Overexposed caustics or light refraction with Path Tracing

Creo
Sun, 09 Nov 2025 16:26:46 +0000
Rendering

Hi,

While using Path Tracer and glass materials I’ve faced with overexposed (or over brightened) caustics from glass.

First of all, to create glass materials I used official documentation

Here is two room renders with glass in windows:

and without glass in windows (you can see in right windows that there is no reflection of the spotlight)

Predictably not that much difference between two of them. In both cases glass has Refraction = 0 that is light is not refracting inside room.

Now let’s take a look on another 2 renders with Refraction = 1.5

With glass in windows:

and without glass in windows:

As you can see if we add glass in first render we get a lot of light over refracted into room. As I know this is not a physically correct behavior of a light.

I tried to toggle Energy Conservation on Material option but it doesn’t solve the problem

As I further examined over brightness is produced by approximated caustics. If you disable them with console command r.PathTracing.ApproximateCaustics 0 you will get no over light, but also all glass will become opaque to directional light:

For glass material I used a Thin Translucent material from documentation:

Same behavior you will get with simple solid materials. Here is an example with Refraction = 1.5 and enabled/disabled approximated caustics:

On this 2 images we see 3 spheres:

Left: Thick sphere (outside geometry + scaled and flipped geometry inside) with one sided solid glass material

Center: Base sphere with one sided solid glass material

Right: Base sphere with two sided thin glass material

As you can see on the first lit render, there are too bright caustics under the spheres, but on the second one we get opaque shadows.

This behavior doesn’t depends on the shape of a geometry:

Does anyone faced with the same problem?

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