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I have been confused by this aspect. I am trying to create a solid sphere of glass and I'm worried about the light not refracting and transmitting realistically like a real sphere of solid glass. What I would like to know is if a UV sphere with a Glass BSDF shader is considered to be a glass shell by Blender or if it is considered a solid sphere of glass?

If it is the former, how can I create a realistic solid ball of glass that refracts and reflects light like a solid glass ball IRL?

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    $\begingroup$ Spheres are considered solid (a filled medium) until you give the outer edges some thickness (2 sided walls). This can be most easily accomplished by using a Solidify Modifier. If you want to keep it solid, leave it "empty" (untouched). Also - Making a solid glass ball that refracts properly only really works in Cycles - It can be approximated in EEVEE, but it never really looks correct and the refractions will never be "physically accurate". $\endgroup$
    – Christopher Bennett
    Jun 13, 2021 at 14:04
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    $\begingroup$ Hi :). To illustrate - a UV sphere will refract light first when it enters, and then second time when it leaves the geometry. Exactly like a solid glass ball. $\endgroup$
    – jachym michal
    Jun 13, 2021 at 14:09
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    $\begingroup$ @Hash If you want the outer edges to have thickness and have a solid sphere inside, then maybe that's exactly what you should do - create an outside sphere that has a thickness, e.g. with the Solidify Modifier, then put a smaller sphere inside without the modifier so that it's a solid sphere. Or I simply don't get what you want... to me it sounds like you want a solid glass ball that's stuck inside a hollow glass ball...? $\endgroup$
    – Gordon Brinkmann
    Jun 13, 2021 at 14:46
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    $\begingroup$ @moonboots That's because in Eevee glass thickness is defined through material settings, not the actual geometry :) $\endgroup$
    – jachym michal
    Jun 13, 2021 at 15:20
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    $\begingroup$ Just wanted to add: While refraction-wise, the glass shader creates a solid, the colour of the glass shader is like surface painted glass. I.e., for coloured glass you want to use volume shading $\endgroup$
    – Hagen von Eitzen
    Jun 14, 2021 at 13:49

2 Answers 2

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Just to illustrate how Cycles treats these cases:

Left: Sphere walls with zero thickness (light refracts twice) > behaves like a solid glass ball
Right: Sphere walls with 1 mm thickness (light refracts four times) > behaves like a soap bubble

enter image description here enter image description here

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  • $\begingroup$ If i were to model a glass cup glass cup, the only way to do that would be to create a cylinder shaped object and solidify it by making its thickness nonzero. Won't that make it similar to a 'bubble' then rather than a solid glass cup? And does the second picture look like a bubble because the thickness is so low, the deviation of the refracted rays is almost nil? $\endgroup$
    – Hash
    Jun 14, 2021 at 15:12
  • $\begingroup$ No, what I meant was the glass being a 'thick' cylinder with a hole in the middle rather than a thin cylinder with a hole in the middle to make a cup? Would the former, according to your diagram result in a thin bubble like 'glass' during render as opposed to realistic glass? $\endgroup$
    – Hash
    Jun 15, 2021 at 9:30
  • $\begingroup$ What I mean is something like this. Lets say I take a cylinder, inset the top face and delete it, and scale down the bottom face to make a 'cup'. Like a frustrum with one end open Now such a cylinder would have 0 thickness unless I use a solidify modifier. But you stated that having non zero thickness would make cycles treat the object like a 'shell'. But a non-solidified frustrum cup would look ugly and thin. So how do I get it behave like a real glass frustrum cup with thickness, but at the same time not act like a 'shell' frustrum cup? $\endgroup$
    – Hash
    Jun 17, 2021 at 20:52
  • $\begingroup$ Hello :). I'm obviously not very good at explaining things. So here's BlenderGuru's Glass Cup tutorial. Look at his model, that's how it should look. There's no caveat. Just model it like a real cup :)). $\endgroup$
    – jachym michal
    Jun 17, 2021 at 21:07
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    $\begingroup$ Yes, exactly :) $\endgroup$
    – jachym michal
    Jul 2, 2021 at 18:23
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If you just create a sphere, it is treated as a solid. It is easy to determine what is inside and outside the sphere. But if you remove a bunch of faces, you break this behaviour. It is no longer easy to determine the inside and outside.

Here is a solid sphere with a glass material. It has a high refracture index. It will actually focus light from the light source onto the plane it is resting. But this is quite noisy. I have switched the renderer to Cycles.

It actually have a tiny highlight from the light source. This is because it has a very low roughness but also that the light source is very small. Change to an area light and make it bigger.

enter image description here

With this 2D sketch I try to explain how non-solid objects work.

enter image description here

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    $\begingroup$ Hi :). I'm not sure the first paragraph is correct. Refraction angle is specified by normals orientation. Having some faces missing doesn't play a role :). $\endgroup$
    – jachym michal
    Jun 14, 2021 at 7:03
  • $\begingroup$ If a ray goes in through a surface and exits through a hole, where does cycles consider the sphere to stop? $\endgroup$
    – Gunslinger
    Jun 15, 2021 at 9:14
  • $\begingroup$ Hey :). The ray will be refracted only once. However, you can always tell what's inside/outside, because that's specified by normals orientation (which will affect the refraction angle). Feel free to correct me, my brain is running on fumes right now :)) $\endgroup$
    – jachym michal
    Jun 15, 2021 at 18:17
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    $\begingroup$ Ok, I think we mean different things. I will try to clarify my answer, no time right now. When it enters the sphere through a face, it will be inside. But if it exits through a hole, cycles will count it as forever being inside the sphere. $\endgroup$
    – Gunslinger
    Jun 16, 2021 at 6:03
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    $\begingroup$ @JachymMichal it took a while, check my sketch and see if you agree. $\endgroup$
    – Gunslinger
    Jun 29, 2021 at 16:58

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