I'm just starting to learn Blender and am following Blender Guru's donut tutorial. I just "filled" the glass cup with water and everything looks correct as far as I know but when I render it, the water doesn't look right and there are some weird horizontal lines going on. As I said, I'm very new and don't really know what to look for.

  • 1
    $\begingroup$ I wasn't able to access the blender file or the image. The links you provided just redirected me to a bunch of irrelevant websites. Stack Exchange allows you to add images directly to the question, and Blend Exchange is a great place for storing .blend files for posts. Link: blend-exchange.giantcowfilms.com $\endgroup$ Nov 21, 2020 at 18:36
  • $\begingroup$ Please use this site tools for images and files... $\endgroup$
    – lemon
    Nov 21, 2020 at 19:03
  • $\begingroup$ @NullPointerException Strange, the links work for me. But I added a download link for BlendExchange, thanks for the info! $\endgroup$
    – Otter
    Nov 21, 2020 at 19:03

1 Answer 1


Lines in transparent materials are a telltale sign of intersections with other objects. This was no exception.

To verify that there were intersections between the liquid and cup objects, I took these steps to generate a mesh that resembles the intersections:

  1. Tab out of edit mode. This only works in object mode.
  2. Select the liquid.
  3. Hit the "Modifier Properties" tab of the properties window.
  4. Create a new Boolean modifier (it's under the Generate group) with the following properties.
  • Operation: Difference
  • Object: Cup
  • Overlap Threshold: 0

Below is the screenshot of the results:


Yep, the cup and liquid definitely intersect. And notice how those lines match the lines in the render.

One way to fix the problem is by shrinking the liquid in the x and y axes. Here's how I did it:

  1. Remove the Boolean modifier (that was just to see the intersections)
  2. Select the liquid and only the liquid.
  3. Tab into edit mode
  4. Hit a to select all vertices
  5. Hit s to scale them
  6. Hit shift and z to lock the transformation in the Z axis.
  7. Enter 0.99.
  8. Tab out of edit mode.

Now the cup should look like this: No noticeable difference since the first image

At first glance, it doesn't look like anything changed. But when you render the scene... Yay, no lines! ...no more lines!

One other thing to note is that the cup has some duplicate vertices. Make sure to use the "Merge by Distance" tool occasionally to clean those up.

  • $\begingroup$ Thank you for the quick reply! This fixed it :) $\endgroup$
    – Otter
    Nov 21, 2020 at 21:43

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