It seems you are using [-pi,pi] as the range of all your angles.I think you need to use [-pi/2,pi/2] as the range for the u argument (in the wikipedia page).
> From: rauldmil...@gmail.com > Date: Fri, 6 Mar 2015 21:35:37 -0500 > To: programm...@jsoftware.com > Subject: Re: [Jprogramming] Plotting a 3D Ellipse > > Hmm... > > I notice that load'graph' no longer provides the steps verb mentioned > in http://www.jsoftware.com/books/pdf/expmath.pdf > > So here's a workalike: > steps=: 3 : 'a+(w)*(i.n+1)%n[w=.z-a[''a z n''=.y' > > and that gives us an example of a surface plot. > > Next, we need a parametric representation of the ellipse, and > http://en.wikipedia.org/wiki/Ellipsoid#Parameterization looks like a > plausible approach there. > > Using that, and http://www.jsoftware.com/help/jforc/graphics.htm as a > starting point, it seems to me that I ought to be able to draw an > ellipse like this: > > a=:3[b=:4[c=:5 > > X=:4 :0~ > a*(cos x)*/cos y > ) > > Y=:4 :0~ > b*(cos x)*/sin y > ) > > Z=:4 :0~ > c*(sin x)*/1: y > ) > > draw=:3 :0 > angles=: steps _1p1 1p1 100 > x=. X angles > y=. Y angles > z=. Z angles > 'surface' plot x;y;z > ) > > draw'' > > Sadly, that's not an ellipse. > > But I do not have enough familiarity with plot to know whether I've > screwed up my math or if there's a defect in plot -- I don't know how > to isolate the problem. > > That said, a quick test with a 3d model of a cube: > > 'surface' plot ;/|:#:i.8 > |NaN error: ncile > > ...suggests that plot isn't really designed to represent 3d solids. > > Still, that does not eliminate any potential errors on my part. > > On the other hand, maybe the right approach would be to use jhs and > generate a distance field renderer along the lines of what you see at > shadertoy.com. > > I'll have to think a bit to see if I can pull off something like that. > (I think that that would only work for people who have adequate > graphics hardware support. But I think you have a suitable graphics > card?) > > Thanks, > > -- > Raul > > > On Fri, Mar 6, 2015 at 7:10 PM, Skip Cave <s...@caveconsulting.com> wrote: > > I want to plot a 3D ellipse (ellipsoid) and then be able to change the > > lengths of the three axes. The equation of an ellipse with axis lengths of > > a, b, & c is: > > > > ((x^2)%(a^2)) + ((y^2)%(b^2)) + ((z^2)%(c^2)) = 1 > > > > How can I arrange this so it can be plotted in a 3-D plot in J, and then be > > able to experiment with the lengths of the axes? > > > > Skip > > > > Skip Cave > > Cave Consulting LLC > > ---------------------------------------------------------------------- > > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
