Reflections in Polished Marble Walls, Floors

[3ddesign]

Most of my interior projects have polished marble pilasters, floors and other elements. I've been experimenting with the reflection in my Vray marble material using falloff in the relection and reflect glossiness slots as well as trying Fresnel variables but nothing I do seems to adequately replicate the properties of the actual marble. For example, in the sample sitting at my desk, the reflections of the overhead light fixtures are much more vivid than the reflections of the room itself which are barely noticable. I've concluded that this is because the light eminating from the fixture is reflected back to my eye whereas the light bouncing off room materials is mostly absorbed. This had led me to wonder if it would be possible to seperate the values of the highlight reflection levels from the standard reflection levels similar to the way glossiness can be seperated from highlight and reflection. Or, perhaps I could select which objects I want to reflect or even give objects a reflection contribution number. Anyway, just my thoughts as I work through the issue. And of course, if there are better solutions I'd love to hear them.

Craig

[GLASS-CANVAS]

Sounds to me that you need to use a combination of render elements and the Vray Blend material to get the effect you're looking for.

[Neilg]

Render out a reflection pass and do some photoshop/cc on it.

[3ddesign]

Yes, I can easily get the effect I'm looking for in Photoshop by comping different passes. It would just be nice if I could have the control to get it right in Vray instead of in post. One less layer in my already bloated PSD files.

[simmsimaging]

I don't think the light from the fixture is reflected/absorbed differently than the rest of the room in reality, or in the render - but Vlado could confirm the physics on that far better than I. I would guess it might have more to do with the level of contrast (dynamic range) in your scene. Are you sure your brightness difference in the scene is really reflective of the real-world example you are looking at?

b

[3ddesign]

It seems that in real life, at least in my photographic experience, there is a far greater light level difference from a direct light source than from the reflected light in a room than what is happening in 3D.

[rivoli]

It seems that in real life, at least in my photographic experience, there is a far greater light level difference from a direct light source than from the reflected light in a room than what is happening in 3D.

that is certainly true, but in your scene the light sources may be too weak to be reflected as strongly as you would expect. you may have an excess of ambient light that is leveling the overall brightness of your environment, or maybe too bright materials that reflect more light than they're supposed to.
in general I find vray's fresnel to work quite well, and I guess vlado can confirm its physical accuracy. I guess the reason for these reflections looking off may depend on the overall contrast of your scene, as brett suggests.

[vlado]

I've concluded that this is because the light eminating from the fixture is reflected back to my eye whereas the light bouncing off room materials is mostly absorbed.

It's the wrong conclusion The marble doesn't magically know if the light that falls on it comes from a light fixture or from the room. It always reflects in the same way. The reason to see the light is because it is so much brighter.

The cutoff threshold for VRayMtl might be at fault here - it could clip away reflections that it thinks are too faint, but which may actually be quite bright (f.e. if a light source is reflected). So in such cases you can try reducing the cutoff threshold (but don't set it to pure zero as this may increase render times too much).

Best regards,
Vlado

[3ddesign]

It's the wrong conclusion The marble doesn't magically know if the light that falls on it comes from a light fixture or from the room. It always reflects in the same way. The reason to see the light is because it is so much brighter.

That is what I meant to say; the real world light is being reflected back to me (from the real marble sample) because it is so much brighter than the light being reflected from the room objects. But in Vray, I can't seem to make the light source bright enough to reflect back the same way without also making the scene too bright. I will look at the cutoff, do some tests and get back. Thanks. Craig

[JeffG2]

Create a blend material. Vraymat as the base material and 2nd vraymat in coat material (check additive mode). Make the 2nd mat pure black and reflective.

[3ddesign]

OK, I think I found the solution but it challenges my assumptions about light and materials in the real world. Here are those assumptions: That the average world RGB value as seen by our eyes as well as digital cameras is 114. I came to that assumption because digital cameras exposure and the old photographers 17% gray card is RGB 114. I also did an experiment by selecting 100 random photos from Flickr and averaging their RGB values in Photoshop and it came out to 114. In my work I create renderings for specific rooms that have a specific color and lighting theme. Some of the rooms are darker and use wood and rich colors and some are quite bright using lots of marble, crystal, art glass windows, etc. I have measure the RGB value of the professional photos of the darker rooms and they come out to... you guessed it, 114. But the lighter rooms average out to 200 RGB. So, based on these assumptions I have always set up my lighting using an override mtl with 114 and 200 RGB. Until now that setup has worked very well. But as I've experimented with the relfections in marble, I had to drop the RGBs down to 85 in the dark room and 120 in the light room for the reflections to work out the way I see them in the real world. It makes sense because to get a proper exposure with these new darker colors, I had to turn up the light multiplier to get a proper exposure and as a result, the lights reflected in the marble are brighter than the room reflections. So now the question: Have I been wrong in my original assumptions? Should I be using much darker colors and brighter lights as a rule? Thanks for anyone willing to explain this to me. Craig

[vlado]

The 114 value is probably after 2.2 gamma correction, so that means that the actual value is in fact about 43 (for the 200 value, it would be about 149. But even if you used the correct values, it follows from nowhere that the colors you get in a photo are representative of the absolute reflectance of the surfaces in the scene.

Best regards,
Vlado

[3ddesign]

But even if you used the correct values, it follows from nowhere that the colors you get in a photo are representative of the absolute reflectance of the surfaces in the scene.

But there should be some correlation. We are trying to mimic in 3D the photos that would be taken in a given space, using similar lights and similar materials. I'm not talking about the reflectance of materials (I don't think) but just the overall average of measurable RGB values resulting from a combination of lights and materials. I guess what I don't know is what the appropriate RGB of my 3D materials should be to mimic a similar material in the real world. Using a colorometer gives me values that are far too bright.

[3ddesign]

Actually, I've been thinking about this subject for a long time. Here's the problem: The interior designer gives me a fabric sample that I need to put on a sofa. How do I get into my model? I usually scan it or take a digital photo. But in each of these cases, the fabric is subjected to a light source and an exposure to create an image that "looks like" the fabric sample. But then you take this already lit sample image, put it in a 3D space then light it again and set up an exposure with a virtual camera. In neither case is the true reflectance of the fabric taken into account: Only a representation of the perceived reflectance based on someone's best guess as to what our eye sees in the real world (often manipulated in Photoshop to make it even more "accurate"). Am I totally off base?

[vlado]

Actually, I've been thinking about this subject for a long time. Here's the problem: The interior designer gives me a fabric sample that I need to put on a sofa. How do I get into my model? I usually scan it or take a digital photo. But in each of these cases, the fabric is subjected to a light source and an exposure to create an image that "looks like" the fabric sample. But then you take this already lit sample image, put it in a 3D space then light it again and set up an exposure with a virtual camera. In neither case is the true reflectance of the fabric taken into account: Only a representation of the perceived reflectance based on someone's best guess as to what our eye sees in the real world (often manipulated in Photoshop to make it even more "accurate"). Am I totally off base?

Perfectly correct, yes. The problem is solved by photographing some samples for which you already know the reflectances, together with your sample. For example, LabSphere provides kits with small discs with known and calibrated reflectances (f.e. 2%, 50%, 99%). You can put these discs on the fabric and take a photo, making sure that the white disc is not over-exposed, but just slightly below pure white. Then when you remove the gamma correction from the resulting photo and compare the RGB values of the fabric to those of the white LabSphere disc, you can get a very good idea of the true reflectance of the fabric.

Best regards,
Vlado