Have you ever had that fleeting sensation that your interior renders look a bit off? It might feel like the light is spraying colors all over the scene. This particular problem is known as excessive Color Bleeding, or Light Leaks as some like to call it.

The Problem of Color Bleeding

This is not generally a problem with how VRay calculates GI, but instead how some materials are harder to simulate realistically. In the real world, some materials despite being quite colorful, don’t bleed as much. This is a testimony to the fact that true CG material simulation is a complex art that requires us to use tricks to get what we want. After all, every rendering technique is just an approximation. That’s why it’s always best to have a reference of how light truly reacts in a similar real world scene. Too much GI bleed can be equally bad as too little of it.

Color bleeding itself is a natural property of light and materials. When a light bounces from a surface during GI calculation, it picks up its color and carries it over onto the next surface. And this process colors your entire scene in ways you cannot foresee. Or so you thought, because VRay already has you covered there as well. Introducing the VrayOverrideMtl (VRay Override Material), a utility material to set custom materials for GI, Reflection and Refraction calculation.

The Solution

So let me demonstrate real quick on how to use this feature to control the color and intensity of the bounced light, while keeping the base material unchanged. As the name suggests, the VRayOverrideMtl gives an object the ability to use different materials for calculating its Global Illumination, Reflection and Refraction properties. It provides an excellent way for you to dial in the perfect color bleed levels that your scene calls for. Let me show you how to use this material.

1. Create a new VRayOverrideMtl instead of the standard VrayMtl.
2. Assign your original material to its base slot. This will only affect the appearance of the object.
3. Create and assign a separate material for the GI slot. VRay will ignore the base material and instead use this material for its GI calculation.
4. You can now manipulate the hue, saturation and intensity values for this GI material and get a physically correct or a pleasing color bleed.
5. If you want to neutralize the effect of the color bleed entirely, I suggest you use a grey material with 128 intensity as your GI override.

So that’s it guys. It’s a pretty simple VRay functionality, but knowing when and how to use it effectively can help you a long way in getting that photoreal render. If you found this helpful, don’t forget to share or check out my other VRay tips and tricks.

You can also refer the Official VRay Resource for the VRayOverrideMtl for further information and examples.Till then, take care.