a Physics Engine in Janet Part 2: Intersections

It's time for more physics! Today I'm going to go through 2 shapes, Circles and Rectangles (with rotation!), and the 3 types of intersections that entails.

  1. Circle vs Circle
  2. Rectangle vs Rectangle
  3. Circle vs Rectangle

Note that 99% of the logic here is based on this amazing physics tutorial on youtube. Thank you "Two-Bit Coding" for making an actual good tutorial with working code and logic I can understand 😅.


blah blah blah

(defn shape/circle [radius]
  {:type :circle
   :radius radius
   :area (* math/pi radius radius)

   :get-vertices (fn [self] nil)
   :draw (fn [self {:pos pos} col]
	   (tic80/circ ;(map math/round pos)
		       (self :radius)
	   (tic80/circb ;(map math/round pos)
			(self :radius)
(defn shape/rectangle [width height]
  {:type :rectangle
   :width width
   :height height
   :area (* width height)

   :get-vertices (fn [self]
		   (let [left (/ (* -1 width) 2)
			 right (+ (/ (* -1 width) 2) width)
			 top (/ (* -1 height) 2)
			 bottom (+ (/ (* -1 height) 2) height)]
		     [(vec/init left top)
		      (vec/init right top)
		      (vec/init right bottom)
		      (vec/init left bottom)]))
   :draw (fn [self body col]
	   (let [[a b c d] (:get-transformed-vertices body)]
	     (tic80/tri ;a ;b ;c col)
	     (tic80/tri ;a ;c ;d col)
	     (tic80/line ;a ;b 7)
	     (tic80/line ;b ;c 7)
	     (tic80/line ;c ;d 7)
	     (tic80/line ;d ;a 7)))})

Circle vs Circle

Rectangle vs Rectangle: the Seperate Axis Theorem

Rectangle vs Circle

Just like last time, heres the complete code!