Internal Patches -- What to Avoid

©1999 By Jeff Cantin aka TinCan on IRC

This tutorial is intended to demonstrate how internal patches can lead to unwanted render ambiguities, how to spot them, and a method to avoid or remove them. I advise newer AM users to actually open the program and try to follow along step by step, do the test renders when called for.

In this example, I want to make a rib for an airplane wing assembly. It looks like Figure 1.

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Figure 1

I intend to extrude this shape, so I will start by drawing it with the "Add Lock" (Shift + A) spline tool. At this point I just want the shape and I am not caring about how many control points to use in making it. I have "Peaked" (P key) several CPs (Control Points) to square off the inside corners, and adjusted the bias handles to maintain a curve for the outside surfaces, using the "Show Bias Handles" button and selecting the appropriate side of the CP. Note: Once you peak a CP each side can be manipulated separately. I ended up with something like Figure 2.

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Figure 2

A test extrude at this point would only make a "ribbon" and would look like Figure 3. This test extrude is to show that the sides won't render because they have too many CPs to make a valid patch.

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Figure 3

So, delete the extruded portion and let's decide how to divide up the shape with legal 4 point patches. I ended up having to add one CP to make all legal 4 point patches as seen in Figure 4.

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Figure 4

There are two ways that I know to make sure I am making legal 4 point patches, turning on normals (Shift + 1) or change the drawing mode to Shaded/Wireframe. The advantage of normals is that you can adjust them so they all point in the same direction, also be aware that both of these solutions can slow down the redraw speed on a very complex model.

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Figure 4a

Now I am ready to extrude. Select any CP and whack the "/ Key" to select everything connected to the CP, then hit the "Extrude" button (E Key). A test render at this point may show rendering anomalies because we just created internal patches, also, one side of the new rib model bulges so we need to peak some of the CPs. You can do this now or later, I do it later. The internal patches were created by the splines we used to make 4 point patches in the last step we did, and can be easily seen if you still have the normals turned on (Figure 5). I marked two of them, can you see the others? (Hint: the normal markers are a giveaway.)

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Figure 5

Not to worry! We are not done yet! Select the section you just extruded and extrude again in the same direction. (I do this automatically now when I am building a model like this--extrude twice, moving each extrude over with the cursor keys.) Using the "Group" tool of your choice, select only the middle set of splines (Figure 6) and hit the "Hide" button (H Key).

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Figure 6

Now delete all the splines you created to "close" the shape in the earlier step, all the "Normals" arrows should vanish as you delete these splines. We just deleted all the internal patches! Hit the "Hide" button again to unhide the entire model.

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Figure 7

A test render will show the creases caused by internal patches are gone! Also, because we were careful with normals (they are all pointing out), so normal aware elements of AM should work as intended!

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Figure 8

The inset is an example of how an RC model glider wing is built using the rib model and stringers passing through the notches we
made in the rib. Also look at this image for a textured wing model.

Have fun and good splining!

If you learned something from this tutorial, please drop me an E-mail at Jeff Cantin
This is an important step, I track the number of visits to "thank you" notes, I need to know my time and my web space is well spent.

I have other tutorials linked from my hobby homepage. Give those a try too when you are ready.