threejs模型UV算法，超級好用

https://stackoverflow.com/questions/20774648/three-js-generate-uv-coordinate

使用模型表面法線來決定使用（xyz）中的哪兩個，對應到UV坐標當中。用起來相當不錯。當然如果能在設計模型的時候進行展UV就更好了。

The other answers here were a great help but didn't quite fit my requirements to apply a repeating pattern texture to all sides of a shape with mostly flat surfaces. The problem is that using only the x and y components as u and v results in weird stretched textures on vertical surfaces.

My solution below uses surface normals to pick which two components (x, y and z) to map to u and v. It's still pretty crude but it works quite well.

function assignUVs(geometry) {

    geometry.faceVertexUvs[0] = [];

    geometry.faces.forEach(function(face) {

        var components = ['x', 'y', 'z'].sort(function(a, b) {
            return Math.abs(face.normal[a]) > Math.abs(face.normal[b]);
        });

        var v1 = geometry.vertices[face.a];
        var v2 = geometry.vertices[face.b];
        var v3 = geometry.vertices[face.c];

        geometry.faceVertexUvs[0].push([
            new THREE.Vector2(v1[components[0]], v1[components[1]]),
            new THREE.Vector2(v2[components[0]], v2[components[1]]),
            new THREE.Vector2(v3[components[0]], v3[components[1]])
        ]);

    });

    geometry.uvsNeedUpdate = true;
}

函數並不會自動適配模型的大小進行貼圖，用在多模型共用一張材質的時候效果更好，因為是基於物體實際三維坐標來做模型uv對應的。對於材質來講，應該設置重復以及縮放比例來更好地表現。

This function doesn't normalise the UVs to the size of the object. This works better when applying the same texture to different sized objects in the same scene. However depending on the size of your world coordinate system, you'll probably need to scale and repeat the texture as well:

texture.repeat.set(0.1, 0.1);
texture.wrapS = texture.wrapT = THREE.MirroredRepeatWrapping;