[pstricks] vanishing plane as function of viewpoint

[Jon Joseph]

Once again I am doing battle with the pst-solides3d package.  Can someone explain to me why the tangent plane vanishes when I change the viewpoint?  


\documentclass[12pt]{article}
\usepackage[letterpaper,asymmetric=true, total={18cm,22cm}, 
		top=3cm, bindingoffset=1cm]{geometry}
\usepackage[svgnames]{pstricks}
\usepackage{pst-plot,pst-infixplot}
\usepackage{pst-eucl}
\usepackage{pst-fun, pst-coil}
\usepackage{pst-solides3d}
\usepackage{pst-fun}
\usepackage{pstricks-add}


\begin{document}

% With this viewpoint everything draws correctly

\psset{viewpoint=30 10 30, Decran=50}
\begin{pspicture}[showgrid=false](-1.5,-3)(6.5,6) 
\psSolid[object=grille,base=-3 3 -1 6,action=draw**, fillcolor=gray!20] 

\defFunction[algebraic]{G5}(t)
{t}{0.5*t^2}{0} 
\defFunction[algebraic]{G6}(t)
{t}{0.5*t^2}{4} 

\psSolid[object=plan,  definition=equation, fillcolor=Aquamarine, args={[0 1 0 0] }, base=-3 3 -4 0]  % Tangent plane


\psSolid[object=cylindre, range=-3 2,h=4, function=G5, axe=0 0 1, incolor=cyan!50, fillcolor=yellow!50, ngrid=3 8]

\axesIIID(0,4.5,0)(4,6,5)
\end{pspicture}


% Changed the viewpoint and the plane disappears

\psset{viewpoint=30 -10 30, Decran=50}
\begin{pspicture}[showgrid=false](-1.5,-3)(6.5,6) 
\psSolid[object=grille,base=-3 3 -1 6,action=draw**, fillcolor=gray!20] 

\defFunction[algebraic]{G5}(t)
{t}{0.5*t^2}{0} 
\defFunction[algebraic]{G6}(t)
{t}{0.5*t^2}{4} 

\psSolid[object=plan,  definition=equation, fillcolor=Aquamarine, args={[0 1 0 0] }, base=-3 3 -4 0]  % Tangent plane - oops? Where did it go?


\psSolid[object=cylindre, range=-3 2,h=4, function=G5, axe=0 0 1, incolor=cyan!50, fillcolor=yellow!50, ngrid=3 8]

\axesIIID(0,4.5,0)(4,6,5)
\end{pspicture}


\end{document}