View Full Version : Radiosity + transparent = black?

01-04-2005, 03:48 PM
Need help to get the right look.

I am puzzling over the best way to produce a hair model. The transparent texture I am using does not work. It produces black.

If I use the transparent texture as a clip map I get closer to what I need but I don`t get the soft fading of the hair.

I need something in between, a nice soft fade at the edge, while still getting the stencil effect of a clip map, e.g. when transparency = 100%, don`t render the surface at all.

This is with raytraced shadow switched off.

1 - picture is with transparent surface.
2 - picture shows DirectX preview... looks good :)
3 - picture shows a clip map, note the lack of nice soft edges.

Thanks in advance... Chris

01-05-2005, 03:22 AM
How about blurring the clip-map ? You can do it even different for every part of the clip-map in PS.

01-05-2005, 03:24 AM
The reason you get black with transparent is due to ray recursion.
If you have multiple layers of transparent surfaces (in z-depth) you will have to increase ray-recursion to 50...100. Warning : It will take forever to render such case.

01-05-2005, 05:13 AM
2 - picture shows DirectX preview... looks good :)

FYI, LightWave uses OpenGL, not DirectX

01-15-2005, 05:31 AM
Thanks for the feedback,

Panikos - I don`t have multiple layers of transparent polygon on the black object, its just one double sided object, so not sure ray recursion is the problem, tried winding it up but I still just get black.

Ingo - The clip map is the same as the transparency map, it is blurred/faded at the edges, it appears that the clip map applies a very sharp cut off, it either clips or it doesn`t. I can`t get it nice and soft at the edge.

Mattclary - Yup - indeed it does, thanks for pointing that out.

I am still struggling though, this must be possible, I just can`t figure out how.

01-15-2005, 06:07 AM
Are you using Raytracing Refraction/Transparency in the Render-Options ?

Also, what is the setting in your Surface / Environment for the Hair Surfaces ?

Raytracing + Backdrop
Backdrop Only
Raytracing + Spherical Map
Spherical Map Only ?

Good luck

01-16-2005, 02:37 PM
I have:

Ray trace transparency = on

Colour = 100% UV mapped texture
Diffuse = 90
Spec = 40
Gloss = 40
Transparency = 100% UV mapped texture
Smoothing on and at 89.53
Double sided

Ray Traced + Backdrop
Reflection Map of Background probe

Saying that, constantly tweaking and still not managed to get rid of the black, it should be completly see through as in the clipmap.

01-17-2005, 03:29 AM
I've never understood why Lightwave can't use Alpha. Clip mpas suck because you can either use black or white but not greys.
Transparency sucks because even though you can get a soft edge you'll still get spec and other light effects off the invisible part of the surface.
I've never discovered what you can do to solve this. Lightwave has alpha but only to reveal a layer below the one you apply it to. I use alpha in the packages we use at my studio all the time to make things invisible to the renderer why the hell can't Lightwave? It's such a useful thing to have.

01-17-2005, 05:40 AM
Peteb, Clip means either black or white so anything inbetween is biased towards the neighbouring edge. Clips is suitable for these cases only and it works as a fast alternative to transparency.

Transparency works this way, i.e thansparent surfaces continue to receive Specular since a 99% Transparency still gets specular highlights.
If I have a Transparent Surface like Glass, it doesnt necessarily mean that the Glass doesnt receive Specularity or Reflection etc because its Transparent.
Transparency is not Zero/Hundred % only but all the range inbetween.

The lack of node-based surfacing for LW forces you to apply the Transparency Map to other Surface channel to block them. In a nodal-based system this is simple cause you use one node applied to multiple channels.

01-17-2005, 05:58 AM
Sorry I didn't mean that you can't use greys I simply meant that there's no point. And yep obvioulsy that's how you want transparency to work, what I meant is it sucks if what you want is parts to be totally invisble.
Thinking about it the only way is to make sure you have image maps for each channel so as to stop any spec or reflect channels from appearing where you want it clear. Although after just saying that I'm sure I've tried this before and you still get a shadow of the whole poly, so how the hell do you solve this?


01-17-2005, 06:30 AM
Shadows are visible where there is Diffuse (mostly).
Generally speaking on non-Lambertian Surfaces (Transparent / Reflective) the Diffuse is low.

If you can attach an image, would be very useful.
Good luck

01-17-2005, 07:20 AM
Ok tested that out an you're right you can do it but it's a real pain in the arse to do. Is it not possible for Lightwave to include the option to use an alpha?
Going back to the original post about blackness. If you look at the attached image you'll see strange black lines where the polys lie. The mesh is simple and is just a grid of 6 polys. There's one area light in the scene and it's set to raytraced. Any ideas what this is and if it's related to the problem that Slivers been getting?

I've just realised why the shadows were a problem before. I was using Shadow maps and Lightwave calculates these (well seems to) based on the poly not the image, which makes sense. So you have to set it to raytraced to get the correct outline of your texture. The only thing with raytrace is I'm getting those black lines.

01-17-2005, 07:51 AM
To be honest I never use ShadowMaps.
Today's CPUs can handle raytracing without a serious penalty. (Of course radiosity excluded from the equation)

Transparent Surfaces cast opaque shadow when used with ShadowMaps. Be Warned about that.
Worley's G2 solves this.

As a rule of thumb, the "anatomy" of a transparent object is important.
Classic problems : unmerged points/polys etc
The simpler and tidy you have your work, it will save you from hair pulling later.
Sometimes on Transparent/Reflective objects its preferable to use Tris instead of quads or n-gons especially on curvy surfaces.

Also, a problem with the refraction, large values shows the backdrop in the calculation even if Backdrop is not included in the Raytracing Environment Settings.

Anyway, generally speaking, once you solve such problems, you cant forget them the next time.


01-17-2005, 08:29 AM
Thanks for the advice but I'm still not sure what those black lines are. The object is welded and is very simple. It was just a box with a few divisions and bent a bit. I'll try tessellating it see if that helps.

just tried the tessellation idea and it worked so well done Panikos.

01-17-2005, 08:41 AM
I am very glad :)

01-26-2005, 11:56 PM
Have you tried applying both a clip map and a transparency map of the same texture? It will soften the edges of your polygons up to the clip point (50%?) and may be a trade-off you can deal with.