Functions

The Fast Mirror operator lets you select an area of the mesh to symmetrize using a mirror modifier.

This operator is what is called a "modal" operation in Blender. Basically, this enters a mode where you have to input designated sets of hotkeys to activate certain functions. While in this mode, Blender's own hotkeys are disabled and only the modal operation hotkeys are accepted.

Depending on what modal operator you are using, usually there is a hotkey to execute or finalize the operation and some can also have multiple stages before finishing.

The Auto Mirror operator consists of two modal stages where you have to select an area you want to mirror in the first stage and select which axis to mirror that area in the second stage.

The gif files below shows Auto Mirror operator being used from Random Flow but this is basically the same as the Fast Mirror operator.

Also, UI and contextual help in modal operators is pretty common so you have all you need to operate them in the 3D viewport.

Figure 1

Figure 1 shows the first stage of the modal operation where you have to select which area of the active object you want mirrored using the Left Mouse Button (LMB) hotkey.

The mouse cursor also changes appearance and becomes an eyedropper icon.

You can also cancel the entire operation anytime using the Escape or Right Mouse Button (RMB) hotkey.

When you pick an area anywhere on the active object's surface by clicking on it using the LMB hotkey, the second stage of the modal operation will start.

Figure 2

In the second stage shown in Figure 2, you can set which axis you want the area you have selected mirrored.

The x, y and z axes are activated using the X, Y and Z hotkeys respectively. You can use any of them at the same time.

You can reset or toggle off the mirror axes using the R hotkey.

Again, the entire operation can be cancelled using the Escape or Right Mouse Button (RMB) hotkey.

After deciding which axis or axes you want the active mesh mirrored, you can use the Space or Enter hotkey to finalize the entire operation.

This will bisect the active mesh leaving only the area you have set to mirror from the first modal stage and adds a mirror modifier to symmetrize that area into the different axis you have enabled in the second modal stage.

Figure 3

When using the x, y and z axis in the second modal stage, it changes color to show that it has been activated. The colors are Red for X, Green for Y and Blue for Z or RGB = XYZ.

Figure 4

Additional UI elements can be seen in the second modal stage too, like the x representing the area you have chosen to mirror from the first model stage.

There is also a striped black line that leads from the x to the origin point of the active object where the 3D axis widget of the modal operator is located.

Figure 5

When trying to symmetrize an object that is already mirrored shown in Figure 5, you will see that the axes in the 3D widget are already colored based on where the mesh is currently mirrored.

Also in Figure 5, the mirror modifier will be applied by using the R hotkey for reset which made the 3D widget white by disabling all the mirror axes.

Alternatively, each axis you disable in a mirrored mesh will have that side of the geometry applied.

Figure 6

The term "real geometry side" is the area you decidied to mirror in the first modal stage with the rest of the faces that make the model whole being provided by the mirror modifier.

Any changes you make to the real geometry side is mirrored to the geometry supplied by the mirror modifier.

You can also relocate the real geometry side anytime by using the operator again then clicking a new area and just leaving the enabled mirror axes alone.

Figure 7

This operator can also be used in edit mode.

You can also mirror multiple objects selected as long as you click on the active mesh from the selection for the first modal stage.

Figure 8

Clicking on other objects will show the "Mirror axis not found!" text at the bottom of the 3D viewport.

And that's it for the basic inner workings of the Auto Mirror modal operator.

Figure 9

Again, in the first modal stage, pick an area you want mirrored by clicking on the active mesh' surface using the LMB hotkey then in the second modal stage, activate the axis you want this area mirrored using the X, Y and Z hotkeys and finally using Space or Enter hotkey to execute or finish the entire modal operation.