### 4.2 A technical drawing

Let's look at a drawing that represents the kind of problem `sketch` was meant to solve—a pair of textbook figures regarding a polygonal approximation of a truncated cone. Here are the pictures we will produce.

The cone shape is just a swept line with no closure tag and culling turned off. Begin by setting up some useful constants.

      def O (0,0,0) def I [1,0,0] def J [0,1,0] def K [0,0,1]
      def p0 (1,2) def p1 (1.5,0) def N 8
      def seg_rot rotate(360 / N, [J])

The points `p0` and `p1` are the end points of the line to be swept. The definition `seg_rot` is the sweep transformation. With these, the cone itself is simple.

      sweep[cull=false] { N, [[seg_rot]] } line(p0)(p1)

The axes are next and include an interesing trick that shows the hidden parts as dotted lines. The secret is draw the axes twice—solid lines with the normal hidden surface algorithm in effect, and then dotted with the option `lay=over` so that no polygons can hide them.

      def ax (dx,0,0) % tips of the axes
      def ay (0,dy,0)
      def az (0,0,dz)
      line[arrows=<->,linewidth=.4pt](ax)(O)(ay)
      line[arrows=->,linewidth=.4pt](O)(az)
      % repeat dotted as an overlay to hint at the hidden lines
      line[lay=over,linestyle=dotted,linewidth=.4pt](ax)(O)(ay)
      line[lay=over,linestyle=dotted,linewidth=.4pt](O)(az)
      special|\footnotesize
              \uput[d]#1{$x$}\uput[u]#2{$y$}\uput[l]#3{$z$}|
        (ax)(ay)(az)

The labels are applied with `PSTricks` special objects as usual.

For the height dimension mark, the power of affine arithetic is very helpful.

      def hdim_ref unit((p1) - (O)) then [[seg_rot]]^2
      def c0 (p0) then scale([J])
      def h00 (c0) + 1.1 * [hdim_ref]
      def h01 (c0) + 1.9 * [hdim_ref]
      def h02 (c0) + 1.8 * [hdim_ref]
      line(h00)(h01)
      def h10 (O) + 1.6 * [hdim_ref]
      def h11 (O) + 1.9 * [hdim_ref]
      def h12 (O) + 1.8 * [hdim_ref]
      line(h10)(h11)
      line[arrows=<->](h02)(h12)
      def hm2 ((h02) - (O) + (h12) - (O)) / 2 + (O)
      special|\footnotesize\rput*#1{$h$}|(hm2)

The general idea employed here is to compute a unit “reference vector” parallel to the xz-plane in the desired direction of the dimension from the origin. The transformation `[[seg_rot]]^2` rotates two segments about the y-axis. When applied to `(p1) - (O)`, the resulting vector points to the right as shown. In this manner, we can pick any vertex as the location of the height dimension lines by varying the exponent of `[[seg_rot]]`. This is only one of many possible strategies.

The computation of `hm2` is a useful idiom for finding the centroid of a set of points.

The two radius marks are done similarly, so we present the code without comment.

      % radius measurement marks
      def gap [0,.2,0]  % used to create small vertical gaps

      % first r1
      def up1 [0,3.1,0] % tick rises above dimension a little
      def r1 ((p1) then [[seg_rot]]^-2) + [up1]
      def r1c (r1) then scale([J])
      def r1t (r1) + [gap]
      def r1b ((r1t) then scale([1,0,1])) + [gap]
      line[arrows=<->](r1c)(r1)  % dimension line
      line(r1b)(r1t)             % tick
      def r1m ((r1) - (O) + (r1c) - (O)) / 2 + (O) % label position
      special |\footnotesize\rput*#1{$r_1$}|(r1m)  % label

      % same drill for r0, but must project down first
      def up0 [0,2.7,0]
      def r0 ((p0) then scale([1,0,1]) then [[seg_rot]]^-2) + [up0]
      def r0c (r0) then scale([J])
      def r0t (r0) + [gap]
      def r0b ((p0) then [[seg_rot]]^-2) + [gap]
      line[arrows=<->](r0c)(r0)
      line(r0b)(r0t)
      def r0m ((r0) - (O) + (r0c) - (O)) / 2 + (O)
      special |\footnotesize\rput*#1{$r_0$}|(r0m)

The second drawing uses the same techniques. Only the method for drawing the elliptical arc is new. Here is the code.

      def mid ((p00)-(O)+(p10)-(O)+(p11)-(O)+(p01)-(O))/4+(O)
      special|\rput#1{\pscustom{
        \scale{1 1.3}
        \psarc[arrowlength=.5]{->}{.25}{-60}{240}}}|
        [lay=over](mid)

We could have swept a point to make the arc with `sketch`, but using a `PSTricks` custom graphic was simpler. Again we computed the centroid of the quadrilateral by averaging points. Note that scaling in Postscript distorts the arrowhead, but in this case the distortion actually looks better in the projection of the slanted face. A `sketch` arrowhead would not have been distorted.

The complete code for this example, which draws either figure depending on the definition of the tag `<labeled>`, is included in the `sketch` distribution in the file truncatedcone.sk.