quad-indices/index.js

creates the indices for a quad (two triangles)

var dtype = require('dtype')
var anArray = require('an-array')
var isBuffer = require('is-buffer')

var CW = [0, 2, 3]
var CCW = [2, 1, 3]

module.exports = function createQuadElements(array, opt) {
    //if user didn't specify an output array
    if (!array || !(anArray(array) || isBuffer(array))) {
        opt = array || {}
        array = null
    }

    if (typeof opt === 'number') //backwards-compatible
        opt = { count: opt }
    else
        opt = opt || {}

    var type = typeof opt.type === 'string' ? opt.type : 'uint16'
    var count = typeof opt.count === 'number' ? opt.count : 1
    var start = (opt.start || 0) 

    var dir = opt.clockwise !== false ? CW : CCW,
        a = dir[0], 
        b = dir[1],
        c = dir[2]

    var numIndices = count * 6

    var indices = array || new (dtype(type))(numIndices)
    for (var i = 0, j = 0; i < numIndices; i += 6, j += 4) {
        var x = i + start
        indices[x + 0] = j + 0
        indices[x + 1] = j + 1
        indices[x + 2] = j + 2
        indices[x + 3] = j + a
        indices[x + 4] = j + b
        indices[x + 5] = j + c
    }
    return indices
}