<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">I'm attaching a file and an output picture. Briefly, here is the code:<div><blockquote type="cite"><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\begin{pspicture}(-3.5,-3)(3.5,3.5)</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\psset{Alpha=90, Beta=30}</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\multido{\rt=0+20}{18}{%</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\pstThreeDLine[arrows=->, arrowscale=2, linewidth=2pt, linecolor=gray]%</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">(\rt\space cos 3 mul, \rt\space sin 3 mul, 0)%</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">(\rt\space cos 3 \rt\space 2 div sin 1.5 mul sub mul,%</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\rt\space sin 3 \rt\space 2 div sin 1.5 mul sub mul,%</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\rt\space 2 div cos 1.5 mul)%</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">}</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\parametricplotThreeD[yPlotpoints=100](-1,1)(0,360){</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">u 0.5 mul cos t mul 3 add u cos mul %</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">u 0.5 mul cos t mul 3 add u sin mul %</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">u 0.5 mul sin t mul%</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">}</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\parametricplotThreeD[plotstyle=curve, linewidth=2pt](0,720){</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">t 0.5 mul cos 3 add t cos mul %</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">t 0.5 mul cos 3 add t sin mul %</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">t 0.5 mul sin%</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">}</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\parametricplotThreeD[plotstyle=curve, linestyle=dashed, linewidth=2pt]%</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">(0,360){t cos 3 mul t sin 3 mul 0}</font></div><div><span class="Apple-tab-span" style="white-space:pre"><font class="Apple-style-span" color="#000000"> </font></span><font class="Apple-style-span" color="#000000">\end{pspicture}</font></div></blockquote><div><br></div><div>
<span class="Apple-style-span" style="font-size: 12px; "><p style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Note the following features: the first multido draws gray arrows, whose starting points should lie on the dashed given by the last parametricplot.</p><div>I don't see any difference between the coding of the curve and of the starting points (other than \rt in place of u). Yet look where the arrow start.</div><div><br></div><div>What have I done wrong?</div><p style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><br></p><p style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><br></p><p style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font face="Helvetica" size="3" style="font: normal normal normal 12px/normal Helvetica; "></font></p></span></div></div></body></html>