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Data Members

Point position Private variable
The location of the Focus in the world coordinate system.

Point direction Private variable
The direction of view from position into the scene.

Point up Private variable
The direction that will be at the top of the projected drawing.

real distance Private variable
The distance of the Focus from the plane of projection.

real angle Private variable
Used for determining the up direction.

char axis Private variable
The main axis onto which the Focus is transformed in order to perform the perspective projection, z by default.

It will normally not matter which axis is used, but it might be advantageous to use a particular axis in some special situations.

Transform transform Private variable
The Transform, which will be applied to the Shapes on the Picture, when the latter is output. The effect of this is equivalent to transforming the Focus, so that it lies on a major axis. 
          Focus f(5, 5, -10, 2, 4, 10, 10, 180);
          =>
          
          f.transform ==
           0.989  -0.00733  -0.148  0
           0       0.999    -0.0494 0
           0.148   0.0488    0.988  0
          -3.4    -4.47      0.865  1

Transform persp Private variable
The Transform representing the perspective transformation for a particular Focus. Let d stand for distance, then 
          persp ==
          1 0 0  0
          0 1 0  0
          0 0 0 1/d
          0 0 0  1