(a, b) = corner1		(coordinates of the side of the rectangle)
(c, d) = corner2

len = sqrt[(a - c)^2 + (b - d)^2]

ac' = (a - c) / len
bd' = (b - d) / len

dotProduct = x1*ac' + x2*bd'

xp = dotProduct * ac'
yp = dotProduct * bd'

xr = x1 - xp
yr = y1 - yp

x2 = xr - xp
y2 = yr - xp