This article explains the possible causes and solutions for the following issues:
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The rendered image appears very dark
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The rendered image appears completely black
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Reflections look incorrect or distorted
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Refractions look incorrect or distorted
These problems are most commonly caused when the camera is unintentionally placed inside a volumetric medium—such as a material with a refractive index (IOR) different from 1.0 and/or one that uses volumetric absorption.
When is this expected?
In some cases, it is expected that the image appearance will change when the camera is placed inside a volumetric medium. Examples include:
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Underwater rendering (e.g., when the camera is placed inside a pool):
Water is a volumetric medium with a refractive index (IOR) of approximately 1.33. Corona accurately simulates how light rays interact with the scene in this situation, which can significantly affect the appearance of reflective and refractive objects underwater.
For example, the water surface viewed from below may appear almost mirror-like. This effect occurs whether the water volume is enclosed in watertight geometry (such as a box) or applied as a material to a flat plane placed above the camera—both are valid and expected setups for underwater rendering.
Example:
Pool scene, no water. Spheres from left to right: dielectric material (IOR 1.5), metal material, glass material (IOR 1.5).
Pool scene, with water.
Observe that the appearance of the spheres has changed. The dielectric sphere (left) appears less reflective. Refraction has changed in the glass sphere (right) - it now shows a different part of the teapot object. The biggest difference is in how the air bubbles are rendered (IOR 1.0). Without the water, they are completely invisible, because they have the same IOR value as the environment they are in (in Corona, the IOR for air is 1.0. In reality, it's slightly higher). This is expected when rendering from inside a medium with its IOR different from 1.0 (1.33 in case of water).
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Camera inside geometry using volumetric absorption and scattering (e.g., fog):
When the camera is placed within a volumetric object (like one using fog effects), the volumetric behavior is expected. This includes how the fog interacts with light in the scene and how it affects the visibility of other objects. In such cases, the dark or muted appearance is intentional and part of the desired effect. In the above pool example, the water material, in which the camera is placed, uses volumetric absorption set to a blue color. This results in the scene becoming darker as it gets farther from the camera. -
Camera passing through volumetric effects (e.g., smoke from Phoenix simulations or Corona Volume Material):
If the camera moves through a heterogeneous volumetric medium like smoke, the resulting changes in lighting and visibility are expected. Corona renders how the camera travels through the layers of volume, affecting the overall appearance.
When does it cause issues?
Problems can occur when the camera is unintentionally placed inside or too close to an object that uses a volumetric effect. Corona may then assume that the camera is submerged in a volumetric medium—similar to the underwater scenario described earlier. This can result in a dark, black, or visually incorrect rendering.
Common scenarios include:
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An object with IOR different from 1.0 placed above or near the camera:
This often happens when a non-watertight mesh is accidentally positioned close to the camera. Corona interprets this as the camera being inside a refractive volume, which can drastically affect lighting, reflections, and refractions. -
Flat or open geometry with volumetric absorption placed near the camera:
Similar to the above, a non-watertight object or a thin surface with volumetric absorption enabled can cause the rendering to go dark or black.
Solutions:-
Ensure the object is closed and watertight.
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Add thickness if it's flat.
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Or disable the absorption effect if it's not needed.
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A flat windowpane (a plane) using a solid glass material:
If a glass material with a non-1.0 IOR is applied to a plane with no thickness, Corona might treat it as a volumetric object surrounding the camera.
Solutions:-
Add thickness to the glass geometry using the Shell modifier (or model it with thickness).
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Alternatively, enable the "Thin shell (no inside)" option in the glass material to prevent volumetric behavior.
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Example:
A small plane object with a water material (IOR 1.33) is placed above the camera. When it's moved closer to the camera, the appearance of the whole scene changes dramatically. We can observe the following:
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Background color changes:
The water material's absorption causes the background to appear darker or take on a tinted hue. Corona assumes the camera is inside an infinitely large volume of water, which absorbs light as it travels through the medium. -
Changes in reflections:
The reflectivity of nearby objects (e.g., a reflective cylinder) changes. This is because the same IOR value will produce different reflection and refraction results depending on the surrounding medium’s IOR. When the camera is in water instead of air, light behaves differently. -
Previously invisible air bubble becomes visible:
An air bubble that was initially invisible (because it had the same IOR as the default environment, 1.0) becomes clearly visible once the surrounding medium changes to water (IOR = 1.33). The contrast between the two IORs reveals the bubble’s shape.
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Troubleshooting Steps
If your scene appears very dark or completely black, or if reflective and refractive objects don't render correctly, follow these steps to identify and resolve the issue:
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Check for objects with volumetric effects near the camera:
Look for any objects close to the camera—especially directly above it—that use refraction or volumetric absorption. These may unintentionally cause Corona to interpret the scene as if the camera is inside a volumetric medium.-
Solution: Either move or delete these objects, or disable the volumetric features in their materials.
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Inspect your windowpane geometry and materials:
Windowpanes using glass materials can cause issues if they are flat (single-plane) and use solid glass settings.-
Solution: Ensure windowpanes have thickness (e.g., by applying the Shell modifier), or enable the “Thin shell (no inside)” option in the material settings.
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See also: Corona renders black screen!