PBR material questions (IOR, opacity)


I have two questions for the PBR material:

  • Where can be found the reflection IOR?

  • How to control an opacity without to get a refraction effect? (I want to blend in an object.)


There is no separate control for reflection IOR.

You’ll want to use the Alpha channel from the Opacity section

Full white = not transparent at all. Full black is completely transparent.

TextureOnPbrAlphaChannel.3dm (357.5 KB)

That’s confusing that the IOR is hidden in the opacity tab, also if it is needed for reflection. Better could be if the IOR would be shown at reflections and opacity.

It’s a pity that there is no continuous transparency option for slightly blend in objects to a scene. The alpha map is a workaround, but will work for stills only. My hope was to animate the transparency during rendering a Bongo animation.

I ask me how an air balloon could be setup, where a single surface sphere should be enough, if the refraction IOR could be set 1 and the reflective IOR = 1.5?

Thank you for your hints.

As said, there is no reflection IOR control, only refraction IOR.

You can set IOR to 1.0 so you don’t get refraction.

That said, if you model a baloon as a closed sphere then won’t be able to get a good effect. You need to give the surface a thickness. Otherwise it’ll be interpreted as a solid object.

Would something like this do?

SimpleBalloon.rmtl (43.7 KB)

Red balloon

Red Simple Balloon.rmtl (45.4 KB)

Here one more illustration why thickness is necessary. On the left side just a sphere with the red balloon material. On the right the sphere with thickness (0.10) and the red balloon material. See how the solid sphere has different ray action than the one with thickness. This is how it works in the real world too.


Ah, I understand, I thought the IOR is used for both, reflection and refraction. So, it’s only used for refraction and the reflection IOR is limited to an internal set fix value. Strange. I like the classic way of full artistic freedom with independent control of refraction and reflection IOR.

Classic render engine allow to render single surface balloons without thickness and to set IOR 1/1,5. It’s easier to build and to render. Your last test shows the problems of the thickness-method - the white ring is black at the right side since more trace depth is needed, which is is less easy to calculate. Also the right balloon doesn’t look like a thin film object without any refractive effects. A single surface with IOR 1/1,5 would be much easier to control and more real looking. :wink:

We don’t have a thin film material no. We may have sometime in the future. In the render the torus is so dark because that is the shadow side. If I move the sun so it comes from the front it looks like this



Some notes aout reflective IOR in Rhino.

For the Custom material, the reflective IOR is set at 1.56. It can’t be changed in the UI, but you can change it through the SDK - this is to allow 3rd party renderers to modify it for their own material types - so that the display can support it.

For PBR - and this is part of the PBR spec - reflective IOR is controlled by Specularity F0. It only has an effect on Dielectrics - ie, if “Metallic” is checked (1.0), then Specularity F0 has no effect on the material. It’s complicated - but 0.5 means that the reflectivity at the camera incident angle is 4%. 1.0 maps to 8% and 0.0 maps to 0%. Reflectivity for a dielectric at glancing angles is always 1.0.

There is a direct relation between Specular F0 and Reflective IOR given by the following formula:


  • Andy

@nathanletwory Do you know why the scene behind the sphere is not visible like expected by a thin surface? Here an example of a V-Ray material where the transparency is without refraction.

@andy Thank you for explanation. I think it would be easier for the user to use IOR values for refraction and reflection and best both independent for the artistic freedom. F0 looks like an academic value, difficult to use.

I’m interested on the PBR materials since I hope they can be controlled by Bongo and finally be used per Bongo and V-Ray. Looks like I stumble at the first step - the setup of the PBR materials.
One effect I’m very interested to get is to blend in and out objects during an animation. Blend in and out means a changing transparency without refraction effects. Best would be if the inner sides of the objects could kept invisible, but I’m not sure this would be possible.

My material has still quite a high roughness. Setting that lower, making the thickness less (say 0.01 or even 0.001) and making the base color less dark will make it similar to what you have.

Darker colors means less light gets through. Lighter colors means more light gets through.

Pick the red you want, then switch to HSL sliders (if you aren’t already) and move the lightness value closer to 1.0

Compare to lower lightness

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OK, it works, only the usage is quite limited.

I did some tests and I found that the alpha options looks promising for blending in and out objects. Also it work with Bongo. :slight_smile:

Thank you,