No transparency with transparent material and background

Why do I get no background transparency in the upper half of the ring?

Ring material is “White Transparent Plastic” (from Material Library).

  • Transparent background: enabled
  • Raytraced (Cycles)
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Hi @Neubauer
Transparent materials in Cycles refract the light that passes through them. What you are seeing through the transparent material is actually not what is right behind it, but rather another part of the scene, as the light is bend as it passes through the material. How much it’s bend depends on the IOR (index of refraction) of the material. Some render engines (including Cycles in Blender) do allow for alpha transparency to be set anyways (although technically it’s cheating), but i don’t think the options is exposed in Rhino. Is that right, @nathanletwory?
What you can do is either a) edit the alpha channel in Photoshop (or similar) to give you the desired amount of transparency or b) turn off the transparent background, set the background color to something that matches the intended background and then turn transparent background back on. This will give you a render that’s somewhat easier to edit into the target background, as the refraction will have the right colour.
HTH, Jakob

Edit Or you can set the intended background as the environment and wallpaper for your rendering, and do the composit directly in Rhino - if that fits your work flow.

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I would like to add that the old rhino renderer (now lagacy renderer) can do that.

I never realized this since transparent or translucent materials aren’t usually something I use in my renderscene. I would be pretty surprised if RhinoCycles couldn’t do this. This should be a pretty basic feature. This kind of scenario is quite common when doing product renderings.

Btw, welcome to the forum @Neubauer

Thank you for the explanation and workaround, @Normand.

I need the “universal” transparency to compose it with different backgrounds later.
I agree with @hitenter, there should be an option / setting for RhinoCycles, because it’s quite a common scenario.

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Our current Cycles implementation, which is trailing behind several years, has glass materials as they are actually implemented in that Cycles version: glass materials are opaque.

It is on my todo list to improve this once I the Rhino Cycles version has been updated.