Fixing Box2D Drawings

Question

I am trying to draw a rectangle which would have an edge next to each of its sides (it should represent a building with a fence) for one building it goes quite well, but when I am trying to add a new building it messes itself up completely.

I have this code to create buildings:

class Building():
    def __init__(self, Box_2_World,shape, position, sensor= None):
        self.Box_2_World = Box_2_World
        self.shape = shape
        self.position = position

        if sensor == None:
            sensor = False

        self.sensor = sensor
        self.footprint = self.Box_2_World.CreateStaticBody(position = position,
                                                           angle = 0.0,
                                                           fixtures = b2FixtureDef(
                                                               shape = b2PolygonShape(box=(self.shape)),
                                                               density = 1000,
                                                               friction = 1000))

        self.Lower_Fence = self.Box_2_World.CreateStaticBody(position=(self.footprint.position[0],self.footprint.position[1] -1.75*self.shape[1]))
        self.Lower_Fence.CreateEdgeChain([(self.Lower_Fence.position[0]-4.25*self.shape[0],self.Lower_Fence.position[1]),
                            (self.Lower_Fence.position[0]-2.25*self.shape[0],self.Lower_Fence.position[1]),
                        ])

        self.Right_Fence = self.Box_2_World.CreateStaticBody(position=(self.footprint.position[0]-1*self.shape[0],self.footprint.position[1]))
        self.Right_Fence.CreateEdgeChain([(self.Right_Fence.position[0],self.Right_Fence.position[1] - 1.25*self.shape[1]),
                            (self.Right_Fence.position[0],self.Right_Fence.position[1]-3.25*self.shape[1]),
                        ])

        self.Upper_Fence = self.Box_2_World.CreateStaticBody(position=(self.footprint.position[0],self.footprint.position[1] -0.45* self.shape[1]))
        self.Upper_Fence.CreateEdgeChain([(self.Upper_Fence.position[0] - 4.25* self.shape[0],self.Upper_Fence.position[1]),
                            (self.Upper_Fence.position[0]- 3.25* self.shape[0]+ self.shape[0],self.Upper_Fence.position[1]),
                        ])

        self.Left_Fence = self.Box_2_World.CreateStaticBody(position=(self.footprint.position[0]-2.25*self.shape[0],self.footprint.position[1]))
        self.Left_Fence.CreateEdgeChain([(self.Left_Fence.position[0],self.Left_Fence.position[1] - 1.25*self.shape[1]),
                            (self.Left_Fence.position[0],self.Left_Fence.position[1]-3*self.shape[1]),
                        ])

Skyscrapers = []
Rectangles = [(pos_X-5, pos_Y-5),(pos_X+15, pos_Y -5),(pos_X - 5,pos_Y + 15),(pos_X+15, pos_Y + 15)]

for i in range(4):
    Skyscrapers.append(Building(Box_2_World,shape = (5,5), position =  Rectangles[i]))

and these functions to draw it using PyGame:

SCREEN_OFFSETX, SCREEN_OFFSETY = SCREEN_WIDTH/16, SCREEN_HEIGHT

def fix_vertices(vertices):
    return [(int(SCREEN_OFFSETX + v[0]), int(SCREEN_OFFSETY - v[1])) for v in vertices]
def _draw_polygon(polygon, screen, body, fixture):
    transform = body.transform
    vertices = fix_vertices([transform * v * PPM for v in polygon.vertices])
    pygame.draw.polygon(
        screen, [c / 2.0 for c in colors[body.type]], vertices, 0)
    pygame.draw.polygon(screen, colors[body.type], vertices, 1)
polygonShape.draw = _draw_polygon

def _draw_edge(edge, screen, body, fixture):
    vertices = fix_vertices(
        [body.transform * edge.vertex1 * PPM, body.transform * edge.vertex2 * PPM])
    pygame.draw.line(screen, colors[body.type], vertices[0], vertices[1])
edgeShape.draw = _draw_edge

And the output is this: Blue rectangles represent buildings, blue lines are fences, first building fits quite nice, but the others are for some reason out of desired positions

Also, if you find out a way how to create the fences using for loop, it would be great (that's the reason why I put for-loop tag into this question)

Any help appreciated

Answer 1

The coordinates which are passed to CreateEdgeChain have to be relative to the body.

The position of the body is set when the object is constructed eg:

self.Lower_Fence = self.Box_2_World.CreateStaticBody(
    position=(self.footprint.position[0], self.footprint.position[1] -1.75*self.shape[1]))

And the edges have to be relative to this position rather than an absolute position.eg:

self.Lower_Fence.CreateEdgeChain([(-4.25*self.shape[0], 0), (-2.25*self.shape[0], 0)])

To create the fences in a for -loop, you've to define a list of the edges. With the list of the edges you can create a list of fences:

fence_edges = [
    [(-4.25, -1.75), (-2.25, -1.75)],
    [(-1.00, -1.25), (-1.00, -3.25)],
    [(-4.25, -0.45), (-2.25, -0.45)],
    [(-2.25, -1.25), (-2.25, -3.25)]
]

self.Fences = []
for edge in fence_edges:
    p1, p2 = edge
    fence = self.Box_2_World.CreateStaticBody(position=self.footprint.position[:])
    fence.CreateEdgeChain(
        [(p1[0] * self.shape[0], p1[1] * self.shape[1]),
         (p2[0] * self.shape[0], p2[1] * self.shape[1])])
    self.Fences.append(fence)

self.Lower_Fence, self.Right_Fence, self.Upper_Fence, self.Left_Fence = self.Fences