Material of the half-transapent fabric lampshades in V-Ray

18 Nov, 2011 George M.
18 Nov, 2011 # Re: Material of the half-transapent fabric lampshades in V-Ray
Hello again Renderstuff,

I'm trying to create a realistic cloth material. With realistic I mean not only that it has the texture look but also has the correct translucency. The purpose is to put this texture on a object that is sitting on a light source (vraylight material is used) and show the light correctly like the light gets threw the tiny holes of the material. The cloth object is a shelled one so I cannot use a vray2sided material. I already tried to make a procedural material with a checker pattern where each checker color has it's level of refraction giving the light behind to pass threw the texture holes. The material looks exactly like I want but the vraylight material must have a multiplier of 35 for enough light to pass throught the cloth material object to be realistically visible from a camera sitting at ~10 meters, so I don't think it's a good solution since it has to work(not to be overbright) also when the cloth is not sitting on it. What do you suggest?

I was thinking of using SSS but the cloth material should not be translucent in non-lighted areas of the object that is sitting behind. Giving a pattern texture to limit that efect I'm afraid it will increase my rendering times considerably.

on another related subject: What multiplier value do you recommend (approximately) not to be exceded on a vraylight material for not having those jagged antialiasing artefacts on overbright sources? (the environment is low illuminated and the light source is a car taillight so it has also transparent plastic above it).
I tried already to reduce the exposure in the HDR render of the light but it doesn't work correctly, the antialising jagged artefacts still remains and it doesn't have the soft edges expected even if the light is reduced in intensity.

I'm aware that this complex advices are usually not free, I only ask for some basic guidance.
Thank you very much and I really appreciate the advices you already gave me last days! 😁

19 Nov, 2011 RenderStuff (Staff Author)
19 Nov, 2011 # Re: Material of the half-transapent fabric lampshades in V-Ray

The first thing to note is that the cloth is not a translucent material in essence. The fact is that translucency is the sub-surface effect, occurring within the material. Translucency is evident when the light illuminates the object, and this actual object not just absorb or let pass the light rays through itself, it bends them within own volume. At this time, such an object is half-transparent so all the light processes inside we see as sub-surface glow. In even simpler words, the translucent material “gathers” the light and becomes self-illuminated. The only difference is that it is not an actual source of light, it gets the light from outside.

Now let’s look on the fabric’s properties. If you would look closely at the fabric, you’ll see that this is just a lot of interweaved threads. They are not ideally close; they have gaps between each other. These are the openings, which let the light rays partially come through cloth material, while the remaining part of light gets absorbed and reflected by the threads. Again, roughly speaking, fabric is the grid with the small cells, through which the light passes.

There is no translucency in fabric just because there are no light effects in the bodies of actual threads. They only absorb and reflect light, as any other opaque material.

Back to your example with the lampshade, we have to realize that the threads of any type of fabric are the thick objects of nearly cylindrical shape. This property visually bends the light. When we look on the fabric from the one side and lit it from the other, we see an illusion of fabric “gathering” light because first, we basically see the reflection of the light to our direction from the cylinders’ sides, and second, light literally skirts a round thread shape to get into our sight. It is the way we see the lampshade fabric glowing and it is the effect you want to imitate.

Let’s come back to the CG world, where everything consist of planar triangles, no matter what their shape is and whether they have actual volume.

The one-sided object is the best candidate to imitate light-transmitting half-transparent fabric. Just get a plane surface and apply material to it, where Diffuse slot has the texture map of cloth color, and in the Opacity slot a b/w opacity map, which imitates the gaps between threads. In such a way, the material becomes partially transparent. However, this material does not imitate the light bending glow yet. Here the VRay2SidedMtl comes to help. The essence of its work is trivial. This material shades the both facing and backfacing sides of the polygons, giving to final image the average mixed result. The strength of this mixing is set in parameter called Translucency.

In the case of one-sided lampshade, the outer surface becomes lit due to aforementioned mixing of brightness with the inner well-illuminated inner surface. That is we see as glowing of the lampshade fabric. But, this is not the true translucency.

If we give the lampshade model the volume and apply the material with the non-black glossy refraction, we will observe the similar effect. That is because we see the light through a diffuse color of the lampshade as the object has transparency. The main evident difference is in the rendering time – the VRay2SidedMtl renders quickly than VRayMtl with refraction. In addition, there will be the artifacts from the light, passed through refractive material.

Obviously, the VRay2SidedMtl option is more preferable because of faster rendering. In other respects, they are both not translucent, just like the real cloth 😁

Regarding the VRayLightMtl with Multiplier set to 35, as we discussed in vray antialiasing specifics, the jags appear not from overbrights, but from the overall rendering approach. The activation of the Clamp output + Sub-pixel mapping, or zonal exposure control on the HDRI post-process stage, leaves us from the jaggedness issue. If you go over that comment, you’ll find the answer 😉

When we need to find compromise solution, there is no hesitation. Just mix the properties of any separate materials using VRayBlendMtl.  For example, it can be used to make nice composite material, which will look as a cloth thanks to usual VRayMtl and, at the same time, have the inner illumination from VRayLightMtl. Moreover, the using of the gradient blend map for zonal mixing of those two materials, may give the additional realism. This way, you can get beautiful glowing lampshade while staying in the RGB brightness range 👌

18 Nov, 2011 George M.
19 Nov, 2011 # Re: Material of the half-transapent fabric lampshades in V-Ray
Well, I have remodeled the cloth without shell modifier so it can be 1 sided and did some tricks to still have some thickness and applied a new cloth material based on VRay2SidedMtl. Conclusion: I have improved the translucency effect and reduced render time by 35-40% !! 👍

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