Dialog Box - called from: Input2 Menu
If perspective is not in effect, or turned off, the drawing is simply a perpendicular or orthographic projection down the observer x axis. If perspective is on, the drawing is projected from a point (xp+xo, yo, zo), where xp is the perspective distance and xo, yo and zo are the projection offsets (below), perpendicularly onto the plane (xo, y,z ), that is a horizontal plane passing through the structure center. Scaling applies to the projection plane, but obviously fixed scaling has limited intrinsic meaning. Generally the scaling modes Universal Maximum or Each View Maximum (set in the Scaling dialog in the Input2 menu) should be used when perspective is in effect. The perspective distance is the distance xp of the projection point from the projection plane, or in other words the distance from which the structure is viewed (distance from the center of the structure to the eye). Smaller distances will accentuate the "perspective"; larger distances will cause the plot to appear more like a straight-on projection. The perspective distance should be considerably larger than the distance of the furthest atom from the center (or the plane x=xo) in the positive x direction. If atoms come within about 1 Angstrom of the x coordinate of the eye they will be omitted, but many of such atoms would be off scale (sideways) anyway.
Projection Offsets. During calculation the midpoint of the structure is determined - this includes all atoms, but does not include crystal edges, unit cell or axes. When projecting in Perspective viewing, the coordinates of this midpoint, xo, yo and zo are used as the coordinates of the projection point (xp+xo, yo, zo), where xp is the perspective distance. This insures that the projection is never oblique to the projection plane. This is equivalent to the Automatic Centering option in previous versions of ATOMS. Although arbitrary specification of the offsets is no longer allowed, you can attain almost any attitude of the structure by suitable rotations. Note that the structure can be displaced in the plotting area with the Centering and Displacements dialog (Input2 menu). In effect, the projection direction moves along with the structure. If you also have structure axes or a unit cell (Display of Axes and Unit Cell dialogs, Input2 menu) which are separated from the structure, they will be projected obliquely, according to their distance in Angstroms from the structure center; that is, if you are looking down, say, the a-axis in the structure itself, structure axes placed to the side will not have that axis exactly parallel to the view direction. You can rotate to effect a compromise between the two orientations.