This is a set of miscellaneous instructions for specifying the drawing dimensions, viewing distance, general lettering orientation, pen displacement for line retracing, and ellipse smoothness.

• 301: Drawing Boundaries, Margin, and Viewing Distance
• 302: Title Rotation
• 303: Retrace Displacement
• 304: Ellipse Smoothness

This instruction defines the maximum x and y dimensions of the drawing and the drawing margin, all in inches. The boundary dimensions must not exceed those allowed by the plotter. The program will prevent the pen from getting closer than 0.1 in. to any boundary. The drawing margin is a constant width border inside the entire boundary. When automatic scaling is used (600 series instructions), the center points of the atoms are prevented from falling in the margin; but the atom representation, which has a finite size, may extend into that area. To accommodate this extension into the margin, the margin width should be large when the overall drawing scale and the ellipsoid scale are expected to be large.

In addition, instruction 301 specifies the perspective viewing distance, in inches, from the plane of the drawing. An entry of 0 for view distance is used to indicate an infinite view distance, and the crystal structure is then mapped in parallel projection normal to the drawing board.

For regular titles and chemical symbols, the title rotation is specified with instruction 302. The lettering base line for all lettering is rotated counterclockwise by an angle theta, in degrees, from the x axis of the plotter. Although any value is allowed, 0 and -90 are the values most often used, so that the finished drawing has either the y plotter axis vertical or the x plotter axis vertical.

Columns	Instruction 302	Primer Constant
7-9	302
10-18	(o)	0.

Instruction 303 directs certain lines to be made heavier than others by retracing over the path several times with slight pen displacements (DISP) from the original path. For example, if DISP is greater than 0., the forward half of the principal plane trace of ellipsoids is drawn heavier than the hidden half so the eye does not confuse the two halves. Also, all regular lettering (but not perspective lettering) is gone over four times to give it a boldface appearance. In ORTEP-III, retracing is turned off by default with the primer constant for DISP set to 0. inches. Retracing can be turned on with the 303 instruction, using a value for DISP greater than 0. (0.005 in. usually works well).

Instruction 304 may be used to adjust the smoothness of the plotted ellipses as a function of their size. Smoother ellipses require more computational time and produce larger illustration files. Smaller smoothness factors produce smoother ellipses, and larger values produce ellipses with more jaggies. Factors in the range 0.5-3.0 are recommended; the default value is 1. A value of 0. will draw all size ellipses with 128 points, the maximum provided in ORTEP.

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