The V-Ray displacement modifier is a tool that generates additional geometry that would otherwise need to be modelled. Patterns such as the ones commonly seen on a car tyre use complex geometric shapes, these can be time consuming to model. This same 3D pattern can be generated from a texture map with the ability to control the overall height and mesh quality. The disadvantage however is that it can be slow to render due to the calculations required at render time.
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The V-Ray displacement modifier has three types of mapping which are 2D, 3D and subdivision. 2D displacement will only work with texture maps and requires UVW coordinates. It also does not support procedural maps such as noise, cellular and smoke. This method is fast because all calculations are carried out in texture space prior to generating 3D geometry. This requires a large amount of RAM as the displacement is first stored and then mapped on to the 3D object.
3D displacement works with both texture and procedural maps. The displacement is calculated and applied in 3D space by subdividing the original geometry in to smaller triangles. The amount of subdivision is controlled via the edge length parameter in the displacement modifier. Smaller edge length values generate more sub triangles but take longer to calculate. To speed up this process, you should add more detail to the mesh using either mesh smooth, tessellate or subdivide modifier below the displacement modifier in the stack. This will enable you to keep the edge length parameter as high as possible.
Subdivide the mesh to speed up displacement
Memory is recycled using this method so there is no need to increase the dynamic memory limit, therefore you are retaining maximum static memory for other render time calculations.
Subdivision displacement divides the mesh using the max subdivisions parameter. Similar to how the mesh smooth modifier works, it will generate a smooth result but whilst using less memory. You can also use the displacement modifier as a mesh smooth by setting the displacement amount to 0 and adjusting the maximum subdivisions.
Setting up the displacement modifier
Add a V-Ray displacement modifier to your object and make sure it is on top of any UVW mapping. In most cases use the 2D mapping type as it offers both speed and quality providing the texture map used for the displacement is of a high standard. The amount parameter controls the height and this is set up in accordance with your system unit setup.
High resolution and good quality textures provide better results
The texture map is really important and must be a high quality image, without any colour banding or noise. It is best to use an uncompressed image file format such as TIF and save out as 16-bit to retain as much colour information as possible. If there is any colour banding, you may notice a stepping effect in the displacement. In the displacement modifier, set the resolution parameter to be the same as the longest edge of your texture to maintain the highest possible quality for the displacement.
Add a black border to your displacement maps to avoid any gaps
If your texture has a white edge, the geometry you wish to displace will end up moving away from the original position therefore creating a gap. To move the geometry back you can adjust the shift parameter. Alternatively, if possible make sure your texture has a black border around it so that the displacement is at 0 on the edge.
Use the VRayHDRI bitmap loader for loading in the texture
The VRayHDRI bitmap loader is not just for HDR maps, it can also be used to load in regular textures. By using this instead of the 3ds Max standard bitmap loader you will notice a small performance increase as well as improved filtering that will result in a much cleaner edge on the displacement.
CGI Quick Tips
How to set up multiple displacement maps
Create a multiple sub-object material and assign different displacement maps to each material. Using the displacement modifier, choose 3D mapping type and tick use object material to gain precise control over the quality via the modifier.
Avoid harsh edges by using blurred texture maps
Jagged edges in the texture will produce an untidy edge in the displacement. To avoid this, blur the edges slightly in Photoshop to create a softer edge. This will also generate a chamfer type effect to catch highlights.
Increase the amount of dynamic RAM to avoid render time crashes
Displacement uses dynamic memory. In the render settings under system, set the dynamic memory limit to around a third of the available system RAM.