//兼容移动端
 


 
 
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 //写字区域
 

  

  

  

  

  

  

 
  
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#canvas{
 display:block;
 margin:0 auto;
}
#controller{
 margin:0 auto;
}
.op_btn{
 float: right;
 margin:10px 0 0 10px;
 border:2px solid #aaa;
 width:80px;
 height:40px;
 line-height:40px;
 font-size:20px;
 text-align:center;
 border-radius: 5px 5px;
 cursor:pointer;
 background-color: white;
 font-weight:bold;
 font-family: Microsoft Yahei, Arial;
}
.op_btn:hover{
 background-color:#def;
}
.clearfix{
 clear:both;
}
 
.color_btn{
 float: left;
 margin: 10px 10px 0 0;
 border:5px solid white;
 width:40px;
 height:40px;
 border-radius: 5px 5px;
 cursor:pointer;
}
.color_btn:hover{
 border: 5px solid violet;
}
.color_btn_selected{
 border: 5px solid blueviolet;
}
#black_btn{
 background-color: black;
}
#blue_btn{
 background-color: blue;
}
#green_btn{
 background-color: green;
}
#red_btn{
 background-color: red;
}
#orange_btn{
 background-color: orange;
}
#yellow_btn{
 background-color: yellow;
}

var canvasWidth = Math.min( 800 , $(window).width() - 20 );//如果屏幕小于800px，则取值为屏幕大小宽度，便于移动端的展示，-20是为了使得米字格不紧贴于边缘
var canvasHeight = canvasWidth;
 
var strokeColor = "black";
var isMouseDown = false; //鼠标按下时候的状态
var lastLoc = {x:0,y:0}; //鼠标上一次结束时的位置
var lastTimestamp = 0; //上一次时间，与笔刷粗细有关
var lastLineWidth = -1; //笔刷粗细
 
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
 
canvas.width = canvasWidth;
canvas.height = canvasHeight;
 
$("#controller").css("width",canvasWidth+"px");
drawGrid();//画米字格
 
$("#clear_btn").click(
 function(e){
  context.clearRect( 0 , 0 , canvasWidth, canvasHeight );
  drawGrid();
 }
)
$(".color_btn").click(
 function(e){
  $(".color_btn").removeClass("color_btn_selected");
  $(this).addClass("color_btn_selected");
  strokeColor = $(this).css("background-color");
 }
)
//适用于移动端触控
function beginStroke(point){
 
 isMouseDown = true
 //console.log("mouse down!");
 lastLoc = windowToCanvas(point.x, point.y); //上一次坐标位置
 lastTimestamp = new Date().getTime();
 
}
function endStroke(){
 isMouseDown = false;
}
function moveStroke(point){
 
 var curLoc = windowToCanvas( point.x , point.y );
 var curTimestamp = new Date().getTime();
 var s = calcDistance( curLoc , lastLoc );
 var t = curTimestamp - lastTimestamp;
 
 var lineWidth = calcLineWidth( t , s );
 
 //draw
 context.beginPath();
 context.moveTo( lastLoc.x , lastLoc.y );
 context.lineTo( curLoc.x , curLoc.y );
 
 context.strokeStyle = strokeColor;
 context.lineWidth = lineWidth;
 context.lineCap = "round";
 context.lineJoin = "round";
 context.stroke();
 
 lastLoc = curLoc;
 lastTimestamp = curTimestamp;
 lastLineWidth = lineWidth;
}
 
canvas.onmousedown = function(e){
 e.preventDefault();
 beginStroke( {x: e.clientX , y: e.clientY} );
};
canvas.onmouseup = function(e){
 e.preventDefault();
 endStroke();
};
canvas.onmouseout = function(e){
 e.preventDefault();
 endStroke();
};
canvas.onmousemove = function(e){
 e.preventDefault();
 if( isMouseDown ){
  moveStroke({x: e.clientX , y: e.clientY})
 }
};
 
canvas.addEventListener('touchstart',function(e){
 e.preventDefault();
 touch = e.touches[0];
 beginStroke( {x: touch.pageX , y: touch.pageY} )
});
canvas.addEventListener('touchmove',function(e){
 e.preventDefault();
 if( isMouseDown ){
  touch = e.touches[0];
  moveStroke({x: touch.pageX , y: touch.pageY});
 }
});
canvas.addEventListener('touchend',function(e){
 e.preventDefault();
 endStroke();
});
 
var maxLineWidth = 30;
var minLineWidth = 1;
var maxStrokeV = 10;
var minStrokeV = 0.1;
function calcLineWidth( t , s ){
 
 var v = s / t;
 
 var resultLineWidth;
 if( v <= minStrokeV )
  resultLineWidth = maxLineWidth;
 else if ( v >= maxStrokeV )
  resultLineWidth = minLineWidth;
 else{
  resultLineWidth = maxLineWidth - (v-minStrokeV)/(maxStrokeV-minStrokeV)*(maxLineWidth-minLineWidth);
 }
 
 if( lastLineWidth == -1 )
  return resultLineWidth;
 
 return resultLineWidth*1/3 + lastLineWidth*2/3;
}
 
function calcDistance( loc1 , loc2 ){
 
 return Math.sqrt( (loc1.x - loc2.x)*(loc1.x - loc2.x) + (loc1.y - loc2.y)*(loc1.y - loc2.y) );
}
 
function windowToCanvas( x , y ){
 var bbox = canvas.getBoundingClientRect();
 return {x:Math.round(x-bbox.left) , y:Math.round(y-bbox.top)}
}
function drawGrid(){
 
 context.save();
 
 context.strokeStyle = "rgb(230,11,9)";
 
 context.beginPath();
 context.moveTo( 3 , 3 );
 context.lineTo( canvasWidth - 3 , 3 );
 context.lineTo( canvasWidth - 3 , canvasHeight - 3 );
 context.lineTo( 3 , canvasHeight - 3 );
 context.closePath();
 context.lineWidth = 6;
 context.stroke();
 
 context.beginPath();
 context.moveTo(0,0);
 context.lineTo(canvasWidth,canvasHeight);
 
 context.moveTo(canvasWidth,0);
 context.lineTo(0,canvasHeight);
 
 context.moveTo(canvasWidth/2,0);
 context.lineTo(canvasWidth/2,canvasHeight);
 
 context.moveTo(0,canvasHeight/2);
 context.lineTo(canvasWidth,canvasHeight/2);
 
 context.lineWidth = 1;
 context.stroke();
 
 context.restore();
}