Getting focal length and focal point from a projection matrix

问题

I have a 4x4 projection matrix

(SCNMatrix4) 
   s = (m11 = 1.83226573, 
   m12 = 0, 
   m13 = 0, 
   m14 = 0,
   m21 = 0,
   m22 = 2.44078445,
   m23 = 0,
   m24 = 0,
   m31 = -0.00576340035, 
   m32 = -0.0016724075, 
   m33 = -1.00019991, 
   m34 = -1, 
   m41 = 0, 
   m42 = 0, 
   m43 = -0.20002, 
   m44 = 0)

I would like to get the focal point and the focal length out of this matrix.

回答1:

From slides 4 and 5 on this GDC presentation:

The focal length is merely the first element in the matrix (m11).

The focal point, however, cannot be extracted from this matrix alone - you need the camera direction D and position P. Once you have them, simply do P + D * m11 to obtain the focal point.

来源：https://stackoverflow.com/questions/46075606/getting-focal-length-and-focal-point-from-a-projection-matrix