(中文字幕，按cc )(Click cc for english caption)
如何為一個無平行邊的4邊形拍張正方形照片？(Followed by english description)
在眼睛前面放個無限大張的螢幕，所有螢幕前的點和眼睛連線都會跟螢幕有交點
此交點就是那點的"像"，此動畫展示如何由三個基本性質：
1. 半圓的圓周角為90度
2. 四個角都是直角的四邊形就是矩形
3. 對角線互相垂直的矩形就是正方形
找到可以照出正方形照片的位置。
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Take a picture of a quadrilateral (which has no parallel sides).
Is it possible to get a square image?
This animation demonstrate how to get the camera position, base on three facts
(1) The circumferential angle of semi-circle is 90 degree,
(2) Rectangle: quadrilateral in which all four angles are right angles.
(3) Square: rectangle in which the diagonals are perpendicular.
