## 1.7.4 Interpolation and Clipping

There are 2 interpolation functions.

Linear Interpolation:
Linear interpolation is performed in the user-coordinate system. It can be specified in all coordinate systems. However, specification in a normal rectangular-uniform coordinate (itr=1) is meaningless; and it is frequently used in drawing axes in curvilinear coordinates and parallels in map coordinates.
Great Circle Interpolation:
Interpolation in map projection coordinates (TC) using a great circle passing through the starting and end points. This is effective only with map projection functions.
In a map projection function, the singular point is always at the pole in TC, so by specifying a great circle, the line segments in the vicinity of the singular point can be more appropriately drawn.

The following are the parameters for sgpget/sgpset for this function.

 llnint (L) A flag specifying whether linear interpolation will be made. (Default value: .false.) lgcint (L) A flag specifying whether a great circle interpolation will be made. (Default value: .true.). rdx, rdy (R) The interpolation interval. The length of a segment in the interpolation will not exceed this value. (Default value: (5., 5.))

Clippings will be performed in the following locations.

• Boundary of a map Map projection coordinates (TC)
• Viewport Virtual rectangular coordinate (VC)
• Boundary of a device Normalized rectangular coordinate (RC)
The clipping is done around a clipping rectangle with sides parallel to the axes of the coordinate system. Therefore, when the pole is moved in a rotation of a map projection, the clipping boundaries in the TC will not coincide with the meridians and parallels.

The following are the parameters for sgpget/sgpset for this function.

 lclip (L) A flag specifying whether clipping will be made in the viewport. (Default value: .false.) vxmin, vxmax, vymin, vymax (R) The viewport. Normally set by sgsvpt. txmin, txmax, tymin, tymax (R) The clipping boundaries in TC. (Default values:  -180.0, +180.0, -90.0, +90.0) irmod (I) A parameter specifying whether the tone boundary lines will be connected in the clockwise or counter-clockwise direction. (See 1.7.5 ) (Default value: 0).

The clipping of a viewport can be switched ON/OFF with 'LCLIP'. Other clippings will always be performed. In orthographic projections, the line segments on the reverse side will also be drawn when the default value for the clipping boundary in TC is used. To draw only lines on the front side, set 'TYMIN' to 0.