dplib.js

// Calc DPI with JS
function dpLib(debug) {
    "use strict";
    var div = document.createElement("div");
        div.style.width    = "1in";
        div.style.position = "abolute";
        div.style.top      = "-100%";
        div.style.bottom   = "-100%";
    document.body.appendChild(div);
    //
    this.version   = "1.0";
    this.cmInInch  = 2.54; // 1 in = 2.54 cm
    this.pixInInch = div.offsetWidth;
    this.pixInCm   = this.pixInInch / this.cmInInch;
    //
    document.body.removeChild(div);
    if (debug) {
        console.info("Version of dpLib: %s", this.version);
        console.info("inch = %s pixels \n centimetr = %s pixels", this.pixInInch, this.pixInCm);    
    }
}
// inches
dpLib.prototype.i = function (i) {
    if (!i) i = 1;
    return this.pixInInch * i;
}
// centimetres
dpLib.prototype.cm = function (cm) {
    if (!cm) cm = 1;
    return this.pixInCm * cm;
}
// convert centimetres to inches
dpLib.prototype.cm2i = function (cm) {
    if (!cm) cm = 1;
    return cm / this.cmInInch;
}
// convert inches to centimetres
dpLib.prototype.i2cm = function (i) {
    if (!i) i = 1;
    return i * this.cmInInch;
}