Desenhar na área de desenho HTML5 com um rato

1. anexar a mousedown, mousemove, e mouseup ouvinte de eventos para a tela DOM
2. em mousedown, obtenha as coordenadas do rato e use o método moveTo() para posicionar o seu cursor de desenho e o método beginPath() para iniciar um novo caminho de desenho.
3. em mousemove, adicionar continuamente um novo ponto ao caminho com lineTo(), e Colorir O último segmento com stroke().
4. em mouseup, defina uma bandeira para desactivar o desenho.

Http://dev.opera.com/articles/view/html5-canvas-painting/

Vê isto http://jsfiddle.net/ArtBIT/kneDX/ isto deve direccioná-lo para a direcção certa

Eu também estava a tentar usar este método para assinaturas, encontrei uma amostra em http://codetheory.in/.

Adicionei o código abaixo a jsfiddle

Html:

<div id="sketch">
    <canvas id="paint"></canvas>
</div>

Javascript:

 (function() {
    var canvas = document.querySelector('#paint');
    var ctx = canvas.getContext('2d');

    var sketch = document.querySelector('#sketch');
    var sketch_style = getComputedStyle(sketch);
    canvas.width = parseInt(sketch_style.getPropertyValue('width'));
    canvas.height = parseInt(sketch_style.getPropertyValue('height'));

    var mouse = {x: 0, y: 0};
    var last_mouse = {x: 0, y: 0};

    /* Mouse Capturing Work */
    canvas.addEventListener('mousemove', function(e) {
        last_mouse.x = mouse.x;
        last_mouse.y = mouse.y;

        mouse.x = e.pageX - this.offsetLeft;
        mouse.y = e.pageY - this.offsetTop;
    }, false);


    /* Drawing on Paint App */
    ctx.lineWidth = 5;
    ctx.lineJoin = 'round';
    ctx.lineCap = 'round';
    ctx.strokeStyle = 'blue';

    canvas.addEventListener('mousedown', function(e) {
        canvas.addEventListener('mousemove', onPaint, false);
    }, false);

    canvas.addEventListener('mouseup', function() {
        canvas.removeEventListener('mousemove', onPaint, false);
    }, false);

    var onPaint = function() {
        ctx.beginPath();
        ctx.moveTo(last_mouse.x, last_mouse.y);
        ctx.lineTo(mouse.x, mouse.y);
        ctx.closePath();
        ctx.stroke();
    };

}());

Https://jsfiddle.net/richardcwc/d2gxjdva/

//Canvas
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
//Variables
var canvasx = $(canvas).offset().left;
var canvasy = $(canvas).offset().top;
var last_mousex = last_mousey = 0;
var mousex = mousey = 0;
var mousedown = false;
var tooltype = 'draw';

//Mousedown
$(canvas).on('mousedown', function(e) {
    last_mousex = mousex = parseInt(e.clientX-canvasx);
	last_mousey = mousey = parseInt(e.clientY-canvasy);
    mousedown = true;
});

//Mouseup
$(canvas).on('mouseup', function(e) {
    mousedown = false;
});

//Mousemove
$(canvas).on('mousemove', function(e) {
    mousex = parseInt(e.clientX-canvasx);
    mousey = parseInt(e.clientY-canvasy);
    if(mousedown) {
        ctx.beginPath();
        if(tooltype=='draw') {
            ctx.globalCompositeOperation = 'source-over';
            ctx.strokeStyle = 'black';
            ctx.lineWidth = 3;
        } else {
            ctx.globalCompositeOperation = 'destination-out';
            ctx.lineWidth = 10;
        }
        ctx.moveTo(last_mousex,last_mousey);
        ctx.lineTo(mousex,mousey);
        ctx.lineJoin = ctx.lineCap = 'round';
        ctx.stroke();
    }
    last_mousex = mousex;
    last_mousey = mousey;
    //Output
    $('#output').html('current: '+mousex+', '+mousey+'<br/>last: '+last_mousex+', '+last_mousey+'<br/>mousedown: '+mousedown);
});

//Use draw|erase
use_tool = function(tool) {
    tooltype = tool; //update
}

canvas {
    cursor: crosshair;
    border: 1px solid #000000;
}

<canvas id="canvas" width="800" height="500"></canvas>
<input type="button" value="draw" onclick="use_tool('draw');" />
<input type="button" value="erase" onclick="use_tool('erase');" />
<div id="output"></div>

Alco verifique este:
Exemplo:
https://github.com/williammalone/Simple-HTML5-Drawing-App

Documentation:
http://www.williammalone.com/articles/create-html5-canvas-javascript-drawing-app/

Este documento inclui os seguintes códigos: -

HTML:

<canvas id="canvas" width="490" height="220"></canvas>

JS:

context = document.getElementById('canvas').getContext("2d");

$('#canvas').mousedown(function(e){
  var mouseX = e.pageX - this.offsetLeft;
  var mouseY = e.pageY - this.offsetTop;

  paint = true;
  addClick(e.pageX - this.offsetLeft, e.pageY - this.offsetTop);
  redraw();
});

$('#canvas').mouseup(function(e){
  paint = false;
});

$('#canvas').mouseleave(function(e){
  paint = false;
});

var clickX = new Array();
var clickY = new Array();
var clickDrag = new Array();
var paint;

function addClick(x, y, dragging)
{
  clickX.push(x);
  clickY.push(y);
  clickDrag.push(dragging);
}

//Also redraw
function redraw(){
  context.clearRect(0, 0, context.canvas.width, context.canvas.height); // Clears the canvas

  context.strokeStyle = "#df4b26";
  context.lineJoin = "round";
  context.lineWidth = 5;

  for(var i=0; i < clickX.length; i++) {        
    context.beginPath();
    if(clickDrag[i] && i){
      context.moveTo(clickX[i-1], clickY[i-1]);
     }else{
       context.moveTo(clickX[i]-1, clickY[i]);
     }
     context.lineTo(clickX[i], clickY[i]);
     context.closePath();
     context.stroke();
  }
}

<html>
<head>
<!--script librarires-->
<script type="text/javascript" src="processing.js"></script>
<script type="text/javascript" src="init.js"></script>

<!--styles -->
<style type="text/css" src="stylesheet.css">
</style> 
</head>
<body>
<!--toolbox -->
<div id="draggable toolbox"></div>
<script type="application/processing">
// new script
int prevx, prevy;
int newx, newy;
boolean cliked;
color c1 = #000000;
int largeur=2;
int ps = 20;
int px = 50;
int py = 50;

void setup() {
size(500,500);
frameRate(25);
background(50);

 prevx = mouseX;
 prevy = mouseY;
 cliked = false;
 }

void draw() {
 //couleur
 noStroke(0);
 fill(#FFFFFF);//blanc
rect(px, py, ps, ps);
 fill(#000000);
 rect(px, py+(ps), ps, ps);
 fill(#FF0000);
 rect(px, py+(ps*2), ps, ps);
 fill(#00FF00);
 rect(px, py+(ps*3), ps, ps);
 fill(#FFFF00);
 rect(px, py+(ps*4), ps, ps);
 fill(#0000FF);
 rect(px, py+(ps*5), ps, ps);
 //largeur
 fill(#FFFFFF);
 rect(px, py+(ps*7), ps, ps);
  fill(#FFFFFF);
 rect(px, py+(ps*8), ps, ps);
 stroke(#000000);
 line(px+2, py+(ps*7)+(ps/2), px+(ps-2), py+(ps*7)+(ps/2));
 line(px+(ps/2), py+(ps*7)+1, px+(ps/2), py+(ps*8)-1);
 line(px+2, py+(ps*8)+(ps/2), px+(ps-2), py+(ps*8)+(ps/2));

 if(cliked==false){
  prevx = mouseX;
 prevy = mouseY;  
 }
 if(mousePressed) { 
  cliked = true;
 newx = mouseX;
  newy = mouseY;
  strokeWeight(largeur);
  stroke(c1);
  line(prevx, prevy, newx, newy);
  prevx = newx;
 prevy = newy;
 }else{
  cliked= false;
  }
}
void mouseClicked() {
 if (mouseX>=px && mouseX<=(px+ps)){
  //couleur
  if (mouseY>=py && mouseY<=py+(ps*6)){ 
   c1 = get(mouseX, mouseY);
  }
   //largeur
  if (mouseY>=py+(ps*7) && mouseY<=py+(ps*8)){ 
   largeur += 2;
  }
 if (mouseY>=py+(ps*8) && mouseY<=py+(ps*9)){ 
   if (largeur>2){
    largeur -= 2;
   }
  }
 }
}
</script><canvas></canvas>
</body>
</html>

Se você tem uma imagem de fundo para a sua tela, você terá que fazer alguns ajustes para que ela funcione corretamente, porque o truque de apagamento branco irá esconder o fundo.

Aqui está um gist com o código.

<html>
    <script type="text/javascript">
    var canvas, canvasimg, backgroundImage, finalImg;
    var mouseClicked = false;
    var prevX = 0;
    var currX = 0;
    var prevY = 0;
    var currY = 0;
    var fillStyle = "black";
    var globalCompositeOperation = "source-over";
    var lineWidth = 2;

    function init() {
      var imageSrc = '/abstract-geometric-pattern_23-2147508597.jpg'
      backgroundImage = new Image();
      backgroundImage.src = imageSrc;
      canvas = document.getElementById('can');
      finalImg = document.getElementById('finalImg');
      canvasimg = document.getElementById('canvasimg');
      canvas.style.backgroundImage = "url('" + imageSrc + "')";
      canvas.addEventListener("mousemove", handleMouseEvent);
      canvas.addEventListener("mousedown", handleMouseEvent);
      canvas.addEventListener("mouseup", handleMouseEvent);
      canvas.addEventListener("mouseout", handleMouseEvent);
    }

    function getColor(btn) {
      globalCompositeOperation = 'source-over';
      lineWidth = 2;
      switch (btn.getAttribute('data-color')) {
        case "green":
        fillStyle = "green";
        break;
        case "blue":
        fillStyle = "blue";
        break;
        case "red":
        fillStyle = "red";
        break;
        case "yellow":
        fillStyle = "yellow";
        break;
        case "orange":
        fillStyle = "orange";
        break;
        case "black":
        fillStyle = "black";
        break;
        case "eraser":
        globalCompositeOperation = 'destination-out';
        fillStyle = "rgba(0,0,0,1)";
        lineWidth = 14;
        break;
      }

    }

    function draw(dot) {
      var ctx = canvas.getContext("2d");
      ctx.beginPath();
      ctx.globalCompositeOperation = globalCompositeOperation;
      if(dot){
        ctx.fillStyle = fillStyle;
        ctx.fillRect(currX, currY, 2, 2);
      } else {
        ctx.beginPath();
        ctx.moveTo(prevX, prevY);
        ctx.lineTo(currX, currY);
        ctx.strokeStyle = fillStyle;
        ctx.lineWidth = lineWidth;
        ctx.stroke();
      }
      ctx.closePath();
    }

    function erase() {
      if (confirm("Want to clear")) {
        var ctx = canvas.getContext("2d");
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        document.getElementById("canvasimg").style.display = "none";
      }
    }

    function save() {
      canvas.style.border = "2px solid";
      canvasimg.width = canvas.width;
      canvasimg.height = canvas.height;
      var ctx2 = canvasimg.getContext("2d");
      // comment next line to save the draw only
      ctx2.drawImage(backgroundImage, 0, 0);
      ctx2.drawImage(canvas, 0, 0);
      finalImg.src = canvasimg.toDataURL();
      finalImg.style.display = "inline";
    }

    function handleMouseEvent(e) {
      if (e.type === 'mousedown') {
        prevX = currX;
        prevY = currY;
        currX = e.offsetX;
        currY = e.offsetY;
        mouseClicked = true;
        draw(true);
      }
      if (e.type === 'mouseup' || e.type === "mouseout") {
        mouseClicked = false;
      }
      if (e.type === 'mousemove') {
        if (mouseClicked) {
          prevX = currX;
          prevY = currY;
          currX = e.offsetX;
          currY = e.offsetY;
          draw();
        }
      }
    }
    </script>
    <body onload="init()">
      <canvas id="can" width="400" height="400" style="position:absolute;top:10%;left:10%;border:2px solid;">
      </canvas>
      <div style="position:absolute;top:12%;left:43%;">Choose Color</div>
      <div style="position:absolute;top:15%;left:45%;width:10px;height:10px;background:green;" data-color="green" onclick="getColor(this)"></div>
      <div style="position:absolute;top:15%;left:46%;width:10px;height:10px;background:blue;" data-color="blue" onclick="getColor(this)"></div>
      <div style="position:absolute;top:15%;left:47%;width:10px;height:10px;background:red;" data-color="red" onclick="getColor(this)"></div>
      <div style="position:absolute;top:17%;left:45%;width:10px;height:10px;background:yellow;" data-color="yellow" onclick="getColor(this)"></div>
      <div style="position:absolute;top:17%;left:46%;width:10px;height:10px;background:orange;" data-color="orange" onclick="getColor(this)"></div>
      <div style="position:absolute;top:17%;left:47%;width:10px;height:10px;background:black;" data-color="black" onclick="getColor(this)"></div>
      <div style="position:absolute;top:20%;left:43%;">Eraser</div>
      <div style="position:absolute;top:22%;left:45%;width:15px;height:15px;background:white;border:2px solid;" data-color="eraser" onclick="getColor(this)"></div>
      <canvas id="canvasimg" style="display:none;" ></canvas>
      <img id="finalImg" style="position:absolute;top:10%;left:52%;display:none;" >
      <input type="button" value="save" id="btn" size="30" onclick="save()" style="position:absolute;top:55%;left:10%;">
      <input type="button" value="clear" id="clr" size="23" onclick="erase()" style="position:absolute;top:55%;left:15%;">
    </body>
    </html>