View Full Version : What am I missing here?

06-27-2003, 07:39 PM
Hi, This is my first post here and I'm asking for help already.
I am building an antique clock in a glass dome using Lightwave.
Having made the glass dome with the correct glass thickness and a refractive index of 1.5, I expected to see it render like a clock in a hollow glass dome, but instead it rendered like a clock in a goldfish bowl, completely distorted. I tried flipping the polygons, making them double sided etc. but no joy. It seemed to me that Lightwave ignores the fact that the dome is hollow so I've settled for drastically reducing the refractive index. Although this helps it doesn't give the correct type of refraction so I was wondering if anyone can tell me the right way to go about this.

As you can see from the image the edges of the base are refracted upwards, which is consistant with looking through a solid transparent object. When looking through a hollow object they should be refracted down.

I haven't been able to figure a way out of this one so any ideas would be welcome. :confused:

06-27-2003, 08:03 PM
If you look where the item appears bent up, you technically ARE looking through a solid object. After all, the glass thickness, when looked at on a tagent, is simply "one" piece of glass. The ray is going into the glass and comin out og the glass without ever going "inside" the glass on the edges.

Does that make any sense?

06-27-2003, 10:34 PM
This is just a wild guess, which might be completely off track. You could try spliting the glass down the middle so that you have a front and back section, then give them two different surface names but the same settings. And or try it without the refractive Index on the rear section?? Sorry just a stab in the dark.

06-28-2003, 02:31 AM
Thanks Mat,
I can see your point but the trouble is I've got the real one right infront of me and when viewed from the same angle as the render it obviously aint right.

Cheers, I can imagine that might work. I shall give it a try when I can fit it in(horrendous render time with all that glass}.

06-28-2003, 03:00 AM
what you need, what does not exist until you put it in, is 'air'

LW continues to bend the light until it hits a polygon with a diifferent refract value, that's why it looks like a fishbowl.

Copy the glass into another layer, flip all the polygons, name the surface "air" copy and paste it back into the layer with the glass. Give the air surface a refrac value of 1.0003 (air does refract light)

06-28-2003, 05:56 AM
Nice one Toby.
That matches up with something I just read on another forum, so I think you've solved my problem.
Thanks a lot.

06-28-2003, 12:08 PM
Please post the image again after adding the air. I'm curious how it will look. Thanks


06-28-2003, 05:26 PM
Will do!
Might be a while though.

06-30-2003, 01:58 AM
OK Marvin here it is.

Much better now.
Thanks for your help everyone, especially Toby.

Strange thing though.
When I re-opened the model, Lightwave had lost all the image maps. The settings for them were still there so it was just a matter of putting them back in. Then, when I rendered it, it took a mere 3.5 Hours instead of 4 days like last time. Something spooky goin on here.


06-30-2003, 02:11 AM
much better! that's what we were looking for.

the dome of the glass needs more polygons to smooth it out - you can see the corners

one thing that can speed up render times is to turn down the ray recursion. Default is 16, but I've never needed that much - try 8, or even lower, but refraction gets cloudy if you go too low.

Nice model btw!