The Override Material is a utility material provided with the V-Ray renderer. It allows a surface to look different depending on whether it is seen through reflections, refractions, or GI. With this material you can get a fine control over the color bleeding, reflections, refractions, and shadows of the objects.
Image by Linda Ferroni demonstrating the reflect material override option.
Base Material – The material V-Ray will use while rendering the object.
Shadows – The material that is used to render shadows cast from the object.
Reflection – The material V-Ray use to render the object with, when the object is seen in reflections. For more information, see Using the Reflect Material example below.
Refraction – The material V-Ray will use to render the object with, when the object is seen in refractions. For more information, see Using the Refract Material example below.
GI – The material V-Ray use while calculating the GI solution. For more information, see Using the GI Material example below.
Example: Using the GI Material
This example shows how the use of a GI material will affect the rendering.
Scene rendered with two VRay materials
Scene rendered with 1 Base + 1 GI Mtl
As you can see, the scene contains a square-type room. There are window openings in one of the wall. There is a light coming through, which simulates the sun. The floor has a texture in the Diffuse map slot. The rest of the scene – the walls, the ceiling and the teapots – have a Default VRay Material with a Diffuse Color (200, 200, 200).
In the first render, it is clear that all the walls, the ceiling, and the teapots have been rendered in some light brown (pale pumpkin) color, although they have a light-gray material assigned. This is due to Color Bleeding, which is generated by the GI calculation.
In the second picture, the scene is rendered with a MtlOverride GI material assigned to the floor.
This Override material contains in itself the initial two V-Ray materials: that of the floor and that of the walls. After assigning it to the floor object, V-Ray will know that while calculating the GI, it will use the GI material. In our case, it is the wall VRayMtl with Diffuse Color of (200, 200, 200). When rendering the actual floor, V-Ray will use the Base Material, the floor VRayMtl with texture in the Diffuse Slot. The result is quite different from the first render as the Color Bleeding has gone. Of course this depends entirely on our choice for the GI material. For instance, if we had chosen a bluish colored material, the final result would also be tinted slightly to blue, like in the first render with the pale brown colors.
In this simple scene, the result of the second render can be produced, with a pre-saved irradiance map, calculated with just the walls' material assigned to all the geometry.
For a much more complex scene, with lots of different geometry, shaders, textures, etc., using the MtlOverride material can be very helpful.
Example: Using the Reflect Material
The scene used in the following examples is very simple. It contains four boxes, a light source, and a studio type environment. Each box has a MtlOverride material assigned, but only the Base Material is active. The rendered boxes are all one and the same in their diffuse and their reflections as well.
As you can see now, each of the boxes has a different material assigned in their MtlOverride Reflect Material. The first one has a red diffuse color, the second ones have green, and the third one has blue. V-Ray uses those materials, when the objects are seen in reflections. In our scene, the environment is actually a reflective surface, so the boxes are being reflected. On the other hand, you can also notice that the base material of the boxes is also reflective (Fresnel type), and the middle boxes are seen with their MtlOverride Reflect material in the right box.
Example: Using the Refract Material
The next render is even more complex as the MtlOverride Refract Material of the boxes is activated as well. From left to right follow: a cyan, a purple and a yellow diffuse color. Those materials are set so that when seen through refraction, V-Ray will consider and render the objects with them. As you can see the Reflect materials are still affecting the render image. If you take a closer look at the lens' edges you will notice the green reflection, which is actually the that reflect material of the middle boxes. While V-Ray had been tracing the rays on the lens' surfaces, those polygons on the edges had first captured a reflection, so that's why there are green traces.
Can be Overridden – When disabled, the material is not overridden by the Material Override option in the Render Settings.