Extrusion is our primary means of adding additional geometry to a mesh in Maya.
The extrude tool can be used on either faces or edges and can be accessed at Mesh → Extrude, or by pressing the extrude icon in the polygon shelf at the top of the viewport (highlighted in red in the image above).
Take a look at the image we've attached for an idea of what a very basic extrusion looks like.
On the left, we started with a plain old default cube primitive.
Switch into face mode, select the upper face, and then press the extrude button in the polygon shelf.
A manipulator will appear, which looks like an amalgamation of the translate, scale, and rotate tools. In a sense, it is—after performing an extrusion, it is essential that you either move, scale, or rotate the new face so that you don't end up with overlapping geometry (more on this later).
For this example, we simply used the blue arrow to translate the new faces a few units in the positive Y direction.
Notice that there's no global scale manipulator at the center of the tool. This is because the translate tool is active by default.
If you'd like to scale the new face simultaneously on all axes, simply click one of the cube-shaped scale handles and a global scale option will appear at the center of the tool.
Similarly, to activate the rotate tool, simply click the blue circle surrounding the rest of the tool and the rest of the rotation options will appear.
Keep Faces Together
The extrude tool also has an option that allows for a totally different set of results called Keep Faces Together. When keep faces together is enabled (it is by default) all selected faces are extruded as a single continuous block, as we've seen in previous examples.
However, when the option is turned off, each face becomes its own separate extrusion that can be scaled, rotated, or translated in its own local space.
To turn the option off, go to the Mesh menu and uncheck Keep Faces Together.
Making extrusions with the option unchecked is extremely useful for creating repetitive patterns (tiles, panels, windows, etc.).
Look at the image above for a comparison between the two types of extrusion.
Both objects began as a 5 x 5 polygon plane. The model on the left was created by selecting all 25 faces and performing a very simple extrusion with Keep Faces Together turned on—for the object on the right the option was turned off.
In each example, the extrusion process was virtually identical (Extrude → Scale → Translate), but the result is completely different.
Performing edge extrusions with keep faces together turned off can produce some very, very messy results. Until you become more comfortable with the tool, make sure keep faces together is turned on if you're doing edge extrusions!
Extrusion is incredibly powerful, in fact, we wouldn't hesitate to call it the bread and butter of a proper modeling work-flow. However, when used carelessly the tool can inadvertently produce a relatively serious topology issue called non-manifold geometry.
The most common cause of non-manifold geometry is when a modeler accidentally extrudes twice without moving or scaling the first extrusion. The resulting topology will essentially be a set of infinitely thin faces that sit directly on top of the geometry they were extruded from.
The biggest issue with non-manifold geometry is that it's virtually invisible on an un-subdivided polygon mesh, but can completely destroy the model's ability to be smoothed properly.
To Troubleshoot Non-Manifold Geometry:
Knowing how to spot non-manifold faces is really half the battle.
In the image above, the non-manifold geometry is clearly visible from face selection mode, and looks like a face sitting directly on top of an edge.
In order to spot non-manifold geometry this way, it's necessary to set Maya's face selection preferences to center rather than whole face. To do so, go to Windows → Settings/Preferences → Settings → Selection → Select Faces With: and choose Center.
We've previously discussed Non-Manifold Geometry in a separate article, where we cover some of the best ways to rid yourself of the problem. In the case of non-manifold faces, the quicker you can spot the problem the easier it'll be to fix.
One final concept before we move on to the next lesson.
Faces in Maya are not inherently two-sided: they're either facing out, toward the environment, or they're facing in, toward the center of the model.
If you're wondering why we're bringing this up in an article that's otherwise focused on the extrude tool, it's because extrusion can occasionally cause a face's surface normals to be unexpectedly reversed.
Normals in Maya are invisible unless you explicitly change your display settings to reveal them. The easiest way to see which way a model's normals are facing is to go to the Lighting menu at the top of the workspace and uncheck Two Sided Lighting.
With Two Sided Lighting turned off, reversed normals will appear black, as shown in the image above.
Surface normals should generally be oriented outward, toward the camera & environment, however, there are situations when reversing them makes sense, modeling an interior scene for example.
To reverse the direction of a model's surface normals, select the object (or individual faces) it and go to Normals → Reverse.
We like to work with Two-Sided Lighting turned off so that we can identify and fix surface normal issues as they crop up. Models with mixed normals (like the one on the right side of the image) typically cause problems with smoothing and lighting later in the pipeline, and should generally be avoided.
That's all for extrusion (for now). In the next lesson, we'll cover some of Maya's topology tools.