Relation between Specular IOR and Reflection

[shawn]

Hi:

I'm creating a scene that includes a thin piece of clear mylar with embedded circuit traces. I'm using a bump map to fake the relief of the traces. I've found two ways to make the edges of that bump map catch the light.:

1. Use Specular with an IOR above 1 to make the reflections visible. The problem with this is the more you crank up the IOR (to make the reflections brighter), the more refraction is applied to anything underneath the mylar. Thin pieces of plastic don't really refract what's under them, so this isn't a realistic approach.

2. Keep the Specular IOR at 1 but increase the Metalness to some value above 0. The bump texture will become brighter as the Metalness is cranked up. This seems closer to the realistic effect I want, but it's not very intuitive to crank up a function called "metalness" on a thin sheet of plastic.

In the built-in C4D shader system the reflectance and refraction are separate controls.

Can anyone offer any insight or suggestions about this workflow? I've attached a demo scene.

Thanks.

Shawn Marshall

Marshall Arts Motion Graphics

ior-democ4d.zip

[lee_griggs]

I tweaked the IOR and bump height and got this. I guess it's going to be difficult as you want an effect that is not physically accurate?

[shawn]

Thanks for replying; I appreciate it.

Perhaps the effect I'm going for isn't physically accurate from a 3D rendering standpoint, but it sure seems accurate to me from a visible, real-world standpoint.

Right now on my desk I have a piece of semi-stiff cellophane that came with a Christmas Card. It has a lot of bumps and wrinkles, as in my sample scene. If I hold that an inch over another object the undulations in the surface catch all sorts of reflections, but the cellophane exhibits no visible refraction of the object underneath it. My observations indicate that for a transparent, real-world object to refract something it has to have a certain amount of thickness. Transparent plastic that's 1/10th of a millimeter thick just won't refract light, but it will sure reflect light. Yet if I make a plane with zero thickness, apply an Arnold glass shader to it and use the parameter tag to disable opaque it will refract the heck out of the light passing through it. That doesn't seem right.

Am I missing something?

Best.

Shawn

[lee_griggs]

Here is a version with Thin Walled enabled.

[shawn]

Thanks again for the reply.

Yes, Thin Walled seems to be the setting I'm looking for. I actually did look up its functions in the documentation, but those descriptions (bubble, paper airplane) didn't appear to match my scenario. I guess I never tried actually clicking that option (though I thought I had).

Further tests indicate that if you add a tiny bit of depth to the flat plane the refraction behaves much more like the real world (with Thin Walled deactivated). Such a plane will increasingly refract light as you make it thicker, which is realistic. However, if you keep it very thin (but with depth) and distort it with a bump texture those bumps will distort the light more than I observe in the real world.

Best.

Shawn