DRAW Reference Manual
Point and Line Construction
: Tangency Point
Tangency Point
This construction generates a point at which a tangent line from a specified reference point meets a specified arc, circle or ellipse.
Examples
1.
NEW STRA
FPT X100 Y100
TPT X300 Y200 TAN @
Figure 19:12.
Use of the TANGENCY Point Construction
This STRA element has an explicit FPT; its TPT is the point on the ARC that, together with (X300, Y200), forms a tangent to the ARC. Here the cursor hit acts as a qualifying point. Notice that the STRA produced here is not a tangent line.
2.
NEW STRA
FPT X200 Y100
TPT X200 Y100 TAN PREV CIRC QUAL @
Here, the STRA has one endpoint defined explicitly and the other defined as the point where the tangent line through (X200, Y100) intersects the previous CIRC in list order which is closest to the cursor-specified qualifying point.
Figure 19:13.
Use of the TANGENT Construction to give a Tangential Line
The qualifier is necessary since in this case there are two possible tangents. (Other tangent shown dashed.) Other cases would not need a qualifier since only one tangent would be possible.
3.
NEW STRA DEF @ @ TAN @
In this case, the first cursor hit defines one endpoint of the STRA, the second defines one end of a tangent, and the third picks the circle and also acts as a qualifier. The resulting STRA intersects the tangent.
Figure 19:14.
Use of the TANGENT Construction to give a Line perpendicular to a Tangent
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