 360, 10)]

def createObstacles(surface):
    w, h = surface.get_size()
    return [((random.randrange(w), random.randrange(h)), (random.randrange(w), random.randrange(h))) for _ in range(5)]

window = pygame.display.set_mode((800, 800))
clock = pygame.time.Clock()

origin = window.get_rect().center
rays = createRays(origin)
obstacles = createObstacles(window)

move_center = True
run = True
while run:
    clock.tick(60)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
        if event.type == pygame.MOUSEBUTTONDOWN:
            obstacles = createObstacles(window) 
        if event.type == pygame.KEYDOWN:
            move_center = not move_center

    if move_center:
        origin = pygame.mouse.get_pos()
        rays = createRays(origin) 
        
    window.fill(0)
    for endpoint in rays:
        pygame.draw.line(window, (128, 128, 128), origin, endpoint)
    pygame.draw.circle(window, (255, 255, 255), origin, 10)
    for start, end in obstacles:
        pygame.draw.line(window, (255, 0, 0), start, end)
        for endpoint in rays:
            pos = intersect_line_line(start, end, origin, endpoint)
            if pos:
                pygame.draw.circle(window, (0, 255, 0), (round(pos[0]), round(pos[1])), 3)
    pygame.display.flip()

pygame.quit()
exit()