The Cylinder topology assumes the container is closed, so it will automatically work as if the glass was closed on top. In this case a Cylinder topology could work better than Irregular topology. In the second glass, the Irregular topology uses the mesh itself to define the initial and end position of each volumetric ray. Since there’s no mesh on top of glass, the ray on the top edge starts in the interior of the glass instead of the top, producing the clipping in the image. Another common solution to non-capped containers consists of putting a liquid volume with a cylinder topology inside your cup or open container. Set the detail setting to “Default No Flask” which will render just the liquid and it will work. Check out demo scene “NonCapped” for an example.