| Re: object graphics - transparent surfaces [message #48858] |
Tue, 30 May 2006 09:49  |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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greg michael wrote:
> I'm trying to make some 3d red/blue anaglyphs using object graphics. I
> manage to blend the red surface onto the blue using the alpha channel
> and /depth_test_disable on the second surface, something like:
>
> oSurface2 = OBJ_NEW('IDLgrSurface', data,x,y, style=2, alpha=.5, $
> color=[255,255,255],texture_map=oImage2,shading=1)
> oModel2 = OBJ_NEW('IDLgrModel',/depth_test_disable)
> oModel2->add,oSurface2
>
> In most cases it works fine, but occasionally I get ugly pure-red bands
> on the tops of steep ridges. Looking more closely at the idlgrsurface
> alpha_channel documentation, it seems to be saying this technique is
> not recommended... some facets may be rendered in the wrong order for
> transparency. So is there a better way?
Yeah, this is a problem. What might happening is that the front of your
peak is rendered before the back so it isn't blended with anything that
lies behind it (because at the time it is drawn nothing is behind it).
Viewing at certain angles you lose part of your surface behind your
peak. This is an intra-object transparency issue.
There are also inter-object transparency problems where an entire object
is draw out of order which the docs address in the first sentence of
that 2nd paragraph.
> I could render one, read it back, then the other, read it back, blend
> them myself, and then display. But that wouldn't be a nice way to make
> an interactive object. By the way, I'm using 6.1, just in case anyone
> happens to know whether it's been changed since.
There are ways around these limitations, but none of them are
particularly elegant. The best thing to do would be to send an email
off to RSI requesting order independent transparency rendering for the
OG engine. You can double the number of people requesting this feature ;)
The simplest fix would be to order your objects in their container
depending on viewing angle. Your problem arises when you most likely
rotate your objects 180 degrees and the back surface is now in front of
the front surface and there is an inter-object issue. The more
sophisticated you get, the better the results. For more complex objects
you will get better results rendering your surface as a collection of
smaller surfaces all of which are ordered dynamically as you manipulate
them. This does not address intra-object rendering issues but it
minimizes them by limiting the number of self-overlapping areas. It
would be best to subset your data but if you can't you can use
MESH_CLIP. It's a little hairy but it works.
This approach will take longer to set up but will render at full speed
since all you will be doing will be changing the order of your objects
in the parent container.
Another approach would be to control the order in which the individual
polygons of each surface are rendered depending on viewing angle. This
addresses the intra-object issue. A crude approach would be to create
maybe 4 polygon connectivity arrays, one for each cardinal direction,
where the surface polys are rendered from back to front. You would then
change the polygons property of your surface depending on the viewing
angle. You wouldn't use IDLgrSurface for this, instead you would write
your own meshing algo and use IDLgrPolygon (I have a simple meshing
routine for surfaces if you want it). This too would render quite fast.
A more complete solution would be to reorder on every draw. Karl
Schultz worked up a dlm which does this using binary space partition
trees called bsp_poly. This was a few years ago and it was a bit raw.
I can't find it on the code contrib site but Karl may be willing to post
it or send it. This would be by far the slowest to render but it would
yield the "truest" results.
You may want to try my camera object. It will simplify tracking the
viewing angle and you may want to try its built in (beta) 3d features.
It supports shutter glasses connected thru the serial port (requires
some simple hardware hacking, schematic included) and it also supports
rendering to single/dual screens or dual projectors for viewing with
polarized glasses. If you are interested in trying this let me know.
-Rick
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