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Topology Pen

When creating high-resolution assets with functions such as multi-resolution sculpting, you may create geometry that would be difficult to rig and animate. You could have geometry with too many vertices or with little attention paid to surface topology. These and similar things can lead to odd, unrealistic deformations. The solution for getting a free-form, high-resolution model into a format where it can be easily animated is to retopologize the model. To re-structure geometry, you take a high-resolution mesh and use that as a base to generate a much lower resolution mesh. The lower resolution mesh can have more thoughtfully placed geometry that is easier to weight and rig. From there, you can bake a displacement map to create in a final asset that is visually the same as the free-form geometry, but a lot lighter to provide better deformations and smoother interactivity when animating.

To facilitate retopologizing models, you use the Topology Pen tool to create that lower-resolution mesh. This combined with the Pen tool and Sketch tool set to a Background Point Constraint should be all that you need to completely retopologize any high-resolution geometry. Modo also has a Topo interface tab that is dedicated to the functions of retopologizing models. This tab provides all the related tools and settings in a single convenient viewport.

NOTE:  Previously the Topology Pen tool was known as the Drag Weld tool. When features were added to increase its usability, it was renamed, but the functionality remains.

Using the Tool

Retopology works with a high resolution mesh as a background element. (It is visible in the Item List, but it is unselected.) Next, make sure Modo is in Polygons component mode (not Items mode). Create a new empty mesh layer by pressing N. Select the new mesh layer in the Item List. (Rename it if you wish.) Place some initial geometry with either the Pen Tool (possibly using Quads mode to lay down strips of quadrangle polygons) or the Topology Sketch Tool (which also lays down a series of quadrangle polygons along a drawn curve that conforms to the background surface). Once you map out the basic contours, you can activate the Topology Pen tool and begin to extend edges and adjust the geometry into the final surface. The Topology Pen tool, depending on settings, offers a broad range of functions. Some functions even duplicate those of other tools to provide a more convenient and fluid workflow.

By default, the Topology Pen tool also activates a Background Constraint type of Point to ensure that all created vertices lie directly on the surface of the background mesh. To use, click the Topology Pen icon in the Topo interface tab's toolbox. In its Properties panel select the proper mode and options, and then manipulate the geometry in the 3D Viewport. You can see the geometric element to edit by the pre-highlighting (drawing a ghosted color over the element as the you move the pointer). Dragging applies the defined action, represented by a small yellow box. When you move the box, Modo displays the distance as the Offset X/Y/Z values.

Modo provides some shortcuts to use when the tool is in Move mode.

Click = Move

Right-click = Move Edge Loop

Middle-click = Split

Shift+click = Duplicate

Shift+right-click = Duplicate Loop

Shift+middle-click = Add Loop

Ctrl+click = Slide

Ctrl+middle-click = Remove

Smart Snapping

The Topology Pen tool has a custom smart-snapping feature that automatically snaps edges together to weld them into a single vertex. Modo determines the snapping based on the proximity of vertices to each other as you drag an element. Modo indicates snapped edges by a purple color. The welding is not permanent until you release the mouse button; therefore, you can continue to drag and adjust the position of the element (and modifying the snapping) should the wrong vertex snap. You can snap individual vertices together by dragging one vertex onto another.

Drag Weld Panel

Topology Pen—

Offset X/Y/Z: Specifies the distance the point of reference (a small yellow box) travels as you drag it in the 3D Viewport. Once you set the location by clicking the mouse, you may also manually adjust these values directly to adjust the target component.

Mode: Provides several modes that directly relate to the process of retopologizing geometry. quad
Move- Moves an element as you drag it, but it remains fixed against the background surface as it slides around. This is useful in editing vertices, edges, and polygons.
Duplicate- Duplicates an edge as you drag it. With Edge Loop enabled or by dragging with the right mouse button, you can duplicate an entire edge loop simultaneously. This mode also duplicates individual vertices to make triangle polygons.
Remove- Removes the element completely from the geometry. This mode does not modify individual outer edges, but removes interior edges, vertices, and entire polygons. Using the right mouse button removes an entire loop.
Split- Creates an edge, which can then be dragged over and snapped to an adjoining vertex, when dragging a vertex. This splits the geometry with a new edge.
Add Loop- Create a new edge across the entire span or loop of geometry until another edge or a triangle is encountered. Dragging interactively adjusts the position of the loop — unless you have Split at the Middle enabled.
Point- Draws a one point polygon when you click the mouse and the pointer is in an empty area. If you click any geometry component in this mode, then the tool works as in the Move mode.
Fill- Looks for nearby edges and points (vertices) and tries to automatically generate a quad or triangle polygon from them. You can use this mode to fill large areas with a grid of polygons after using the Point mode used to place the vertices. After placing vertices, switch to Fill mode and click in the center of where each quad (or triangle) polygon should appear to create the geometry. Modo pre-highlights the polygon to indicate what geometry it creates when you click.

Edge Loop: When enabled, modifies an entire continuous loop of geometry until an opposing edge or triangle is encountered (instead of a single geometric element) when making any adjustments.

Edge Slide: When enabled, restricts the moving of edges and vertices to the positions of adjoining edges. This slides edges and vertices along the edges of neighboring geometry. Pressing Ctrl while in Move mode provides the same functionality.

Backface: When enabled, snaps geometric components against the back side of a surface. Normally, when disabled, Modo ignores backward-facing polygons for snapping.

Innersnap: When enabled, snaps any vertex to any other vertex regardless of its location (in the same layer). Normally, when disabled, Modo does not snap interior vertices for a surface (those with no open edges) to other interior faces because snapping only occurs between perimeter vertices.

Slip UVs: When enabled, edits applied to the geometry do not change the existing UV map. UV values are generally fixed to specific vertices; therefore, further edits to the geometry may warp, deform, or otherwise distort the UV values in undesirable ways. When this happens, you may need to adjust the map or to redo it altogether. To avoid this undesirable result, you enable Slip UVs so as to not disturb any existing UV mapping applied to the geometry.

Slip UVs A

Slip UVs function disabled (notice the texture warping)


Slip UVsB

Slip UVs function enabled (texture remains even)

Raycast Inactive Surfaces: When enabled, hides certain foreground geometry components behind the background geometry where you cannot select them. This is useful when retopologizing layered elements. For example, when working from a side view with a character that has fingers, several fingers could overlap in the viewport to make it difficult for the Topology Pen tool to discern which edge to indicate for duplicating. With this option enabled, Modo ignores the foreground retopology geometry that is behind the nearest surface and occluded by the background object (the other fingers). In this case, only the retopology surface that is the closest to the viewer is active. This does slow the tool's interactivity, which is why this option is disabled by default.

Add Loop—

Position: When enabled, determines the position along an edge where Modo adds the new line of polygon edges. Modo computes the position as a percentage along each edge encountered. After adding the loop by clicking in the 3D Viewport, you can directly modify this value by adjusting the resulting position.

Split at the Middle: When enabled, creates new edge loops fixed at the 50% (center) position.


The Fill options are only available when the Topo Pen tool is in the Fill mode.

Quad Only: When enabled, tries to force only four-sided (quad) polygons.

Range: Determines the search area around the pointer when looking for vertices to convert to polygons.