Setting the scene
Example 1: Using the Reflect material
Example 2: Using the Refract material
Example 3: Using the GI material
The scene is very simple. It contains 4 boxes, 1 light source and a studio type environment. Each box has a VRayOverride 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 VRayOverride 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 ones are seen with their VRayOverride Reflect material in the right box.
The next render comes even more complex as the VRayOverride Refract material of the boxes is activated as well. From left to right follow: a cyan diffuse color, purple and yellow ones. 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.
This example shows how the use of a GI material will affect the rendering.
Scene rendered with 2 VRay materials Scene rendered with 1 Base + 1 GI Mtl
As you can see the scene represents a square-type room. There are window openings in one of the wall. There is a Direct Light coming through, which simulates the Sun. The Floor has a texture in the Diffuse map slot. All the rest - the walls, the ceiling and the teapots have a Default VRayMtl with a Diffuse Color (200,200,200).
On the first render, it is absolutely visiable 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 because of the Color Bleeding, which is generated by the GI calculation.
On the second picture, the scene is rendered with a VRayOverride GI material assigned to the Floor.
This material contains in itself the initial 2 Vray materials - the Floor's and the Walls' ones. So now assigned on the Floor object, V-Ray will know that while calculating the GI it has to use the GI material /in our case: Walls - VRayMtl with Diffuse Color(200,200,200)/ and during rendering it will use the Base material /in our case: FLOOR - VrayMtl with texture in the Diffuse Slot/. Result of that is quite different from the previous render as the Color Bleeding has gone. Of course this depends entirely on our choise 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.
But for a much more complex scene, with lots of different geometry, shaders, textures etc., using the VRayOverride material can be very helpful.