The viewing or normal direction plus the fixed point and distance
only define the viewed image to within an arbitrary viewing angle,
there is still a choice of the orientation of the image on the
screen. There are a number of possibilities here. One is to consider
the 3D image as an object in space and display a projection of the
viewing plane as seen by an observer in the same 3D space. This has the
merit of providing clear feedback of the position of the plane within the
whole but is not ideal for painting because for some angles the projection
will introduce perspective distortion of the image.
There are two solutions adopted by the paint program both of which display
the view-plane ``flat'' on the screen, the view orthogonal to the
viewing direction. The difference in the two is the model used to establish
the rotation around the viewing direction. In the first versions of the
program the plane was displayed as if the viewer was ``walking'' around
the structure on the x-y plane. This proved confusing and therefore
a different model has been included which is to ensure that the projection
of an arbitrary but predefined vector
u (up) onto the viewing plane
will be parallel to the y-axis of the displayed plane (actually
``up'' on the screen is in the direction (0,-1) in screen
coordinates because traditionally raster coordinates start at the top
left corner of the screen). Both of these have
their advantages and disadvantages and therefore the either model can be
selected.
