球

例えば球の頂点データを作るのにたったこれだけっていうのは、やっぱり短いし楽だと思う。

(defun circle-xy(index step)
  (let ((radian (/ (* 2 pi index) (- step 1))))
    (list (sin radian) (cos radian))))

(defun sphere-xyz(circle-xy index step)
  (let ((yz (let ((radian (/ (* pi index) (- step 1))))
	      (list (sin radian) (cos radian)))))
    (list (* (elt yz 0) (elt circle-xy 0))
	  (elt yz 1)
	  (* (elt yz 0) (elt circle-xy 1)))))

(defun sphere-vertices(radius slice-dim)
  (let ((radius radius))
    (apply #'append
	   (loop for i from 0 below (elt slice-dim 0) collect
		(loop for j from 0 below (elt slice-dim 1) collect
		     (let ((seed (sphere-xyz (circle-xy i (elt slice-dim 0)) j (elt slice-dim 1))))
		       (list (mapcar #'(lambda (x) (* x radius)) seed)
			     seed
			     (list (/ i (- (elt slice-dim 0) 1)) (/ j (- (elt slice-dim 1) 1))))))))))

(defun sphere-indices(slice-dim)
  (let ((x (elt slice-dim 0))
	(y (elt slice-dim 1)))
    (loop for i from 0 below (- x 1) collect
	 (apply #'append
		(loop for j from 0 below y collect
		     (list
		      (+ (* i y) j)
		      (+ (* (+ i 1) y) j)))))))