Owner's Manual

Appendix B: Technical Reference 941
Based on your x and y Window variables, the distance between xmin and
xmax and between ymin and ymax is divided into a number of grid lines
specified by xgrid and ygrid. These grid lines intersect to form a series of
rectangles.
For each rectangle, the equation is
evaluated at each of the four corners
(also called vertices or grid points)
and an average value (E) is
calculated:
E =
z
1
+ z
2
+ z
3
+ z
4
4
The E value is treated as the value of the equation at the center of the
rectangle.
For each specified contour value (C
i
):
¦ At each of the five points shown to
the right, the difference between
the point’s z value and the contour
value is calculated.
¦ A sign change between any two adjacent points implies that a contour
crosses the line that joins those two points. Linear interpolation is used
to approximate where the zero crosses the line.
¦ Within the rectangle, any zero
crossings are connected with
straight lines.
¦ This process is repeated for each
contour value.
Each rectangle in the grid is treated similarly.
Contour Levels and Implicit Plot Algorithm
Contours are calculated and plotted by the following method.
An implicit plot is the same as a contour, except that an
implicit plot is for the z=0 contour only.
Algorithm
z
1
=f(x
1
,y
1
)
z
3
=f(x
2
,y
1
)
E
z
2
=f(x
1
,y
2
)
z
4
=f(x
2
,y
2
)
z
1
ìC
i
z
3
ìC
i
EìC
i
z
2
ìC
i
z
4
ìC
i