CODEWITHSUNDEEP
mathtrigonometry
Chris Hansen
Chris Hansen

Reputation: 335

Find corners of a rotated rectangle given its center point and rotation

Can someone give me an algorithm that finds the position of all four corners of a rectangle if I know its center point(in global coordinate space), width and height, and its rotation around that center point?

clarification edit: The width and height I am referring to is the length of the sides of the rectangle.

Upvotes: 10

Views: 17485

Answers (4)

tCot
tCot

Reputation: 357

Code Python with matrices :

import numpy as np
import math
import matplotlib.pyplot as plt
from matplotlib.patches import Rectangle, Circle

#center of rectangle
X = 2698.77 
Y = 1283.01
center = np.array([[X],[Y]])

angle_deg = 83.5694 #angle rectangle
angle = math.radians(angle_deg)

# rectangle's dimension
width = 2022.23
height = 1978.78

R_lt = np.array([[np.cos(angle),-np.sin(angle)],[-np.sin(angle),-np.cos(angle)]])
A = np.dot(R_lt,np.array([[width/2], [height/2]])) + center

R_rt = np.array([[np.cos(angle),np.sin(angle)],[-np.sin(angle),np.cos(angle)]])
B = np.dot(R_rt,np.array([[width/2], [height/2]])) + center

R_rb = np.array([[-np.cos(angle),np.sin(angle)],[np.sin(angle),np.cos(angle)]])
C = np.dot(R_rb,np.array([[width/2], [height/2]])) + center

R_lb = np.array([[-np.cos(angle),-np.sin(angle)],[np.sin(angle),-np.cos(angle)]])
D = np.dot(R_lb,np.array([[width/2], [height/2]])) + center

corners = [A,B,C,D]

Upvotes: 0

Tonia Sanzo
Tonia Sanzo

Reputation: 355

The coordinates of each vertices:

 Center point = (center.x, center.y)
 Angle        = angle
 Height       = height
 Width        = width      



TOP RIGHT VERTEX:
Top_Right.x = center.x + ((width / 2) * cos(angle)) - ((height / 2) * sin(angle))
Top_Right.y = center.y + ((width / 2) * sin(angle)) + ((height / 2) * cos(angle))



TOP LEFT VERTEX:
Top_Left.x = center.x - ((width / 2) * cos(angle)) - ((height / 2) * sin(angle))
Top_Left.y = center.y - ((width / 2) * sin(angle)) + ((height / 2) * cos(angle))



BOTTOM LEFT VERTEX:
Bot_Left.x = center.x - ((width / 2) * cos(angle)) + ((height / 2) * sin(angle))
Bot_Left.y = center.y - ((width / 2) * sin(angle)) - ((height / 2) * cos(angle))



BOTTOM RIGHT VERTEX:
Bot_Right.x = center.x + ((width / 2) * cos(angle)) + ((height / 2) * sin(angle))
Bot_Right.y = center.y + ((width / 2) * sin(angle)) - ((height / 2) * cos(angle))

This algorithm is a compressed version of these 3 steps:

Step 1: Center your rectangle around the origin

Step 2: Apply the rotation matrix to each vertex

Step 3: Move the rotated rectangle to the correct position, by adding the center point to each coordinate

This is explained in more depth here https://math.stackexchange.com/questions/126967/rotating-a-rectangle-via-a-rotation-matrix

Upvotes: 9

Tomáš Sláma
Tomáš Sláma

Reputation: 122

If you need all of the corners, it just might be faster to create two perpendicular vectors from the center of the rectangle to both of its sides, and then to add/subtract these vectors to/from the center of the rectangle to form the points.

This might be faster, since you don't need to repeatedly call sin() and cos() functions (you do so only once for each).

Assuming we have a Vector library (for cleaner code - only helps with vector arithmetic), here is the code in Python:

def get_corners_from_rectangle(center: Vector, angle: float, dimensions: Vector):
   # create the (normalized) perpendicular vectors
   v1 = Vector(cos(angle), sin(angle))
   v2 = Vector(-v1[1], v1[0])  # rotate by 90

   # scale them appropriately by the dimensions
   v1 *= dimensions[0] / 2
   v2 *= dimensions[1] / 2

   # return the corners by moving the center of the rectangle by the vectors
   return [
      center + v1 + v2,
      center - v1 + v2,
      center - v1 - v2,
      center + v1 - v2,
   ]

Upvotes: 6

MBo
MBo

Reputation: 80287

Top right corner has coordinates w/2, h/2 relative to the center. After rotation its absolute coordinates are

 x = cx + w/2 * Cos(Phi) - h/2 * Sin(Phi)
 y = cy + w/2 * Sin(Phi) + h/2 * Cos(Phi)

Upvotes: 14

Related Questions