Displacement maps create high-resolution details by physically displacing the geometry at render time. The model is sliced into millions of triangles by the renderer in a process called tessellation. The surface is then pushed in or out based on a grayscale value in the displacement map. Some renderers perform subpixel displacement, which is a re-tessellation of the surface into micropolygons or microtriangles to support even finer details. The important thing to understand is that displacement mapping requires that the geometry be heavily tessellated at render time.

Because these maps create actual geometry, they are heavy on processing power and can involve long render times. They also offer superior results compared with any of the other mapping types since the rendered mesh is extremely dense and carries all the details of the surface and silhouette. This is in contrast to normal mapping, which only creates the look of a highly detailed surface without changing the underlying geometry or polygon count. See Figure 9.2 for an example of normal mapping versus displacement.

Figure 9.2. Two renders of the same character. On the normal mapping example at the top, notice how the internal forms appear highly detailed while the silhouettes still display polygon faceting. The image in the upper right shows the polygon mesh itself. The displaced render at the bottom contains all the fine details on the surface as well as a displaced profile with no faceting.

ZBrush also enables you to create bump maps by using the bump viewer material or by generating a displacement map for use as a bump map. In technical terms, bump maps and displacement maps are identical—it is the renderer that reads and displays them differently. Both are grayscale maps with white values representing height and dark areas representing depth. In application, however, bump maps differ from displacement maps in that they do not change the overall silhouette of the surface. Bump maps give the impression of surface detail where there is none by perturbing the surface normal of the mapped faces. Because bump maps don't require heavy subdivision at render time, they render much faster than displacement maps but do not provide high-quality results since the profiles are not altered. Bump maps can be extremely successful when combined with displacement maps. See the section "Applying Bump Maps in Maya" later in this chapter for more information.

Normal maps are similar to bump maps in that they do not alter the profiles but they offer more realistic shading on the surface details. While bump maps perturb the surface normal, a normal map replaces it entirely with an RGB value. This vector value can represent the X Y Z coordinates of the surface normal of the high-res mesh and allows for highly realistic detailed surfaces but won't affect the low-poly faceting of the shape's edges. Normal maps are used heavily in video game applications.

When generating displacement maps in ZBrush, it is important to be sure the UVs do not overlap at all. Overlapping UVs (which are easy to miss) will cause unexpected crashes when you are extracting your displacement map. You can avoid this problem entirely by using ZBrush AUV tiles or other automatic options for your mapping. This approach will remove any possibility of overlapping UVs, but as you will see, having UVs laid out in Maya instead will open up several workflow possibilities not otherwise available. AUV tiles are not human readable, which makes editing color maps in Photoshop nearly impossible.

Figure 9.3 is an example of overlapping UVs in the toes of a model. This problem must be corrected before the displacement map can be extracted. Otherwise, this small overlap could cause a crash. If at any point you crash while generating a displacement map, chances are extremely good that you have a minor overlapping UV.

Figure 9.3. Overlapping UVs

Note that unless you are painting color maps with UV projection techniques, ZBrush is not concerned with UV coordinates This leaves you free to model and paint in ZBrush and then lay out UVs later or change your UV layout without losing all the work you applied to your model. ZBrush also offers a tool to check for overlapping UVs under the Tools palette: select Tools → Texture and click the UV Check button (Figure 9.4). Clicking this button will highlight in red on your model any overlapping UVs, so you know where to look for errors back in Maya. When performing an inspection like this in ZBrush, be sure to rotate your models to view all surfaces.

Figure 9.4. UV Check button

It is also possible at this phase to place your UVs outside the 0 to 1 texture space. This will allow you to use polygroups to organize your mesh in ZBrush for easy masking and hiding as well as enable you to maximize texture space and extract maps for each body part instead of a single map for the entire model.

Figure 9.5 shows an acceptable UV layout for a model. The UV shells are all in the dark gray square, which represents the 0 to 1 range. This will generate a single map in ZBrush that applies to the entire mesh. The multiregion layout in Figure 9.6 will allow you to interactively hide parts of the mesh using the UV Groups option (as discussed in Chapter 3). This layout will also enable you to generate multiple displacement maps for your mesh so that you can spread the body parts out across multiple maps, thus giving more texture space to the displacements. For example, the head, torso, legs, and tail may each have a displacement map at 4096x4096 resolution as opposed to spreading the detail for the entire body over one 4096 map.

Figure 9.5. Single-region UV mapping

Figure 9.6. Multiregion UV mapping

If for any reason you need to change the UV coordinates of the model before generating maps, this is no problem. The process of transferring a model from ZBrush to Maya and back to ZBrush seamlessly is as follows.

You can move the OBJ file between applications as long as the vertex order remains unchanged. Vertex order is changed by cutting new faces, deleting faces, or reordering on import. Use the following process any time you transfer your model between Maya and ZBrush:

If you import a model into ZBrush and the vertex order has changes, it may appear to import fine. The problem manifests when you try to step up subdivision levels. The mesh will explode, as shown in Figure 9.8.

Figure 9.8. Changing the vertex order outside of ZBrush can cause meshes to explode.

If you plan on using a displacement map for detail transfer, as in Chapter 3, you must use a 16-bit map. For all other rendering, I recommend using 32-bit, which is described in the next section. There are situations where a 16-bit map may be needed in a pipeline. These cases include pipelines that don't support 32 bit (these are rare these days) and the increased overhead of multiple 32-bit maps in a single render. For your own personal renders, I recommend using 32 bit as much as possible; the results will always be far superior to 16-bit displacement.

Figure 9.9. The Create DispMap button under Displacement

Generating a 16-bit map in ZBrush is simple. To do so, follow these steps:

You have now generated a 16-bit map from your ZTool and prepared it for rendering in Maya. You can move ahead to "Setting Up Single 32-Bit Displacement Renders in Maya" or read the next section for details on generating a 32-bit floating-point map.

You can only export 32-bit floating-point maps using the Alpha Displacement Exporter menu found under ZPlugin → Multi Displacement They can be exported from ZBrush but not reimported, so if you plan on using a displacement map for detail transfer as in Chapter 3, you must use a 16-bit map. For all other rendering I recommend using 32 bit whenever possible.

You can access this plug-in from two places in the ZBrush interface: under the Alpha menu (Figure 9.11) and in ZPlugin → Multi Displacement 3 (Figure 9.13). Although it is possible to export 32-bit maps from the Alpha menu, we won't use this button to do so in our example. This is because if you generate your map using the facilities in Tool → Displacement and then export it from the Alpha → DExporter menu, you will get a 32-bit map without real-world scale.

Figure 9.13. The Displacement Exporter can also be accessed via the ZPlugin menu under Multi Displacement 3.

If you generate a map from this menu for an object imported into ZBrush, the map will not appear to render correctly in the outside application. The reason is that ZBrush scales your model to one ZBrush unit multiplied by your Unify Scale setting in your Preferences palette. When you export the geometry, it will return to the original scale as the imported model. Maps generated using the Tool → Displacement menu will be baked with the internal ZBrush scale. On the other hand, to generate a map with the original object scale baked in, we need to use the Multi Displacement 3 plug-in found under the ZPlugin menu.

The original function of Multi Displacement 3 is to generate multiple maps based on UV region, but if your UVs are all in 0 to 1 space, it will create a single map as you'd expect.

This method is also useful if you have set up UV regions outside 0 to Multi Displacement 3 will generate maps automatically based on each separate region. For example, a model with four UV shells in four separate regions will get four separate maps. Since the Alpha Gain is consistent between the maps, there will be no seams when these maps are applied to the mesh and rendered in Maya. This allows for higher-quality displacements since more texture space is allotted to each part instead of the whole character's displacement being contained in a single map.

Multi Displacement 3 has several menu options:

The Displacement Exporter allows you to export alphas from ZBrush as various kinds of displacement maps and even normal maps. Even though you can create normal maps with Multi Displacement 3, I recommend using ZMapper for the higher level of control it offers. Each option slot can be turned on independently and allows you to specify settings for each map type.

To create a 32-bit floating-point map with Multi Displacement 3, follow these steps:

It is important to remember that with the Displacement Exporter there is no reason to open your map in an external image-editing software and resave it. In many cases this will destroy the floating-point data and render the map useless.

Maya provides a nice utility called imf_info.exe in the Maya/bin directory. You can use this program at the command line to read detailed information about your displacement map.

imf_disp.exe is another utility that will display maps and offer information on them. This utility features a graphic interface.

mental ray comes with another command-line utility called imf_copy, which will allow you to convert TIFF files to the .map format. To run this utility, open a command window in the folder where your TIFF files are located. Use the following command line to convert to .map format:

imf_copy –p originalfile.tif newfilename.map

In this section, we'll import the level 1 mesh to Maya, set up displacement shaders, and start rendering. We'll use a single displacement map to represent the entire character. In the next section, we'll look at using multiple displacement maps for a single character.

addAttr -ln "miExportCCMesh" -at bool mentalraySubdivApprox1;

Setting Up the Render Settings

Next, we need to change a few settings in the Maya Render Settings window. Select Window → Rendering Editors → Render Settings to open your Render Settings window. If it is not already set, choose mental ray from the Renderer dropdown box.

On the mental ray tab is a rollout option called Translation. Open this rollout and set Export Verbosity to Progress Messages. This will tell mental ray to report useful information to the output window when it renders. This includes the final Triangle count, which when divided by two is the face count of your subdivided model at render time. Compare this to the final face count in ZBrush to determine how close to the ZBrush subdivision you are getting with your Approximation settings (Figure 9.19). This can assist in setting your N Subdivision value accurately. Now open the render view and initiate a render. You can keep the image in render view for comparison (Figure 9.20). Figure 9.21 shows how raising the Approximation settings will increase the quality of the fine details on the surface.

Figure 9.19. The output window: this mesh was divided to 452,932 triangles or 226,466 faces at render time.

Figure 9.20. The final render with a single displacement map

Figure 9.21. This image illustrates how a Subdivision Approximation setting that is too low will reduce the visible details.

In this section we'll look at rendering a character using multiple displacement maps. Multiple maps are desirable when you need higher resolution in the finished render so you can spread the entire character's detail across multiple maps. With this technique, you can have 4096x4096 pixels of displacement detail for the head, arms, and body as opposed to spreading the texture space across the entire model.

To generate multiple maps with Multi Displacement 3, it is necessary to set up multiple UV regions in Maya. This simply means your UVs lie outside the standard 0 to 1 range, which is displayed in Maya by the upper-right quadrant in the UV Texture Editor (Figure 9.22). When your mesh has UVs that lie outside the 0 to 1 region, clicking the Create All button in Multi Displacement 3 will start generating and exporting maps for each of these regions. Follow these steps to set up a ZTool with multiple UV regions:

Figure 9.22. The UV Texture Editor

Figure 9.24. The final render with multiple displacement maps

Because a 16-bit map does not have real-world scale baked in, you must adjust the Alpha Gain and Alpha Offset values until the render appears correct. By using an expression for Alpha Offset, you can just adjust the Gain value and Offset will update. Set Alpha Offset to = - filenodename.alphaGain /2;. In this expression, replace filenodename with the name of the current file node. You can find this at the top of the Attribute Editor window when you select the file node (Figure 9.25). If the file node is called file2, use the expression = - file2.alphaGain /2.

Test-render your displacement. If your Approximation settings are correct, you will just need to raise the Alpha Gain value until the height of the displacement looks correct. Figure 9.26 shows the Pixologic displacement test head. This figure illustrates a ZBrush render of the test head on the left and a Maya render of the same head on the right. By adjusting Alpha Gain to increase the level of displacement, we can dial into the right settings. The render scene file for this test head is available on the DVD. Simply open the scene file in Maya and render.

Figure 9.25. The file node name can be found at the top of the Attribute Editor. This node is named file2.

Figure 9.26. Pixologic test head rendered in Maya

In some cases you may see artifacts in your renders. If you see bloating in your render, then there is a mistake with the Alpha Gain and Alpha Offset values or the Alpha Detection setting in Maya 2008 is wrong (Figure 9.30). Usually you have forgotten to set one or both, or the Alpha Offset is not set to –½ Alpha Gain.

Figure 9.30. The bloating in this render is due to incorrectly set Alpha Gain and Alpha Offset settings.

Small spikes on UV seams can be remedied by using the Smooth UV option under Tools → Displacement. Use this only when creating an adaptive map. It has no effect in DPsubPix mode. Many artifacts are caused by bad topology in the underlying mesh. Triangles, poles, or faces with more than four sides can result in surface errors (Figure 9.31). These artifacts should be hidden in inconspicuous places during the modeling process or removed completely. If you find such an artifact on your render, applying a smooth to the mesh before rendering can help (Figure 9.32). Doing so will increase render time, and you may want to lower your Approximation settings as a result.

Figure 9.31. An example of problematic topology

Figure 9.32. Mesh artifacts in render

In this chapter, we explored the two major displacement map types that can be exported from ZBrush, and you learned how to set up models and shaders for displacement in Maya. By combining your texture map, displacement, diffuse, and bump maps with a subsurface scattering shader, you can achieve renders like the one shown in Figure 9.3The ability to capture the high detail of your characters in other renderers allows you to take your work to the next level, breathing life and animation into your work. In the next chapter, we'll continue looking at difference maps in ZBrush by discussing normal maps.

Figure 9.33. The Stingerhead character rendered with displacement and subsurface scattering

Featured Artist: The Art of Damien Canderle

Damien is a freelance ZBrush artist whose striking renders have inspired many of us in the ZBrush community. I especially love the attention to color, light, and atmosphere in his final presentation.