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rendering paintfx, backface display, glow fx reflection

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dgarcia
rendering paintfx, backface display, glow fx reflection
on Apr 19, 2006 at 7:55:07 am

I'm having some problems with transparency/raytracing/backface culling.

First, if I have a paint effect, let's say, on a NURBS surface,
it won't render behind a transparent blinn mat'l unless antialiasing
is set to a low setting (i think medium is the highest I can go
while still keeping it visible). i think it's not rendering in
front of the transparent material- said material is surrounded
by a fully opaque blinn shader and the paint effect can't be seen.
(more precisely - this is a problem with the tutorial on the
Alias learning tools that's based on the short movie blue, the
lesson where you build the space station and the paint effect
is for the stars outside. the room has a window, and outside you
can see planet earth and the stars are painted on a large-scale
nurbs sphere) i've read some of the forums and there's talk
about depth buffers and scanline but I don't really follow.

the second is: can post effects (like glow) show up in
raytraced reflections? obviously not since the raytrace
happens during rendering. how do you manage that
special effect? just staying away from it? i wouldn't
know how to add this even using compositing.

third question: I built a room, so I made a polygon cube
and reversed its normals. then I turned off component display
of back faces so i could see in the room. when i switch to
"high quality rendering" in the viewport shading tab,
the block goes solid again - regardless of the setting
in component display. so i went to the cube's shape node
and in its attributes I switched it to "single sided" - so
now the only face normals pointing in to the room are the
ones that are renderable right? the camera being outside
now renders perfectly in the software renderer.

so now if I bring my camera into the room, the floor
doesn't render, but the walls do. then at different
camera angles the floor might render, but only half of
the triangle (it's not tessellated, just one segment
on each dimension). I guess this is a complicated
question in the sense that I can't explain myself...
so basically the question is: how do I force maya to
render only the faces that have the normal on the side
of the face the camera sees, regardless of how many
other faces are in the way?

it seems the triangles on the floor render depending on the angle
of attack of the camera? yet the floor will ALWAYS render if the camera's
on the outside. if the camera is inside a shallow angle will render fine but
anything more acute (>45degrees?) will show through. yet the
wall on the right always renders fine! it's had some extrusion
done on it. more specifically, an inset followed by a slight
push extrusion, to make a window ledge and a face for the window itself.

I've used the commmand edit poly->normals->reverse to reverse
all the normals in the cube. when I go to the normal node attributes
there's several different normal manipulations, and the help file
doesn't help a lot. is the component display setting considered at
render time or is it just a viewport thing? should I keep the polygons
double sided when I build them? What's the point of a normal if it's
double sided, wouldn't there be two normals to a face, both opposite?

I often feel the help file for Maya is very high level for me. if
it's not in the tutorial book chances are I won't pick it up. the
help is excellent for features I'm familiar with and just want to
learn more deeply though. I'm sorry if I can't explain myself any
better, I've spent a lot of time rereading and rewriting this
question. Thank you for your effort and time.



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cow
Steve Sayer
Re: rendering paintfx, backface display, glow fx reflection
on Apr 25, 2006 at 3:16:27 pm

Hi, dgarcia. I'll take a stab at a few of these, hopefully others will be able to fill out what I miss. My usual caveat: I'm on an older version of Maya, so some details might vary from my experience to yours.

Not sure about the Blue tutorial question.

For the second question (glow in reflections), Maya's software renderer won't add glow to reflections as far as I know. In a production workflow this would almost certainly be handled with render passes and compositing. You would render out a reflection pass of your glowing object, then apply a glow effect to that layer at the compositing stage. By the time a compositor gets ahold of the image, it doesn't know or care whether the pixels it's affecting are due to reflection or refraction or direct visibility--it just adds glow to them, end of story.

Disappearing cube walls: this sounds to me like it might actually be a clipping issue, where the center of a face is behind the camera, so the whole face doesn't render... I'm not sure. In any case, you say the problem arises when you switched a viewport setting... is it possible that it was software rendering okay at that point, even though it glitched in the viewport?

[dgarcia] "is the component display setting considered at
render time or is it just a viewport thing"

It's just a viewport thing.

[dgarcia] "What's the point of a normal if it's
double sided, wouldn't there be two normals to a face, both opposite?"

Normals are not binary, i.e. they don't just indicate whether the surface is facing 'forwards' or 'backwards'; they indicate the specific XYZ direction the surface is facing. This is needed for smoothing and shading calculations, separate from backface culling and so on.

Sorry I don't have more to offer than that...

-Steve



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