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View Full Version : How come most of LW SSS shader need the material to be double sided?



ufo3d
01-27-2007, 07:12 AM
I heard from someone that double sided polygon is much time consuming , but how come most of LW SSS shader need the material to be double sided but other renderer such as Mental ray and modo don't? It seems double sided polygon will cause some unexpect, weird result sometimes .

Bytehawk
01-27-2007, 01:13 PM
because SSS is a form of translucency. Just like glass it needs an in - and out surface. By making it double sided the nodal system can determine when a ray hits the surface and when it exits the surface.

ufo3d
01-28-2007, 12:07 PM
but how come other renderers such as mental ray or modo don't require double sided polygon, nor ray-trace shadow/ refraction.etc? If this is possible, i hope NT they will improve those sss shaders of LW9.

Exception
01-28-2007, 03:08 PM
It gives you more control.
Can you assign a different material to each side of a polygon in modo or mental ray? That's a really powerful feature in LW.

But it would be nice if nodes could enable thos feature by themselves, so that the ones that need it, can assure its proper state.

Chuck
01-29-2007, 10:17 AM
I heard from someone that double sided polygon is much time consuming , but how come most of LW SSS shader need the material to be double sided but other renderer such as Mental ray and modo don't? It seems double sided polygon will cause some unexpect, weird result sometimes .

Double-sided polygons have been functional in LightWave pretty much from the beginning. When used in their design-intended functions they do not produce any weird results, they just do their job and very dependably. Those node shaders that require it do so because they make use of the ability to apply differing shading and texturing to the opposing sides of a double-sided polygon. This is a feature of the LightWave geometry pipeline and renderer.

Other geometry pipelines and renderers may not offer double-sided polygons or may not require them for similar shading issues, or they may but it may not be apparent to the user. In any case, the fact is that if the shading method is to simulate layers for subsurface scattering, then the renderer is going to have to account for those layers and their behaviors in some fashion. Double-sided polygons and "air polygons" are common approaches and they may be handled either by requiring the user to toggle an option or create the necessary geometry, or by the shader or renderer covering the work without requiring additional user intervention. There are other approaches as well that may be used, such as the renderer supporting a volume stack concept.

One way or another, the issue has to be addressed, and every approach for addressing it has its costs. The major difference is simply whether or not the user may be aware of the approach in use or have to toggle controls or create geometry to support the approach in use.

lw3d23
01-29-2007, 10:30 AM
Thanks for your reply, Chuck.
I hope we will have some fast , effective sss shaders in next release. :D