clipka <ano### [at] anonymousorg> wrote:
> Am 26.07.2012 16:31, schrieb Chaanakya:
> > clipka <ano### [at] anonymousorg> wrote:
> >> Am 26.07.2012 16:02, schrieb Chaanakya:
> >>> "Chaanakya" <nomail@nomail> wrote:
> >>>> Warp <war### [at] tagpovrayorg> wrote:
> >>>>> Chaanakya <nomail@nomail> wrote:
> >>>>>> I was trying to create half of a hollow cylinder (both ends open).  I tried
> >>>>>> using CSG with an intersection, and that gave me a half a hollow cylinder,
but
> >>>>>> I'd like to not have a plane on the end.  That is, I'd like essentially a
curved
> >>>>>> plane (the side of the cylinder without the endcaps).  Is there some
> >>>>>> transformation I could use on, for example, a box to achieve this?
> >>>>>
> >>>>> Just make a difference of the cylinder and another cylinder with a slightly
> >>>>> smaller radius and which is a bit longer ther the first cylinder.
> >>>>
> >>>> Thanks!  That lets me control the thickness of the resulting object.  How would
> >>>> I cut it in half?
> >>>>
> >>>> - Chaanakya
> >>>
> >>> This is a bit weird....the shell seems way too thick when I render this:
> >>>
> >>> #version 3.6;
> >>> #include "colors.inc"
> >>>
> >>> global_settings {
> >>>     max_trace_level 256
> >>>     ambient_light White
> >>>     assumed_gamma 1.0
> >>> }
> >>>
> >>> camera {
> >>>     location <10,0,0>
> >>>     look_at <0,0,0>
> >>>     //rotate <0,0,-clock*90>
> >>> }
> >>>
> >>> light_source {
> >>>     <0,100,0>
> >>>     color White
> >>> }
> >>>
> >>> plane {
> >>>     y, -10
> >>>     pigment { hexagon Green, White, Blue }
> >>> }
> >>>
> >>> difference {
> >>>     cylinder { <-1,0,0>,<1,0,0>,1}
> >>>     cylinder { <-1.00003,0,0>,<1.00003,0,0>,0.99999999}
> >>>     pigment { color Red }
> >>> }
> >>>
> >>> The shell should be barely visible, but it's quite thick.  Why?  Is it floating
> >>> point error?
> >>
> >> What you're seeing is just a perspective effect; change the camera
> >> location to e.g. <10,3,0> and light source position to e.g. <100,100,0>
> >> to better see what's goung on.
> >
> > Ah I understand.  Thank you!  Is there any way to now cut that object in half
> > (without ending up with a plane "capping" it)?
>
> Yes: Introduce a plane to the difference.
>
> Note how the plane only "caps" the object at the resulting cuts, in this
> case the walls of the tube. Before that, what you cut was a solid cylinder.
>
> You need to think in terms of solid bodies (after all it's called CSG =
> Constructive Solid Geometry): If you have a solid cylindrical object,
> and a big planar tool to "disintegrate" part of it, your remaining
> object will be a solid semi-cylinder. OTOH, if you have a solid
> cylinder, use a cylindrical tool to "disintegrate" a cylindrical part at
> its center (giving you a tube with a measurable wall thickness), and
> then use that big planar tool, your remaining object will be a half-pipe
> (with a measurable wall thickness).

Okay that makes sense.  Thank you so much clipka and warp! :)  You guys are
awesome :D

- Chaanakya

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