tupali/librerias/firma/app.js

var wrapper = document.getElementById("signature-pad"),
   // clearButton = wrapper.querySelector("[data-action=clear]"),
    
    saveDataButton = wrapper.querySelector("[data-action=save-data]"),
    canvas = wrapper.querySelector("canvas"),
    signaturePad;

// Adjust canvas coordinate space taking into account pixel ratio,
// to make it look crisp on mobile devices.
// This also causes canvas to be cleared.
function resizeCanvas() {
    // When zoomed out to less than 100%, for some very strange reason,
    // some browsers report devicePixelRatio as less than 1
    // and only part of the canvas is cleared then.
    var ratio =  Math.max(window.devicePixelRatio || 1, 1);
    canvas.width = canvas.offsetWidth * ratio;
    canvas.height = canvas.offsetHeight * ratio;
    canvas.getContext("2d").scale(ratio, ratio);
}

window.onresize = resizeCanvas;
resizeCanvas();

signaturePad = new SignaturePad(canvas);

document.getElementById('clear').addEventListener('click', function() {
  signaturePad.clear();
});


saveDataButton.addEventListener("click", function (event) {
    if (signaturePad.isEmpty()) {
        alert("Por favor firme primero.");
    } else {
       saveViaAJAX();
    }
});