Replacing Parts of a Model
ZBrush is able to literally fuse different models together, creating clean topology to connect them. This operation will only affect the polygons located at the connecting borders while the rest of the model will remain unchanged. The process can seem similar to how DynaMesh can merge volumes, except that with this process your model will retain as much of its original topology as possible. It is important to note that while this process may look like a boolean operation, it is not. It is simply a bridging operation and so it cannot be used to create differences or negative insertions. For those effects, you must use the Remesh All function. This feature allows you to quickly and easily create an advanced base mesh with only a few clicks, simply by bridging various pre-existing mesh elements together.
Creating a Connection
The process to create a connection is simple and is based on polygroups:
1. Create a PolyGroup on the support model, where you want to create the connection. The fastest way to do this is to hold CTRL and paint a mask on the area you wish to change into a new PolyGroup. Now click Tool >Polygroups > Group Masked Clear Mask or use its CTRL+W hotkey. ZBrush will convert the masked area into a new PolyGroup. (To display the PolyGroups, don’t forget to turn on PolyFrame (Shift+F hotkey).
2. Select an InsertMesh brush with the geometry that you wish to insert. Click on the new PolyGroup that you just created to add this new mesh to the model. It is really important to do the insertion by clicking on the PolyGroup of the support which will be used to create the connection because this is used to define the border of the polygon bridge. Please also remember that this new geometry must have an open surface; the replacement feature cannot work with volumes.CTRL+click+drag on any open area of the document.
3. ZBrush will now connect the two meshes and weld the difference with quad geometry
An alternative is to tap first on an existing PolyGroup and then doing a CTRL+click+drag on any open area of the document to create the bridge. This method allows you to do a replacement part when working with PolyGroups which have been created previously.
Note: If your Insert brush is composed of more than one part, you must create a PolyGroup which will have the same number of parts. When inserting the mesh, you must click on the polygons of one part of this split PolyGroup.
Instead of masking you can use ZBrush’s mesh visibility controls. Hide everything except the polygons that will define the border of the bridge. Use InsertMesh on the visible polygons. Upon restoring full mesh visibility, ZBrush will weld the two surfaces together.
Notes: In addition to preparing the support mesh, it is advised to prepare your inserted mesh (the one that will be added via the MeshInsert brush) by creating a PolyGroup to define its connection area.
The model must have open topology (unattached edges) at the bridge location. Otherwise you will get topology problems such as non-manifold surfaces. Do not use mesh replacement to insert a solid object!
As with the DynaMesh update operation, holding CTRL while clicking and dragging on a portion of the empty document is what tells ZBrush to calculate the new mesh and bridge between the PolyGroups.
If part of your model has been masked, you first need to clear the mask. In this case you can CTRL+Click+Drag twice (once to clear the mask and a second time to complete the mesh replacement) or by pressing CTRL+Shift+A before executing the remesh command.
If you used hidden polygons instead of PolyGroups, the model will be restored to full visibility upon executing the remesh operation.