Plastic Cup is making shadows inside of itself that cannot be removed I want to make a plastic cup. I made it transparent, almost glassy, by using a Principled shader. I want the shadows that are trapped inside of it to be gone, but moving the light will not work and I have no clue where to start to fix it.

Here's what the problem looks like:



And here's the file to tinker with: File
This is a smoothing issue. In Data Properties, under Normals, enable Autosmooth.

You shaded your object smooth cause you didn't want to see the verticals faces as individual faces, but as a curved surface. The smoothing is done (more or less) by averaging the normal between two faces, and it works really well on the side of the cylinder. But it also average the normals between the bottom and the side faces, and betwen the "collar" and the side, which looks quite ugly.

Autosmooth smooth the faces below a specific angle (30° by default), and keep sharp the edge between faces above that angle.