MODELLING CONCEPTS
Extrusion.
Extrusion is one of the most common tasks in modelling but in subdivision surface modelling is is generally accepted that we should not extrude polygons or groups of polygons unless they have been protected with a boundary.
This normally means insetting or outsetting an area before extrusion.
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When making the decision to extrude, you have made the decision to create a new local geometry. You are not attempting to create new curvature within an area, you are creating a brand new logical shape attached to the mesh. This means that this new shape should be connected to the original shape with a boundary; often using inset or outset..
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If this is not done then 5-spoked poles (E-poles) will be created in several areas of maximum distortion and controlling the curvature into these new shapes will interfere with everything else on your mesh.
The four red control loops in the example below will spread out across the entire mesh affecting everything in their way. They all need to be controlled individually to change something which should be a single aspect of curvature.
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Extruding without creating a boundary region
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The necessary boundaries are (or certainly can be!) significantly more complicated that you may expect so a few rules and techniques are worth a bit of a description here.
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Extruding a section on a flat plane is trivial. Simply inset or outset the required polygons before extrusion. A control loop can then be placed within the new border to control the curvature going up into the extrusion.
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Simple extrusions within a local geometry
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Extruding on an edge requires that additional polygons are selected around the edge of the required area during the insetting stage before the extrusion is made. the polygons on the opposite side and some on the floor running into the new shape should then be inset post extrusion!
This can become complicated! termination of faces can be difficult in this situation. I have not made one yet but this would definitely benefit from a video tutorial. The loops here look complicated but the simple set of rules and key-presses to achieve them are really not!
I will create a video tutorial in due course to more easily explain the process of extruding on edges.
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Extruding around the top or bottom of a hollow cross section must be performed before insetting (a better solution is to extrude the top ring of faces along their normals) and it is a commonly made mistake not to fix these extrusions once they have been created.
Once the extrusion is made, the entire ring of faces across the top of the hollow cross section is then selected, the selection inverted so that every other polygon is selected and then this is inset which pushes the E-poles to a safe area and correctly terminates the polygons at the top of the mesh. This will often be a late stage fix once the detail above the extrusions is finalised.
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A Fully rigged Edge extrusion
Hollow Cross Section Extrusion
A Much better solution is to build the cross section using arrays and modifiers and a good example of this workflow can be seen in the tutorial to create The Rook for the chess set.
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Correctly dealing with E-Poles is one of the most difficult aspects of subdivision surface modelling and extrusion is the worst offender when placing them in undesirable areas.
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