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I was doing the coffee cup tutorial that made by Blender Guru,and this happened:
The shadow looks weird when there is liquid in the cup.(Look like dimanods?)enter image description here But everything looks right when there is nothing inside the cup. Don't know why is that happening.enter image description here Here is my compositing.(I didn't turn on the denoising in the render properties.)enter image description here Here is how it looks like in rendered view mode enter image description here The wireframe of the glass and the liquid.

Cup's wireframeenter image description here LIquid's wireframe enter image description here

Have a nice day.

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  • $\begingroup$ Please add your blend file to your question (Grab the URL of the question. Go to blend-exchange.com. Select the blend file. Add the url of the question. Grab the url that results. Go back to the question and edit it. Add the new url to the bottom of the post.) My guess is that the problem has to do with the way you made the object that is the liquid, so if you can't include your blend file please at least add a wireframe of the liquid and glass $\endgroup$
    – Marty Fouts
    Commented May 28, 2021 at 16:07
  • $\begingroup$ This might be caused by less geometry at the top of water! try adding loop cut s(ctrl+r) at the top of water geometry. As marty fouts said it's best to share a blend file or a wireframe it makes easy for people to look into the issue. Have a nice day! $\endgroup$
    – Nand 27
    Commented May 28, 2021 at 17:02
  • $\begingroup$ So I think it was the way I made the liquid is causing the problem,it has the weird flower look on the top.(Also,I add the wireframe in the question.) $\endgroup$
    – Simon Lin
    Commented May 29, 2021 at 6:20
  • $\begingroup$ Blender Guru clearly shows in the tutorial how to inset faces, make loop cuts etc. to improve the liquid, also he scales it into the cup. Your sceenshot looks like you stopped modeling before he is doing all that. $\endgroup$
    – Gordon Brinkmann
    Commented May 29, 2021 at 13:46

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It looks like your liquid mesh is intersecting very slightly with the cup. It's possible they have different render values for their Subdivision Surface modifiers, or a common fix (and one given by Price) you could try is scaling the liquid mesh up very slightly.

This intersects the liquid's outer faces into the glass completely which gets you a pretty good render.

Additionally, there are several posts on this site and others regarding the proper physically correct method of rendering liquids in glasses, which I will link here.

Liquid / glass interface IOR and normals in Cycles - BSE

fluid in a glass - BSE

Fluid in a Glass - Greg Zaal's blog

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