Constant Cross Section

In the Options area of the Skin Parameters rollout is a new checkbox, labeled Constant Cross-Section. When this is checked (the default), the cross-sections are scaled at angles in the path to maintain uniform path width. When this is not checked, the cross sections maintain their original local dimensions, which causes pinching at path angles. The unchecked method is the way cross-sections were handled originally. Note that acute (small) angles of the path can cause problems when this item is checked.

How To

To use Constant Cross-Section

1. Enlarge the Front viewport to full screen, and the use the Rectangle tool with CTRL held down to create a square approximately 20 x 20 units.
2. Create another rectangle beside it that’s 200 x 100 units.
3. Apply a Skew modifier to the large rectangle, but don’t alter the Skew parameters.
4. Create a loft object in which the larger rectangle is the path and the square is the shape.
5. In the Modify panel, open the Skin Parameters rollout, and check Skin in the Display area.

You can now see the wireframe structure of the lofted rectangle, whose cross-sectional sides remain parallel right up to its corners.

Note: Make sure the color assigned the loft object is easily visible. Change it, if necessary.

6. Uncheck Constant Cross-Section, observe the corners

When Constant Cross-Section is unchecked, the corners become pinched.

7. Check Constant Cross-Section again to restore the corners.

Acute angles can cause problems when the cross-sections formed by the path steps intersect at the corners. You can mitigate this either by avoiding acute angles, or by reducing the path steps.

8. Press H (Select by Name), and choose Rectangle 2 (or whichever name was given to the larger rectangle used as the path).
9. In the Skew panel, change the Skew Axis to Y, and then set the Amount spinner to about 95.
10. Use Region Zoom to zoom in on the upper-left corner of the rectangle so you can see the mesh in detail.

At a skew of less than 100, the acute angle still works because the path cross-sections haven’t intersected.

11. Set the Skew Amount to about 300, and examine the same corner.

At this angle, the path cross-sections intersect, causing problems in the mesh.

12. Select the loft object, and set the Path Steps to 1.

The cross-sections no longer intersect, and the corner is clean.

You can see that, when creating straight-edge molding for architectural modeling, and such, you can avoid mangled corners by simply reducing the path steps to 0.