Here’s why you need a cage:
Take a polymesh object and divide it a few times. Return to subdivision level 1. Notice how it’s smaller/thinner? This is because of how multi-resolution subdivision editing works. Changes made at one level become translated across all other levels as soon as you switch levels. Subdividing also smooths your model, which causes it to contract. When you return to level one, that contraction at the higher levels affects your level one mesh. If you created a displacement map at this point, it would work perfectly for what you see in front of you – but would be wrong for the original, unsmoothed mesh. The displacements would be too strong and your render would come out really puffy.
As a result, you need to calculate your displacement map for the unsmoothed model – aka, the Cage.
After you have finished detailing your model, there are three ways to restore your level one mesh to a cage state so that you can calculate an accurate displacement map:
Import: If your model was originally imported from another program, then you can return to level one and press Tool>Import. Select the original model. It will replace the smoothed level one, restoring this level to its unsmoothed state so that you can create an accurate displacement map. After calculating the map, simply select it in the Alpha palette and export it. You do not need to export the mesh again, since you already have an OBJ of it. Note: If you think you may need to continue sculpting the high res model, you should save your model as a ZTL immediately before importing the original mesh again. Otherwise, restoring the base mesh will cause the upper levels to change as soon as you go to them, making it difficult to continue working. When you wish to return to those levels, you’d simply load the saved ZTL.
Morph Target: Another option is to store a morph target (click Tool>Morph Target>StoreMT) before you first subdivide the model. After you have done the high resolution detailing, you return to level one and press Tool>Morph Target>Switch. This switches to the stored (usnmoothed) state, ready for you to create an accurate displacement map. If you wish to return to the higher levels again, first press Switch again to get back the smoothed version of level one so that your upper levels are unchanged. If you use this technique on an imported mesh, there’s no need to export anything other than the displacment map. If you use it on a mesh that was created in ZBrush, you will need to export the unsmoothed low resolution mesh in addition to the displacement map.
Cage: This third option is sort of a last resort. You would use it if you don’t have an original mesh to import and also forget to store a morph target before dividing the mesh. After detailing the high resolution model, return to level one and activate Tool>Geometry>Cage. ZBrush will modify level one to create an unsmoothed mesh that’s as close as possible to the high resolution version. You can then calculate your displacement map and export both it and the cage. Before returning to higher subdivision levels, turn Cage off again. This method is not preferred because it does not restore your original level one. Instead, it’s an approximation and does not always give the best results. In fact, you may sometimes find that that Cage gives better results if you activate it at level two instead of level one. Whatever level you’re at when you activate it, that is the mesh that must be exported for use in your other programs.
So there you have it. As you can see, these are three UNIQUE methods of restoring an unsmoothed mesh before creating your displacement map. Many people try to mix and match methods by doing things like switching to a morph target, importing the original mesh and then turning on the Cage. That’s bad news. You should only use ONE of the three choices. Which one is best for you will depend on the situation you find yourself in when you’re ready to create the displacement map.
Have an imported mesh? Use the first method. Have an original ZBrush mesh? Store a morph target before dividing and use that. Forget to store a morph target? Use the Cage.
Hope that helps.
Import: If your model was originally imported from another program, then you can return to level one and press Tool>Import. Select the original model. It will replace the smoothed level one, restoring this level to its unsmoothed state so that you can create an accurate displacement map. After calculating the map, simply select it in the Alpha palette and export it. You do not need to export the mesh again, since you already have an OBJ of it. Note: If you think you may need to continue sculpting the high res model, you should save your model as a ZTL immediately before importing the original mesh again. Otherwise, restoring the base mesh will cause the upper levels to change as soon as you go to them, making it difficult to continue working. When you wish to return to those levels, you’d simply load the saved ZTL.
