The Select tool is perhaps the most frequently-used tool in Meshmixer. Many tasks can only be accomplished with carefully-constructed selections, so it is worth spending the time to learn the ins and outs of this tool.
The main panel is shown on the left. At the top you can toggle between Brush and Lasso mode - see info about the Surface Lasso mode below. Most of the time, you will use the Brush mode, where you will be "painting" a set of selected faces.
There are three Brush Modes - Unwrap, Sphere, and SphereDisc. Unwrap selects a connected patch of the surface around the brush - ie what you see in the grey highlight. Sphere mode replaces this grey patch with a semitransparent Sphere, and selects everything inside the sphere. The key thing with Sphere mode is that you can select disconnected regions, so it is good if you are selecting in areas where there are lots of disconnected meshes. Finally SphereDisc mode is a sort of hybrid, that uses a Sphere but limits the selection to those faces that are connected (via some path on the 3D surface) to the point under the cursor.
After you paint some faces with a single brush stroke, you can make additional strokes to add to the selection. If you wish to remove from the selection, ie "un-paint" some faces, hold down shift or ctrl/cmd while applying the brush.
Next we have the Size slider, which just changes the size of the Brush/Sphere. Note that you can also change the brush size with the [ and ] hotkeys, or by using the Hotbox.
When the Symmetry box is checked, then the brush stamp will be applied on both sides of the symmetry line (which will become visible as a while line through your model). When you have symmetry checked, you can click on the tools icon on the right to access a mode where you can change the Symmetry plane, see the Sculpting Brush page for details.
This mode is shown on the right. Use the gizmo to translate/rotate the plane, and the Reset button to restore the original position.
The Filters panel includes controls for a few different extra features of the Select Tool. The first is the Expand Mode. This mode expands/contracts the existing selection, using different criteria. You apply this tool by interactively right-click-dragging, with the Ctrl or Command key held.
The default mode is Geodesic Distance, which means the selection tries to expand by a constant distance across the surface. This is useful to adding/removing a buffer to the current selection. The Crease Angle mode grows the selection across edges where the angle between the two triangles is below the crease angle. This is useful mainly for quickly selecting nearly-flat regions. Finally the Vertex Color Similarity mode uses the model's Vertex Colors, and grows the selection to connected regions with a color that is within some threshold from the selected colors.
The Allow Back Faces checkbox allows the brush to be applied when over the back-side of surfaces (the areas with the stripe pattern). Normally we do not allow this as it makes it difficult to select around holes - if you accidentally slip off the edge of the hole, you can end up selecting on the other side of the model.
Finally, the Crease Angle Thresh slider, when non-zero, specifies the maximum crease angle that the selection will grow over. So, when it is small the brush can select over all but the sharpest creases, while when it is large, the brush will only add faces that are nearly-coplanar with the face under the cursor. Note that the face under the cursor is always added, regardless of this setting.
Inside the Select Tool, double-clicking with the left mouse button invokes various shortcut actions that are both context-sensitive and modal. If you are in Facegroup Color mode, then double-clicking will append the entire facegroup that is under the cursor to the selection. Doing this action with the ctrl/cmd modifier key held down will de-select the facegroup.
In Vertex Color mode, this action will do a sort of selection flood-fill - all triangles connected to the triangle under the cursor will be selected, but the flood-fill will stop at triangles that were already selected. So, using this action you can paint a selection boundary and then quickly select the interior.
Boundary Selection Regardless of color mode, if you double-click on a triangle that is adjacent to a boundary (highlighted blue edge loop), the selection will grow to include all triangles with a vertex connected to the boundary loop, as shown on the right. This is an efficient way to select hole boundaries, which is particularly useful when cleaning up scans and "problem" meshes.
Select Intersecting Double-clicking is overloaded with one more interaction, which allows you to use other objects in the scene as selection tools. If you double-click on some other object in the scene, all the faces fully contained within that object will be appended to the current selection. Note that since only fully-contained faces are selected, you won't actually be able to see any change until you hide the object you double-clicked. As with other selection tools, the ctrl/cmd modifier key will invert this operation, removing the contained faces from the selection.
Since the Select tool is so critical, there are a large number of shortcut and modifier keys which are useful. Learning these keys can make a big difference in how quickly you can operate in Meshmixer. Note that some of these controls are available from the Modify submenu that is available once you have made an initial selection.
s - Select Tool. The s hotkey jumps to the Select tool while you are in just about any other Tool in Meshmixer. Note that this hotkey will also work inside the Select tool - it will restart the tool with no active selection, giving you a quick way to start over if your selection didn't work.
ctrl/cmd+a - Select All. Note that this combo works outside of the Select tool, as long as you have a single mesh object selected.
Esc - Exit Tool. Esc exits the Select tool (and many other tools in Meshmixer)
q - Cycle Brush. Repeatedly hitting this key cycles through the different Brush Modes.
> and < - Expand/Contract Ring. This combo grows/shrinks the selection to include/exclude the next "ring" of triangles connected to the selection boundary
e - Expand to Connected. Grows the selection to include any triangles connected to the current selection (ie all partially-selected connected components are fully selected)
g - Expand to Groups. Grows the selection to fully select any partially-selected face groups.
i - Invert. Replaces the current selection with a selection of all currently-unselected faces
Shift+i - Invert Connected. Same as Invert, but limits the change to partially-selected connected components.
ctrl/cmd + g - Create Facegroup. This hotkey assigns a new facegroup to the selected triangles.
shift + ctrl/cmd + g - Clear Facegroup. This hotkey clears the facegroup assigned to the selected triangles, resetting them to the default facegroup (ie the standard gray color, if you are facegroup-color mode).
o - Optimize Boundary. This very useful action tries to "clean up" the boundary of the selection. In some cases "ear" triangles (ie selected triangles with two un-selected neighbours) and "fins" (un-selected triangles with two selected neighbours) will be problematic for the tool you wish to apply. Optimize Boundary will automatically select/de-select fins and ears, and it will repeat until no more ears or fins are found. This will improve your results in many cases, as ears and fins often introduce artifacts and even prevent tools from running. For example if you are trying to fill a complex hole are getting the dreaded pink band of failure, try hitting 'o' and running it again.
Surface Lasso Mode
Surface Lasso Mode was one of the first tools in Meshmixer, going back to version 1.0. It is an alternative to Brush selection mode that will allow you to select surface regions without having to do a large amount of painting.
You enable Surface Lasso mode using the Brush/Lasso toggle at the top of the properties panel. In Lasso mode, you make a selection by drawing one or more curves on the surface of the object. To draw multiple curves, you must chain them end-to-start (ie each curve needs to begin very close to the end-point of the previous curve). You can change the viewpoint between curve segments. To complete the selection you simply close the loop, ending the final segment near the starting point of the first segment.
Once you close the loop, the faces "inside" the loop will be selected, as shown in the image above. Note that "inside" is defined as the side with fewer triangles, so if you get the "wrong" side, try the Invert (i) or Invert Connected (shift-i) hotkeys to switch sides. Once the selection is made, you will enter Brush mode and can paint to edit the selection. It is not possible to make multiple surface-Lasso selections.
Note that the Surface Lasso may fail in some cases, particularly on meshes with large or very skinny triangles. It will help to draw slowly.
Screen-Space Lasso Mode
In addition to the Surface Lasso, you can draw traditional 2D lassos. You begin a 2D lasso by pressing the left-mouse down somewhere on the background (ie not on top of the selected model). This will initialize the lasso.
There are two modes to the lasso. If you click-and-hold, then you will create a free-form lasso that follows the cursor, which will complete when you release the mouse. If you click-and-release, you will begin a polygonal lasso, and each additional click will add vertices to the lasso, until you complete it by clicking again on the starting point.
In the polygonal lasso, the resulting selection will be all triangles whose vertices are fully within the lasso. However in the freeform lasso, the curve does not have to be closed, as shown on the right. In this case you can think of the curve as "cutting" the model into two sides, and the selection is determined as the side which has the smaller number of faces. This may sometimes select the "wrong" side, which you can quickly correct with the Invert (i) or Invert Connected (shift-i) hotkeys.
By default, each time you draw a screen-space lasso, it will replace the existing selection. Holding the shift modifier while drawing the lasso will append to the current selection, while holding ctrl/cmd will de-select.
One selection action that is only available from the Modify menu is the Select Visible command. This command will select all triangles visible in the current viewpoint. This is particularly useful for quickly selecting the "outside" shell of a complex 3D scan, which may have interior cavities that are connected to the exterior.
To be able to see the Modify menu you must make an initial selection. Then Select Visible will add all visible faces. As with the screen-space lasso, each time you run this command it will replace the current selection, however by holding the shift modifier you can append, or hold ctrl/cmd modifier will subtract.
The Meshmixer 101 video series contains a short tutorial that covers basic operation of the Select tool: