Calculate the bounding box of STL file with JavaScript

Question

So I am using this npm package: node-stl

And its working great. However the regexp syntax, mathematics and geometrical calculations are somewhat confusing to me. Especially all at the same time.

Basically what I want to achieve is to extend the script to calculate the bounding box of the STL.

Here is the main file that calculates the volume and weight of the STL being parsed/read.

var fs = require('fs');

// Vertex
function Vertex (v1,v2,v3) {
    this.v1 = Number(v1);
    this.v2 = Number(v2);
    this.v3 = Number(v3);
}

// Vertex Holder
function VertexHolder (vertex1,vertex2,vertex3) {
    this.vert1 = vertex1;
    this.vert2 = vertex2;
    this.vert3 = vertex3;
}

// transforming a Node.js Buffer into a V8 array buffer
function _toArrayBuffer (buffer) {
    var 
    ab = new ArrayBuffer(buffer.length),
    view = new Uint8Array(ab);

    for (var i = 0; i < buffer.length; ++i) {
        view[i] = buffer[i];
    }
    return ab;
}

// calculation of the triangle volume
// source: http://stackoverflow.com/questions/6518404/how-do-i-calculate-the-volume-of-an-object-stored-in-stl-files
function _triangleVolume (vertexHolder) {
    var 
    v321 = Number(vertexHolder.vert3.v1 * vertexHolder.vert2.v2 * vertexHolder.vert1.v3),
    v231 = Number(vertexHolder.vert2.v1 * vertexHolder.vert3.v2 * vertexHolder.vert1.v3),
    v312 = Number(vertexHolder.vert3.v1 * vertexHolder.vert1.v2 * vertexHolder.vert2.v3),
    v132 = Number(vertexHolder.vert1.v1 * vertexHolder.vert3.v2 * vertexHolder.vert2.v3),
    v213 = Number(vertexHolder.vert2.v1 * vertexHolder.vert1.v2 * vertexHolder.vert3.v3),
    v123 = Number(vertexHolder.vert1.v1 * vertexHolder.vert2.v2 * vertexHolder.vert3.v3);
    return Number(1.0/6.0)*(-v321 + v231 + v312 - v132 - v213 + v123);
}

// parsing an STL ASCII string
function _parseSTLString (stl) {
    var totalVol = 0;
    // yes, this is the regular expression, matching the vertexes
    // it was kind of tricky but it is fast and does the job
    var vertexes = stl.match(/facet\s+normal\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+outer\s+loop\s+vertex\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+vertex\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+vertex\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+endloop\s+endfacet/g);

    vertexes.forEach(function (vert) {
        var preVertexHolder = new VertexHolder();
        vert.match(/vertex\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s+([-+]?\b(?:[0-9]*\.)?[0-9]+(?:[eE][-+]?[0-9]+)?\b)\s/g).forEach(function (vertex, i) {
            var tempVertex  = vertex.replace('vertex', '').match(/[-+]?[0-9]*\.?[0-9]+/g);
            var preVertex   = new Vertex(tempVertex[0],tempVertex[1],tempVertex[2]);
            preVertexHolder['vert'+(i+1)] = preVertex;
        });
        var partVolume = _triangleVolume(preVertexHolder);
        totalVol += Number(partVolume);
    })

    var volumeTotal = Math.abs(totalVol)/1000;
    return {
        volume: volumeTotal,        // cubic cm
        weight: volumeTotal * 1.04  // gm
    }
}

// parsing an STL Binary File
// (borrowed some code from here: https://github.com/mrdoob/three.js/blob/master/examples/js/loaders/STLLoader.js)
function _parseSTLBinary (buf) {
    buf = _toArrayBuffer(buf);

    var 
    headerLength    = 80,
    dataOffset      = 84,
    faceLength      = 12*4 + 2,
    le = true; // is little-endian

    var 
    dvTriangleCount = new DataView(buf, headerLength, 4),
    numTriangles    = dvTriangleCount.getUint32(0, le),
    totalVol        = 0;

    for (var i = 0; i < numTriangles; i++) {
        var 
        dv          = new DataView(buf, dataOffset + i*faceLength, faceLength),
        normal      = new Vertex(dv.getFloat32(0, le), dv.getFloat32(4, le), dv.getFloat32(8, le)),
        vertHolder  = new VertexHolder();
        for(var v = 3; v < 12; v+=3) {
            var vert = new Vertex(dv.getFloat32(v*4, le), dv.getFloat32((v+1)*4, le), dv.getFloat32( (v+2)*4, le ) );
            vertHolder['vert'+(v/3)] = vert;
        }
        totalVol += _triangleVolume(vertHolder);
    }

    var volumeTotal = Math.abs(totalVol)/1000;
    return {
        volume: volumeTotal,        // cubic cm
        weight: volumeTotal * 1.04  // gm
    }
}

// NodeStl
// =======
// > var stl = NodeStl(__dirname + '/myCool.stl');
// > console.log(stl.volume + 'cm^3');
// > console.log(stl.weight + 'gm');
function NodeStl (stlPath) {
    var 
    buf = fs.readFileSync(stlPath),
    isAscii = true;

    for (var i=0, len=buf.length; i<len; i++) {
        if (buf[i] > 127) { isAscii=false; break; }
    }

    if (isAscii)
        return _parseSTLString(buf.toString());
    else
        return _parseSTLBinary(buf);
}

module.exports = NodeStl;

If anyone could help me with this it would be great. I know and it feels like it simple. That I just need to know max/min of the different directions(x,y,z) and could then calculate the bounding box.

But I do not understand what the max/min for x,y and z is here. Please answer if you have an idea.

Answer 1

I've made a new branch https://github.com/johannesboyne/node-stl/tree/boundingbox could you please verify whether the applied algorithm works?

Best, Johannes

Edit: If the branch is stable -> works I'll push it into v.0.1.0 (don't know why it is still 0.0.1)