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I'm making my first project and realised I got a weird circle on the glass on my windows looking like a coffee stain or something (the circle only appears on the windows). I don't know when it appeared and if it helps at all I'm using windows from the Archipack add-on.

Thank's for any help! I'll link some pictures.

Weird circle on window

Weird circle on window zoomed in

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  • $\begingroup$ hello, are you sure there's not an object than is invisible in preview but visible in render? $\endgroup$
    – moonboots
    Jul 8 '21 at 13:26
  • $\begingroup$ Hey thanks for the answer. Yeah, the circle is visible in the preview aswell. when I move the circle change place on the windows. $\endgroup$
    – Alex
    Jul 8 '21 at 13:34
  • $\begingroup$ If you can move the circle, why not simply delete it or hide it in rendered view so that it's no longer visible? $\endgroup$ Jul 8 '21 at 13:40
  • $\begingroup$ Sorry I might have worded that wrong, when I move the camera* the circle change position on the window. I can't move the circle and it's not an object (I think). $\endgroup$
    – Alex
    Jul 8 '21 at 13:54
  • $\begingroup$ I'm guessing it has to do with the IOR of your glass. Since windows are flat, I think you can get away with setting the value as low as 1.05 or so. Does that make the circle disappear? $\endgroup$ Jul 8 '21 at 14:26
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The circle is a reflection of the object Plane.005, which is the empty room behind the camera. For whatever reason, the material has a Background set to black as a surface instead of a Principled BSDF, Diffuse BSDF or any other "real" object shader and I was wondering why it's not reflected as black room inside the window.

Then I looked at the glass material of the window and saw that there is a Fresnel node set to 40(!) as mix factor between a Glossy BSDF and a Transparent BSDF. This causes the reflection to somehow distort spherical between the front and back face of the glass. You can see that more clearly if you change the material of Plane.005 to white for example, you have simple reflection on the front face and the sphere on the backface - the IOR makes the reflection bend that much inside the glass.

When I set the IOR to something "normal" like 1.45, the window simply (strongly) reflects a pitch black room like a mirror, that's because there is a Glossy BSDF in there instead of a Glass BSDF. So the IOR is definitely way off. I don't know why the Archipack window has such a strange material... When I simply delete all nodes and put a Glass BSDF in there instead of this complex node setup, there is no circle and no mirrored pitch black room.

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  • $\begingroup$ Hey, thank you so much! Im new to all this and what I learned is if I use a normal Glass BSDF the room will be filled with a lot of noise. I used the glass recommended by blender guru and clearly must have missed a step :D. Thank you so much again for that answer! $\endgroup$
    – Alex
    Jul 10 '21 at 4:50
  • $\begingroup$ @Alex Yes, interiors are always problematic with noise etc. when you use real glass materials. If I don't care for realistic reflections of the interior on the windows, for a simple solution I put no window in there and use only the frame in the render. Or take a mix shader to make the glass only visible for the camera and completely transparent to the scene. $\endgroup$ Jul 10 '21 at 8:23

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