[03/13/14][HW2] Image Rotation

(1) nx , ny 先平移 (-ox,-oy)，得到 nx2, ny2 

    nx2 = nx - ox , ny2 = ny - oy

(2) nx2, ny2 經由旋轉矩陣，旋轉 theta 度，得到對應的 x, y

     x = nx2 * cos(theta) + ny2 * sin(theta) + ox

(3) 再將 x, y 平移 (ox, oy)

    y = -nx2 * sin(theta) + ny2 * cos(theta) + oy

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