Liquid in Glass with Redshift (Cinema 4D): Modeling, IOR and Trace Depth
Liquid in a glass is one of the hardest everyday shots for any renderer. What matters in Redshift, based on Maxon's documentation:
- Model the boundary on purpose: A gap adds extra surfaces, touching faces cause seams, and an overlap relies on nested dielectrics, which Redshift has supported since version 2.0.
- Give rays enough depth: One straight path through a filled glass crosses four surfaces. Maxon's example needs a refraction depth of 16 to see through 8 panes of glass.
- Raise depth only where needed: Combined caps GI, reflection and refraction rays together; materials such as the Standard Material can override trace depth for the hero glass only.
- Let light through: Dome light rays bend through glass when Affected by Refraction allows it, and caustics need Casts Caustic Photons enabled on the object.
- Rendering beverage shots: Deep refraction is GPU-heavy; iRender 2x, 4x or 8x RTX 5090 or RTX 4090 servers split each frame across all GPUs, and you can tune depth in the RenderView on the same machine.
Beverage, cosmetic and product shots often come down to one hard detail: liquid inside glass. Dark outlines, black glass and noisy caustics usually trace back to how the boundary is modeled and how deep Redshift is allowed to trace. This guide covers both, based on Maxon’s Redshift documentation and the standard nested dielectrics approach.
Checked against the Cinema 4D and Redshift documentation, November 2026.
1. Why does liquid in a glass render wrong?
Light changes direction at every boundary between two transparent media, and the amount depends on both IORs. A renderer has to know which medium a ray is in at every moment, and the way you model the glass-liquid contact decides whether it can (Autodesk Arnold: nested dielectrics).
How to model the boundary between glass and liquid
The approaches renderers deal with, as described in the nested dielectrics literature and renderer documentation.
| Approach | What happens | Result |
|---|---|---|
| Gapliquid smaller than the inner wall | An extra thin layer of air sits between glass and liquid, so rays refract at two extra surfaces. | Extra reflections and a visible outline around the liquid. |
| Coincident facesliquid touches the wall exactly | Two surfaces at the same position; the renderer cannot reliably tell which one the ray hits first. | Speckles and dark seams along the contact. |
| Overlap + nested dielectricsliquid pushed slightly into the wall | The renderer treats only one medium as present inside the overlap, so the glass-liquid boundary behaves like a single surface. | The clean, standard approach in renderers with nested dielectric support. |
2. How deep does Redshift need to trace a glass of liquid?
Every surface a ray crosses uses one refraction step. The figure counts them for a single straight path.
The ray refracts entering the glass, entering the liquid, leaving the liquid and leaving the glass: four steps before it reaches what is behind the glass. Ice cubes, a second glass, a bottle behind, or light reflecting inside the walls all add more. If Redshift runs out of depth first, the ray ends early and those areas render too dark. Maxon’s own example shows 8 panes of glass with thickness needing a refraction depth of at least 16 (Maxon: Render Settings, Basic).
Depth settings that decide whether glass looks right
From Maxon's Redshift render settings documentation.
| Setting | What it limits | For glass and liquid |
|---|---|---|
| Combined | The maximum for GI, reflection, refraction and volume rays together; individual depths above it are still capped by it. | Raise together with Refraction. |
| Refraction | How many times a refraction ray can bounce or pass through objects. | Count the surfaces in the deepest path you see; each glass with thickness needs 2. |
| Transparency | Opacity-based transparency, which is not limited by Combined and renders much faster than refraction. | For cut-outs and labels, not for real glass. |
| Per-material override | Trace depths can be overridden in the Optimizations section of materials such as the Standard Material. | Give only the hero glass a higher depth. |
3. How to set up glass and liquid in Redshift (step by step)
Settings checklist (copy or follow line by line):
Geometry ................ closed, thick glass; closed liquid pushed slightly into the wall Redshift Standard Material (glass and liquid) Transmission .......... 1 IOR ................... ~1.5 glass, ~1.33 water-based liquid Transmission color/depth color build-up with thickness Thin Walled ........... only for single-sided surfaces Redshift Render Settings › trace depth Refraction / Combined . enough for the deepest visible path Lights Affected by Refraction On Objects that throw caustics Casts Caustic Photons = On (off by default)
Step 1: Model closed, thick geometry
Give the glass real wall thickness and close both the glass and the liquid. A full refraction calculation needs closed, double-sided geometry; for single-sided surfaces such as a thin window, the Standard Material’s Thin Walled option is the alternative.
Step 2: Set up the materials
Use a Standard Material with full Transmission for both. Set typical IORs, around 1.5 for glass and 1.33 for water-based liquids, and use the transmission color and depth to control how color builds up with thickness.
Step 3: Model the contact
Push the liquid slightly into the glass wall rather than leaving a gap or snapping faces together, and check the result in the RenderView with a close-up of the contact line.
Step 4: Set trace depths
Raise Refraction and Combined depth to cover the deepest path visible in the shot, or override depth on the glass material only.
Step 5: Let light through
Make sure dome and area lights are allowed to bend through refraction (Affected by Refraction), and enable Casts Caustic Photons on objects that should throw caustics; it is off by default.
Quick check: if the liquid looks black or flat near the contact line, check for a gap or overlap with the glass wall, then raise Refraction and Combined trace depth.
4. Where should you render beverage and glass shots: SaaS or IaaS farm?
Deep refraction makes every frame heavier, so GPU speed matters. A SaaS render farm suits a finished, approved shot. An IaaS render farm such as iRender suits shots still being tuned: you open the scene on the render machine, test trace depths and the liquid contact in the RenderView, and render on the same multi-GPU server.
These videos show Cinema 4D and Redshift jobs on iRender multi-GPU servers.
On iRender you rent a whole server with 2x, 4x or 8x RTX 5090 or RTX 4090 GPUs, with Cinema 4D and Redshift pre-installed; Prime members get activated Cinema 4D and Redshift licenses. Related: Redshift car paint for product and automotive materials. Real render times are in our Redshift benchmark, and plans are on the Cinema 4D Redshift render farm page.
5. Liquid in Glass FAQ
Why does the liquid in my glass render with a dark outline?
Usually because the liquid and glass are modeled with a gap or with coincident faces, which adds extra refracting surfaces or confuses which surface a ray hits. Modeling the liquid slightly into the glass wall and relying on nested dielectrics avoids this, and a refraction depth that is too low can add dark areas too.
What refraction depth does a glass of liquid need in Redshift?
At least one refraction step per surface a ray crosses. A straight path through a filled glass crosses four surfaces; ice, a second glass or internal reflections add more. Maxon’s example with 8 panes of glass needs a depth of 16.
Does Redshift support nested dielectrics?
Yes, Redshift has supported nested dielectrics since version 2.0. Test how your Redshift version resolves overlapping transparent objects with a test render before committing to a sequence.
Why is my glass black in Redshift?
Most often the refraction or combined trace depth runs out before the ray leaves the glass. Raise Refraction and Combined depth, or override trace depth on the glass material only, and check that the glass geometry is closed and double-sided.
How do I get caustics through glass in Redshift?
Enable caustics in the render settings and turn on Casts Caustic Photons in the Visibility tab of the Redshift Object tag for each object that should cast caustics; the option is off by default.
Should I use transparency or refraction for glass?
Use refraction for real glass and liquid. Transparency (opacity) is much faster and not limited by the Combined depth, but it does not bend light, so it suits cut-outs and labels rather than solid glass.
