Tangent and normal

Add tangent/normal to curve

Key n. Draws a short line touching a curve (a tangent) or crossing it at a right angle (a normal) at the point you click. Useful for marking slopes and construction lines.

• Left, “add tangent”: click a point on the curve; a line along the tangent is added, centred on that point.
• Middle, “add normal”: the same with the line perpendicular to the curve.
• Right: nothing.

The curve can be a line, polygon, box, spline, arc, circle or ellipse (also a turned ellipse). The new line is an ordinary three-point polyline in the current line attributes, depth and arrows, so you can change or delete it like any line. It is one undo step.

The length is the indicator's Tangent/Norm Length (in inches or centimetres as the drawing's units say; the new line is this long in total, centred on the point; start value 1.0; a button between Line Style and Join Style in this mode, steps of 0.1).

When several objects lie under the click, the search order is: ellipses and circles, then lines, then splines, then arcs, the last drawn first. At a corner of a polygon or a line's vertex the tangent is not defined: the message panel says "singularity, can't draw tangent/normal". The message "added tangent" or "added normal" confirms success. On splines the tangent is that of the short curve piece found at HIGH_PRECISION, as in xfig, so at the lowest point of a curve it is horizontal. A click that is not on any curve does nothing.