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Remaking the Topology

Retopo (“remake topology”) is a tool for remaking the topology of a mesh. It’s the opposite of the sculpting (or deforming) tools: instead of reshaping the model but leaving the topology the same, it reforms the topology, but maintains (as much as possible) the same shape as the original model. You will not change the geometry of the original mesh in any way, you will be creating a new mesh that is projected upon an existing mesh. There are three ways to use retopo:

  • Retopo paint, where you paint lines on the mesh and painted intersections become vertex locations.
  • Creating new mesh via standard mesh editing methods (the new vertices will “stick” to the reference object’s surface).
  • Projecting an existing mesh onto the object.

In practice one usually uses all three together.

Usage

Retopo is controlled from the Mesh panel in the Editing context (F9).

The Retopo button appears when a mesh is selected and Edit mode is active.

When the Retopo toggle button is pressed, any change to a vertex will cause it to be snapped to the surface of another model. Note that this effect is view dependent: from the view you are working in, the vertices won’t appear to have moved, because they are falling straight back along the axis until they hit the surface of the model.

Note also that your view must be in Solid or Shaded draw mode for this tool to work.

Retopo Paint Overview

Mesh panel with Retopo disabled.
Mesh panel with Retopo enabled.

Once you click Retopo and then Paint, you will see the following options in the header:

The Retopo header.
Drawing ellipses.
Pen
A freehand line that exactly follows the location of the mouse from your start click (LMB Template-LMB.png) until you release the mouse button.
HotSpot
When enabled, this option allows you to easily connect a new line to one end of another one, by clicking when your mouse pointer is sufficiently close to this end (you see a small grey circle around it). You can toggle this option with H.
Line
A line segment from your start click (LMB Template-LMB.png) to where you release the mouse button.
LineDiv
This setting controls the number of sub-segments in a line, the number of points used to project it onto the object. Thus with two points only the endpoints are used, and the rest of the line does not follow the surface but instead passes through the object. And higher levels of LineDiv mean the projection will more accurately fit on the object’s surface.
Ellipse
Draws a ellipse between a rectangular area with its center defined by your start click (LMB Template-LMB.png) and a corner defined by where you release the mouse.
EllDiv
This setting control the number of sub-segments in the ellipse line. Thus a setting of 4 for EllDiv results in a diamond being draw instead of an ellipse.


To extend a previous Pen stroke, enable HotSpot, go to an endpoint of the stroke and a circle will appear – click and drag to continue the stroke.

You can delete previous strokes of any kind by first activating the Pen tool, then going to an endpoint (or for an ellipse along the edge) and when the circle appears click to turn the stroke red, then X or Del to delete it.

Sometimes the option to extend can be an annoyance if you are trying to paint a new stroke near the endpoint of an existing stroke. In which case you can toggle off the hotspot for end point extension with the H.

After painting your ellipses and lines press ↵ Enter to convert them to mesh. At each intersection of lines a vertex is formed. Two intersections on the same line forms an edge also. Three or four edges in a loop form a face.

Drawing different shapes (mouse strokes).
Drawing different shapes (results).


Limitations / Notes

  • When you press ↵ Enter the mesh faces are derived based on your current view; thus you can only paint one side at a time.
  • Lines cannot self intersect.
  • Can only autofill quads and tris, if an area has more than four surrounding vertices then a face will not be created.
  • New mesh from paint and previous mesh from paint must be stitched together manually.
  • Each stroke is sequential.
  • Paint strokes can only be viewed and edited in the 3D view you begun retopo paint in.


Retopo Mesh Overview

Circle before projection.
Circle projected onto carved sphere.

If you use Retopo without Paint you can use standard mesh editing tools to either create a new mesh, or project an existing mesh onto the surface you wish to remake the topology of.

To project an existing mesh select the vertices you wish to project in the view you plan to project from, and then press Retopo All.

You may wish to hide (H) the vertices you have already projected in order to avoid accidentally re-projecting them from a different view.

Limitations

  • In order to avoid effecting mesh on the backside of the object you must hide (H) these vertices (i.e. selection is not “clipped” even if it is not visible). Or, if you are using Solid or Shaded draw type, activate the “cube” button just right to the “select mode” buttons – This will prevent selection of elements occluded by the geometry…
  • To re-topologize part of your current mesh you must either duplicate the entire object in Object mode or duplicate and separate part of the object in Edit mode.


Tutorial

Base Shape.

Since retopo modifies topology, not shape, we must first create the shape we will be using. This shape can be almost any 3D object: a mesh, a NURBS surface, metaballs, or even bones. One useful workflow is to quickly block out a shape in Sculpt mode, then use retopo on that mesh.

For this example, I’ve chosen a simple UVsphere.

Step One

Step One.

In Object mode, add a new mesh. It doesn’t matter what you choose. After you create it, press X in Edit mode. From the Erase pop-up menu that appears, choose Vertices to erase all vertices (you don’t need those vertices, because you’re about to make your own vertices in the next steps…).

Note: if the popup instead says “Erase selected object(s)”, you’re not in Edit mode. You want to delete the unnecessary vertices (“obdata”), but not the object itself!

Step Two

Step Two.

Go in Edit mode (⇆ Tab) and turn on Retopo (Mesh panel, Editing context – F9). You should also check that you have viewport shading (aka draw mode) set to Solid and that “occlude background geometry” is off (that’s the cube icon next to the vertex/edge/face selection buttons, in the 3D view header).

Step Three

Step Three.

Start adding points by clicking CtrlLMB Template-LMB.png. This is a normal Edit mode operation, except that if you now rotate your view, you can see that the vertices you just added are on the surface of the UVsphere. You can also use extrude, duplicate, grab, rotate, and scale – all of these operations will continue to snap vertices to the surface of the object behind them.

Blender3D FreeTip.gif
Tips
To make using Retopo easier, make sure you’re taking advantage of Blender’s theme settings. You can use them to increase the size of vertices or to give better contrast to vertices and edges. Note you can’t modify default theme – you should add and edit a new copy, see this page.

You may also find it helpful to turn on the X-ray button in the Object context and sub-context (F7), Draw panel. This way, your new mesh will always be visible, even when behind another object.

And you may find it useful to have multiple 3D viewports open so that you can see whether the vertex placement is as you desired without needing to rotate your working view around the model.


Step Four

Step Four.

Continue adding points around the rest of the model. Make sure to connect vertices with edges so that you can see the topology you are creating.

Step Five

Step Five.

Once you have all the vertices and edges created, you can turn off Retopo and hide the UVsphere. Still in Edit mode, start selecting groups of edges and use F to add faces (you can select your whole mesh and select Auto in the Make Faces menu popped-up by F – all edges-drawn triangles and quads will be automatically filled). When you’re done, you should have a complete surface.

Step Six

Step Six.

Add a Mirror modifier and, optionally, a Subsurf one.