Weird geometry error when using transform matrices and line/polyline

[kchanqvq]

Test program:

(defsketch test
    ((title "test") (width 640) (height 640))
  (let ((size 48))
    (scale (/ size 2))
    (translate 4 4)
    (with-pen (make-pen :stroke +white+ :weight (/ 2 size))
      #+nil (line 0.4 0.4 1 1)
      #+nil (line 1 1 0.4 0.4)
      (polyline 0.8 0.8 -1 -1 -0.8 -0.8))))

Problem:

1. the output from the polyline is wrong, it flies outside the screen.

2. The output of the two (commented out in the snippet) line has different position. When composing larger algorithms I find geometry gets put in wrong positions everywhere.

[kchanqvq]

Some more issues:
3. Turns out polyline breaks completely when I use (with-pen (make-pen :stroke +black+ :weight 2) ...), even without any transform.
4. The thickness calculation when using line and (with-pen (make-pen :stroke +black+ :weight 2) ...) looks fishy. I see visually inconsistent thickness when there are overlapping lines, could be the triangulation introduced some tiny offset so lines don't overlap exactly.

Update: according to my partial understanding the issue comes from the treatment of collinear or overlapping line segment, which causes intersect-lines to return abnormal values. Not sure about how to solve it...

[Kevinpgalligan]

Relevant article shared by @Gleefre: https://mattdesl.svbtle.com/drawing-lines-is-hard