Vray render different through glass than in sketchup export

[sarah@rengadesign]

Hi, this is my first ever post to a forum so please be patient. I can't find a thread on the topic, or am likely to not be searching the right terms, so here goes.

I'm getting a situation where the render is showing the view beyond two glass sliding doors in a different location to where it is when seen through the open part of the door. The view is not lining through. I've attached the sketchup export and render for comparison. If you look at the line of the paving it's not meeting up across the view in the vray as it is in the sketchup. I've also attached my glass settings (this is just sketchup glass not vray glass) in case it has something to do with the view being partially seen through glass. Any thoughts as to why this is happening?


Above. Sketchup export- view lines up across glass and open doorway



Above. vray export: view becomes out of line look at fence and paving.



Above. glass settings

[georgi.georgiev]

Hi sarah@rengadesign

Do the glass doors have any thickness?

Please be informed that for refractive materials to be rendered correctly, they cannot be a single surface. In cases where a single surface needs transparency, it is recommended to use the Opacity parameters.

[sarah@rengadesign]

georgi.georgiev I've just checked and they didn't have a thickness so I've now extruded by 30mm and painted all inside and out in same matching glass. This has given a partial fix wherby the view on the right matches the view on the left through the open door. Here's a screen grab of the part render where I stopped it. I've since been playing with reversin or not reversing faces and can't quite seem to make the magic combination to get teh view straight. Thanks for the reply- much appreciated.

[georgi.georgiev]

sarah@rengadesign

When you extruded the surface, did you group or make it into a component?

Additionally, you could decrease the IOR in the Refraction rollout.

[sarah@rengadesign]

georgi.georgiev They weren't componenents when I tried last so I made them into componenents (within a group still) and that still didn't work so I then reduced the IOR and that totally fixed it. I can now start adding all of the planting so that's a big relief as I have a client meeting at the end of the week. In the future should I avoid reducing the refraction IOR unless necessary? I really appreciate your help- thank you so much.

[georgi.georgiev]

sarah@rengadesign

When adjusting a material's IOR, it is important to consider the viewing angle (camera) and the geometry's properties (volume, shape, etc.), but most of all it is the user's preferences which prevail in respect to the desired render output.

[konstantin_chaos]

Hi sarah,

I just want to give you some additional information on what is going on here.

1. SketchUp materials don't bend light. You can see directly through them.
The SU viewport shows alpha/opacity without calculating actual refraction.
Because of this single-sided and double sided materials appear similar:

2. In V-Ray, on the other hand, light is refracted much like in reality - it bends when it enters a different medium and it bends once again when exiting.
*In the real world this bending is a result of light being slowed down by the glass.
In V-Ray this behavior is controlled by the Refraction IOR value of the material - the bigger the value, the more the light ray bends.
As a result if you apply refraction to a single sided plane the image you see through the glass is quite different:

3. Adding thickness to the window object should resolve the issue because the ray is bent once when entering and once again(in the opposite direction) when exiting.
At the end it is shifted a little but is still parallel to the original one.
The shift in the illustration is exaggerated. In your scene the effect is gonna be subtle (since the window is really thin).

4. Adding two or more glass sheets does not change the behavior much.
The final ray is still parallel to the original one.

5. Reversed surface normal will produce incorrect results and should be avoided but will still give you similar results.


As a conclusion, I'd suggest you double check the geometry of the glass sheets and make sure the're not intersecting each other.
Avoid snapping multiple sheets of glass together too.
Overlapping multiple glass surfaces with different normals will lead to unpredictable results.

A quick fix if you need such (it should work with any geometry...):
- Enable the Reflection IOR of the material - this will let you use separate IOR values for reflections and refraction.
- Set the Reflection IOR to 1.5 (or something similar) - it controls the strenght of the fresnel reflections of the material.
- Set the Refraction / IOR to 1.0 - this way light will not bend when it passes through.

Hope that helps,
Konstantin

[sarah@rengadesign]

konstantin_chaos thank you for taking the time to write such a thorough description- the images really help. I'm sure I'll need to refer to this in the future. I'm a fairly old hand at sketchup but new to vray. In this instance the building was more of a device to frame the garden garden but this will be very useful going forward when I try to achive more realistic renders.