Re: [Visualpython-users] transparency, no-shadow objects, r --> infinity

I emphasize that I myself know no more about the transparency issue than you
or anyone else who read David Scherer's analysis! So I can't address
questions on that.

Concerning a backdrop a long ways away, the only thing you can do is put
very large objects a very long ways away. Just like in real life!

Here's a little program that shows the Earth going around the Sun, with the
Sun shining uniformly brightly and the Earth always lit only on the side
facing the Sun. It's based on a note from Dave Scherer a while back that if
you deliberately set a color to have components greater than 1, you happen
to get a behavior rather like what you want. The only other thing that's
needed is to continually move the light. This won't work if you want to show
the whole Solar System, because the single faraway light source can
continually point at only one of the planets.

from visual import *
from __future__ import division

scene.lights = [vector(-1,0,0)]
G = 6.7e-11 # gravitational constant
L = 1.5e11 # distance from Sun to Earth
# For the Sun, deliberately set color intensity much larger than 1:
Sun = sphere(pos=(0,0,0), radius=L/10, color=(10,10,0))
Earth = sphere(pos=(L,0,0), radius=L/20, color=color.cyan)
Sun.mass = 2e30
Earth.mass = 6e24
Earth.p = Earth.mass*vector(0,3e4,0) # initial momentum
dt = 24*60*60 # time step of one day

while 1:
    rate(100)
    r = Sun.pos-Earth.pos
    scene.lights = [norm(r)]
    F = (G*Sun.mass*Earth.mass/mag(r)**2)*norm(r)
    Earth.p += F*dt
    Earth.pos += (Earth.p/Earth.mass)*dt

Bruce Sherwood

For the other questions,
----- Original Message -----
From: "John Keck" <joh...@ho...>
To: <vis...@li...>
Sent: Friday, February 14, 2003 1:17 PM
Subject: [Visualpython-users] transparency, no-shadow objects, r -->
infinity


> Bruce,
>
> Thanks for the reply on opacity.  It was helpful, but it opens up new
> questions for me.
>
> <<These problems can be solved for triangles, but the solution involves
> things
> like clipping.  Solving it for entire Visual primitives seems impractical,
> so we would have to do something like:
>
> 1.  Render all opaque objects
> 2.  Break all transparent objects into triangles, and store the triangles
> 3.  Find any pairs of triangles not separated by a plane and split them
> 4.  Sort the triangles (using separating planes)
> 5.  Render the triangles
>
> If you want to do some research on the topic, you will usually see it
> referred to as "Painter's algorithm" or "depth sorting".
> >>
>
> It sounds like the real difficulty with implementing transparency is
> intersecting objects.  But isn't this also a difficulty for completely
> opaque objects as well?  How does VPython get around that difficulty for
> opaque objects?  How is this solution different from that for transparent
> objects?
>
> While we're on a similar subject, I'm guessing it would be simple to
> implement objects whose display color doesn't depend on the scene
lighting.
> I was trying to produce the Sun in a gravity demo I'm writing, but then I
> realized that it would look silly for the Sun to have a shaded side.  Is
> there perhaps an option for this already?  (I'm guessing that having light
> actually coming from a point inside the scene instead of off-stage from
> infinity would be difficult to realize.)
>
> Also, it is possible to have a wall-paper background at infinity, or at
> least to put objects of finite angular size at infinity?  I wanted to
> implement a star-field in my gravity demo, so that there'd actually be
some
> semi-realistic way to "see" a black hole against the star-field (plus also
> to give a sense of orientation to movement through the vacuum of space).
>
> Thanks,
>
> John Keck
>
>
> _________________________________________________________________
> MSN 8 with e-mail virus protection service: 2 months FREE*
> http://join.msn.com/?page=features/virus
>
>
>
> -------------------------------------------------------
> This SF.NET email is sponsored by: FREE  SSL Guide from Thawte
> are you planning your Web Server Security? Click here to get a FREE
> Thawte SSL guide and find the answers to all your  SSL security issues.
> http://ads.sourceforge.net/cgi-bin/redirect.pl?thaw0026en
> _______________________________________________
> Visualpython-users mailing list
> Vis...@li...
> https://lists.sourceforge.net/lists/listinfo/visualpython-users
>
>