Why extrudeGeometry are relate on binormal of spline?

I found code in ‘/three.js/src/geometries/ExtrudeGeometry.js:464’

normal.copy( splineTube.normals[ 0 ] ).multiplyScalar( vert.x );
binormal.copy( splineTube.binormals[ 0 ] ).multiplyScalar( vert.y );

It means that coordinate in extrud are almost only related the direction of normal and binormal. But not the world coordinate.

In some specical cases, such as pull along Axis X or Z.
When
Value of x or z changed less than y, binormal towards to horizontal.
Value of x or z changed more than y, binormal towards to Vertical.

I think only draw an extrude with a horizontal shape, should we deal it’s coordinate carefully.
In other time, coordinate of shape should not be relate on normal or binormal of spline.

Author: Fantashit

1 thought on “Why extrudeGeometry are relate on binormal of spline?

  1. Thx a lot. I’v read some papers about it. Now I know there’s no contact between extrude’s Y direction and world coordinate. Coordinate of a shape (or frames) to an extrude is only related on extrude path’s derivative.

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