blurry translucency

[matd]

I'm trying to create an effect similar to a shadow puppet theatre - namely an opaque object behind a translucent screen (a polygon set to 80% translucent white) lit with a spotlight to cast it's shadow on the screen. All well and good. The problem is that everything is too sharp. When an item is further away from the screen it should be blurrier, for instance when an object is edge on to the screen, the shadow should not be uniformly sharp (i.e the edge furthest from the screen should be less focussed than the closest edge). Is there any way around this because it just doesn't have any realism at the moment.

[Dodgy]

Are you using area lights? r maybe shadow mapped spotlights?

[matd]

Well, i carried out a few tests with surprising results. the test conditions were:

1) A white polygon filling the view in front of the camera with 0% diffuse, 100% translucency and double-sided on.
2) A cut glass tumbler - glass with 1.55 RI and back-facing air polys set to 1.003 RI
3) A light casting a shadow of the glass on the screen, positioned at the exact centre of the view pointing straight towards us.
4) A camera is positioned pointing directly at the backside of the translucent screen poly and another snoop camera is positioned to look at the front side of the poly to see what's happening on the other end.
5) Low-AA renders at 640x480 with no shading noise reduction.

The results were:

A) RT spotlights cast a uniformly sharp shadow from the front to back of the glass with the structure of the cut glass pattern clearly visible.

B) A shadow map size of 2000 and a fuzziness of 4.0 gives a uniform fuzziness from front to back (i.e. the furthest edge from the screen is as blurred as the near edge). The problem here was that the shadow of the glass is no longer shown as a transparent object but as an opaque solid object.

C) Turning on caustics with the ray-traced spotlight had the effect of turning the glass object opaque, so instead of transparent shadows they rendered out black. A "snoop" camera pointing at the reverse shadow catching side revealed that in spite of the view from the back, caustics were being generated quite strongly.

D) Changing the light to a Point light cast a uniform transparent shadow. Turning on caustics changed the shadow to an opaque one. As above caustics only appear on the side they are cast, but don't transmit through to the back side of the transllucent screen.

E) Changing the light to a Linear light created a strange soft dark spot in the centre of the frame. Changing the scale of the light alters the sharpness fall-off; with smaller scales the depth of focus increases whereas larger scales create shadows with less sharpness. At a scale of 20.00 even the point where the glass was touching the translucent screen was defocussed. Turning on caustics preserves the sharpness fall-off but renders shadows as opaque and no caustics appear as for D.

F) Changing the light to an Area light gives a similar effect to the Linear light in E, but with some differences. Firstly the lighting is even and the dark spot no longer appears. Secondly the shadows are more diffuse when comparing an Are and Linear light of identical scales (which stands to reason as the Area light is less focussed). Setting the Area light to the same scale as the Linear light gives a similar result give-or-take.

G) Whether the translucent screen is double-sided or not seems to make no difference.

In summary - area lights give sharpness fall-off, which is a shame as they are so slow and noisy, and caustics don't transmit through to the backside of translucent surfaces. Does anyone know of a work-around for this?

[Dodgy]

Yeah, I'm not over impressed with how translucency is implemented in LW, I think there must be something wrong there....

[matd]

Thanks for your input Dodgy. Do you or anyone else here know of any plugins to overcome this? Alternately, is there an email address at Newtek for feature requests?

btw - I didn't mention that I'm using LW 7.5d. I don't know if anyone out there with 8.1/8.2 gets any different results?

[Dodgy]

There's been no change there as far as I know...

LWfeatures@newtek.com is the address :)

[matd]

Has anyone here got G2, which seems to add some SSS features. Would that solve my problem?

[Lamont]

I thought I'd give this a shot.

I got it to work with an area light, and an object/screen that is double sided. Nothing else has been done, other than to turn on the usual suspects in the render options panel.

http://www.digitalweaponx.net/FTP/tranz.mov

http://www.digitalweaponx.net/FTP/sheild.rar

[starbase1]

Don't forget the spinning light trick...

If you are going to use motion blur anyway then rotating the light will soften shadows for no extra render time as it moves between each pass.

Not an answer as such, but it can help!

Nick