Viewpoint boxes

These three boxes determine the position of the viewer looking directly towards the geometrical centre of the model. The ``View distance'' only affects the appearance of the graph if the model is viewed with perspective. It gives the radial distance from the viewer to the centre of the model, while the angles follow the spherical co-ordinates as defined in Figure [*]. The values in these boxes can be changed by clicking on the spin buttons to the side of each box. The increments (when the spin button is clicked) of θ and φ are fixed at 5o and that of r are determined from the size of the model with the r-value being reset each time the model is drawn to full scale. Values for θ, φ and r can also be typed in manually into the Viewpoint boxes (any integer value is allowed).

Note:— Each time a key is pressed, new viewpoint information is calculated and evaluated, and the displayed graph is updated. The values for the angles are tested and reset if they lie outside the allowable limits for θ, φ and r. This means that it will usually be necessary to delete characters in the Viewpoint boxes, before trying to enter new ones.



Figure: The definition of the angles to the viewpoint, P.
\begin{figure} \centerline{\picbox{assefig.pcx}{65}{70}} \end{figure}