Logic of vertex ordering of box builder

andreasplesch · December 12, 2021, 11:53pm

I am trying to understand the MeshBuilder and its strategies because I want to decide if I may be better off just using vertex data.

First I want to understand the reason why vertices are ordered the way they are for the box builder:

https://playground.babylonjs.com/#ICLXQ8#235

The order is +z, -z, +x, -x, +y, -y as explained in Map Materials to Individual Mesh Faces | Babylon.js Documentation

Is there a reason why z is first ? Perhaps because it is the forward direction.

github.com

BabylonJS/Babylon.js/blob/40f0ba2cc8a7acbd9dbdc81492a305fa781a41bc/src/Meshes/Builders/boxBuilder.ts#L22

    
      
           * * width sets the width (x direction) of the box, overwrites the width set by size, optional, default size

           * * height sets the height (y direction) of the box, overwrites the height set by size, optional, default size

           * * depth sets the depth (z direction) of the box, overwrites the depth set by size, optional, default size

           * * faceUV an array of 6 Vector4 elements used to set different images to each box side

           * * faceColors an array of 6 Color3 elements used to set different colors to each box side

           * * sideOrientation optional and takes the values : Mesh.FRONTSIDE (default), Mesh.BACKSIDE or Mesh.DOUBLESIDE

           * * frontUvs only usable when you create a double-sided mesh, used to choose what parts of the texture image to crop and apply on the front side, optional, default vector4 (0, 0, 1, 1)

           * * backUVs only usable when you create a double-sided mesh, used to choose what parts of the texture image to crop and apply on the back side, optional, default vector4 (0, 0, 1, 1)

          * @returns the VertexData of the box

          */

          export function CreateBoxVertexData(options: { size?: number, width?: number, height?: number, depth?: number, faceUV?: Vector4[], faceColors?: Color4[], sideOrientation?: number, frontUVs?: Vector4, backUVs?: Vector4, wrap?: boolean, topBaseAt?: number, bottomBaseAt?: number }): VertexData {

             var nbFaces = 6;

             var indices = [0, 1, 2, 0, 2, 3, 4, 5, 6, 4, 6, 7, 8, 9, 10, 8, 10, 11, 12, 13, 14, 12, 14, 15, 16, 17, 18, 16, 18, 19, 20, 21, 22, 20, 22, 23];

             var normals = [0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, -1, 0, 0, -1, 0, 0, -1, 0, 0, -1, 1, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, -1, 0, 0, -1, 0, 0, -1, 0, 0, -1, 0, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, -1, 0, 0, -1, 0, 0, -1, 0, 0, -1, 0];

             var uvs = [];

             var positions = [];

             var width = options.width || options.size || 1;

             var height = options.height || options.size || 1;

             var depth = options.depth || options.size || 1;

             var wrap = options.wrap || false;

             var topBaseAt = (options.topBaseAt === void 0) ? 1 : options.topBaseAt;

shows the construction.

The next question is why are sides rotated (with default uvs, and no wrap) with respect to each other ? Is that ordering systematic ? Probably, but what is the principle ?

I think I understand the purpose of the wrap option (reorient faces upright on the sides) but the implementation is hard to follow. Is is possible to explain that procedure ?

Finally, I could not find examples for the top/bottomBaseAt options. I think they are linked to wrap, and may define how wrap reorients sides.

useRightHandedSystem seems to result in mirroring of textures, perhaps because there is no attempt at taking the option into account (which may be the most transparent option).

Deltakosh · December 13, 2021, 2:21pm

Haha! I wrote it this way because it was the clearer in my mind at that time. There is no big strategy unfortunately

I found that representation easy to write and maintain

Deltakosh · December 13, 2021, 2:22pm

I did not write the topAt/bottomAt though. I think @JohnK did if my memory serves

andreasplesch · December 13, 2021, 2:30pm

Thanks. Mostly, I needed to figure out how to adjust faceUVs for RHS and wrap, and could accomplished that more or less by experimenting.

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Logic of vertex ordering of box builder

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