在three.js的ExtrudeGeometry.UVGenerator.generateSideWallUV中条件语句的目的是什么？

Question

In three.js's ExtrudeGeometry.UVGenerator.generateSideWallUV there is the conditional:

if ( Math.abs( a.y - b.y ) < 0.01 ) {

        return [
            new Vector2( a.x, 1 - a.z ),
            new Vector2( b.x, 1 - b.z ),
            new Vector2( c.x, 1 - c.z ),
            new Vector2( d.x, 1 - d.z )
        ];

    } else {

        return [
            new Vector2( a.y, 1 - a.z ),
            new Vector2( b.y, 1 - b.z ),
            new Vector2( c.y, 1 - c.z ),
            new Vector2( d.y, 1 - d.z )
        ];

    }

This makes weird discontinuities in the uvs, for example (just displaying the uv coordinates at the red and green channels on this shape):

Removing this block (and always using the else block) makes everything run as expected. Can anyone tell me why this block exists in the first place?

Answer 1

The default UV generator for ExtrudeGeometry sets the UVs based on the position of the vertices.

Note that the sidewall of ExtrudeGeometry consist of a series of rectangles chained together.

Where the sidewall is mostly vertical in XYZ-space, then the UVs are determined from the YZ-coordinates of the position; otherwise, the XZ coordinates are used.

This is not an easy problem. The UV generation here is reasonable, but can result in stretching.

You are free to specify your own UV-generator if you have a better algorithm.

three.js r.82