another yucky geometry question

TJ Frame tjframe at gmail.com
Mon Sep 26 17:35:05 EDT 2005


Richard,
 Yes the poly is only 2D. I'm working on and the ability to generate a 2D
ellipse that respects perspective for my current project Here is a snippet
of an email that I sent to Jim Hurley The included portion should explain my
unsual request:
 ----------------------------------
 I'm deep into developing my 2D perspective utility for industrial designers
and illustrators like myself and I'd really like to include support for one
of the major hassles of drawing in perspective - ellipses. As it stands now
in my application I can create and move around unlimited vanishing points
(vps) with two handles and those points can be aligned to existing vps of a
background image plate via two handles so in essence I can recreate the 3
perspective vanishing points of an existing photograph (in 2D) this is very
helpful when painting matte paintings and matching new artwork to existing
images... I've also implemented the ability to draw lines that "snap" to the
vps, and I can further have those lines "linked" to a vp so that when I move
the vp, all the linked lines track it. I've also inlcuded the ability to do
4 sided polygons that also snap and link, and the polygons can be filled
with perpspective-tracking gridlike meshes and "windows" via fancy polygon
points scripting.. This is a super help when matte painting or drawing stuff
like windowed buildings. Anyway..I'd like to include support for elipses
which can be a serious pain to do freehand, The easiest method I thought of
for the user would be to click the points of a four sided polygon and then
have the ellipse fit inside.. Since I can easily snap the 4 points to a vp
layout as well as restrict the poly to a "valid" shape, the resulting
ellipse would be in correct perpsective. Dragging the poly edges would allow
for reshaping the ellipse as well. The advantage this has over simple
drawing an ellipse and letting the user tweak it via handles or sliders is
that the ellipse may not be in correct perpsective to a particular vanishing
point.
 Ive uploaded another example: http://www.tjframe.com/EllipseExample.htm
and a brief description of the subject is here:
http://www.drawthrough.com/tutorials/ellipses.html
 This feature would be a fantastic thing to include. The only way to even
remotely duplicate the type of stuff my app does where perspective is
respected when drawing is to use a full 3D program, and a lot of 2D artists
aren't willing or able to delve into that subject. And even then, a lot of
the tools are I'm creating would be hard to fiddle with in a 3D package
becuase they arent really geared for that in the first place.
-----------------------------

So it not that I just need an ellipse aligned along the horizontal or
vertical axis, I need to be able to fit an ellipse shape within the larger
framework of a perspective-respecting 2D drawing utility.. admittedly not a
trivial task. But hey, that why my app will be so useful to us artist types
:)
 As for Rev and 3D.. I can think of a whole slew of stuff I'd like to able
to do with 3D support! :)
 - TJ

 On 9/26/05, Richard Gaskin <ambassador at fourthworld.com> wrote:
>
> > Does anyone know a good place to look for ( or know of any
> > example stacks) that might shed light on how to plot an
> > ellipse inscribed within a 4 sided polygon
>
> Is it necessary to _plot_ an ellipse, or do you really just want to
> _have_ an ellipse?
>
> For 2D, you could take the nearest point of the poly and use that as the
> bounds for an ellipse primitive.
>
> For 3D, far smarter minds than yours and mine have already crafted
> specialized libraries to do that and much more (I think there's a 3D
> engine in the works for Rev).
>
> 3D rendering is such a big wheel it's hardly worth reinventing, and
> unlikely to be satisfying in the long run if reinvented in a scripting
> language.
>
> --
> Richard Gaskin
> Managing Editor, revJournal
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