HTML5 Canvas画板画图工具可定义笔刷和画布

本文作者html5tricks，转载请注明出处

HTML5 Canvas还有一个比较实用的应用，那就是网络画板，这样我们就可以在网页上直接进行画图操作。今天要分享的这款HTML5 Canvas画图工具就可以简单实现网络画图的功能，我们可以自定义笔刷的类型、粗细、颜色，也可以定义画布的大小和背景颜色等。我们也可以对这款HTML5画图工具进行扩展，让它的画图功能更加完善。

html5-canvas-draw

下面我们来简单地分析一下实现这个HTML5网页画板的原理及代码，代码由HTML以及Javascript组成，主要还是Javascript代码。

HTML代码：

<div style="width:530px;margin:10px auto">
    <div id="canvasDiv"></div>
  </div>

HTML代码非常简单，仅仅是构造了一个canvas容器，我们的画板就在这个地方生成。

Javascript代码：

首先我们通过一组变量来定义画板的样式，以及一些数据的初始化：

var canvas;
var context;
var canvasWidth = 490;
var canvasHeight = 220;
var padding = 25;
var lineWidth = 8;
var colorPurple = "#cb3594";
var colorGreen = "#659b41";
var colorYellow = "#ffcf33";
var colorBrown = "#986928";
var outlineImage = new Image();
var crayonImage = new Image();
var markerImage = new Image();
var eraserImage = new Image();
var crayonBackgroundImage = new Image();
var markerBackgroundImage = new Image();
var eraserBackgroundImage = new Image();
var crayonTextureImage = new Image();
var clickX = new Array();
var clickY = new Array();
var clickColor = new Array();
var clickTool = new Array();
var clickSize = new Array();
var clickDrag = new Array();
var paint = false;
var curColor = colorPurple;
var curTool = "crayon";
var curSize = "normal";
var mediumStartX = 18;
var mediumStartY = 19;
var mediumImageWidth = 93;
var mediumImageHeight = 46;
var drawingAreaX = 111;
var drawingAreaY = 11;
var drawingAreaWidth = 267;
var drawingAreaHeight = 200;
var toolHotspotStartY = 23;
var toolHotspotHeight = 38;
var sizeHotspotStartY = 157;
var sizeHotspotHeight = 36;
var sizeHotspotWidthObject = new Object();
sizeHotspotWidthObject.huge = 39;
sizeHotspotWidthObject.large = 25;
sizeHotspotWidthObject.normal = 18;
sizeHotspotWidthObject.small = 16;
var totalLoadResources = 8;
var curLoadResNum = 0;

接下来开始准备画布，也就是初始化Canvas对象：

function prepareCanvas()
{
	// Create the canvas (Neccessary for IE because it doesn't know what a canvas element is)
	var canvasDiv = document.getElementById('canvasDiv');
	canvas = document.createElement('canvas');
	canvas.setAttribute('width', canvasWidth);
	canvas.setAttribute('height', canvasHeight);
	canvas.setAttribute('id', 'canvas');
	canvasDiv.appendChild(canvas);
	if(typeof G_vmlCanvasManager != 'undefined') {
		canvas = G_vmlCanvasManager.initElement(canvas);
	}
	context = canvas.getContext("2d"); // Grab the 2d canvas context
	// Note: The above code is a workaround for IE 8 and lower. Otherwise we could have used:
	//     context = document.getElementById('canvas').getContext("2d");

	// Load images
	// -----------
	crayonImage.onload = function() { resourceLoaded(); 
	};
	crayonImage.src = "images/crayon-outline.png";
	//context.drawImage(crayonImage, 0, 0, 100, 100);

	markerImage.onload = function() { resourceLoaded(); 
	};
	markerImage.src = "images/marker-outline.png";

	eraserImage.onload = function() { resourceLoaded(); 
	};
	eraserImage.src = "images/eraser-outline.png";	

	crayonBackgroundImage.onload = function() { resourceLoaded(); 
	};
	crayonBackgroundImage.src = "images/crayon-background.png";

	markerBackgroundImage.onload = function() { resourceLoaded(); 
	};
	markerBackgroundImage.src = "images/marker-background.png";

	eraserBackgroundImage.onload = function() { resourceLoaded(); 
	};
	eraserBackgroundImage.src = "images/eraser-background.png";

	crayonTextureImage.onload = function() { resourceLoaded(); 
	};
	crayonTextureImage.src = "images/crayon-texture.png";

	outlineImage.onload = function() { resourceLoaded(); 
	};
	outlineImage.src = "images/watermelon-duck-outline.png";

	// Add mouse events
	// ----------------
	$('#canvas').mousedown(function(e)
	{
		// Mouse down location
		var mouseX = e.pageX - this.offsetLeft;
		var mouseY = e.pageY - this.offsetTop;

		if(mouseX < drawingAreaX) // Left of the drawing area
		{
			if(mouseX > mediumStartX)
			{
				if(mouseY > mediumStartY && mouseY < mediumStartY + mediumImageHeight){
					curColor = colorPurple;
				}else if(mouseY > mediumStartY + mediumImageHeight && mouseY < mediumStartY + mediumImageHeight * 2){
					curColor = colorGreen;
				}else if(mouseY > mediumStartY + mediumImageHeight * 2 && mouseY < mediumStartY + mediumImageHeight * 3){
					curColor = colorYellow;
				}else if(mouseY > mediumStartY + mediumImageHeight * 3 && mouseY < mediumStartY + mediumImageHeight * 4){
					curColor = colorBrown;
				}
			}
		}
		else if(mouseX > drawingAreaX + drawingAreaWidth) // Right of the drawing area
		{
			if(mouseY > toolHotspotStartY)
			{
				if(mouseY > sizeHotspotStartY)
				{
					var sizeHotspotStartX = drawingAreaX + drawingAreaWidth;
					if(mouseY < sizeHotspotStartY + sizeHotspotHeight && mouseX > sizeHotspotStartX)
					{
						if(mouseX < sizeHotspotStartX + sizeHotspotWidthObject.huge){
							curSize = "huge";
						}else if(mouseX < sizeHotspotStartX + sizeHotspotWidthObject.large + sizeHotspotWidthObject.huge){
							curSize = "large";
						}else if(mouseX < sizeHotspotStartX + sizeHotspotWidthObject.normal + sizeHotspotWidthObject.large + sizeHotspotWidthObject.huge){
							curSize = "normal";
						}else if(mouseX < sizeHotspotStartX + sizeHotspotWidthObject.small + sizeHotspotWidthObject.normal + sizeHotspotWidthObject.large + sizeHotspotWidthObject.huge){
							curSize = "small";						
						}
					}
				}
				else
				{
					if(mouseY < toolHotspotStartY + toolHotspotHeight){
						curTool = "crayon";
					}else if(mouseY < toolHotspotStartY + toolHotspotHeight * 2){
						curTool = "marker";
					}else if(mouseY < toolHotspotStartY + toolHotspotHeight * 3){
						curTool = "eraser";
					}
				}
			}
		}
		else if(mouseY > drawingAreaY && mouseY < drawingAreaY + drawingAreaHeight)
		{
			// Mouse click location on drawing area
		}
		paint = true;
		addClick(mouseX, mouseY, false);
		redraw();
	});

	$('#canvas').mousemove(function(e){
		if(paint==true){
			addClick(e.pageX - this.offsetLeft, e.pageY - this.offsetTop, true);
			redraw();
		}
	});

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

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

看起来很复杂，前面主要是初始化canvas的背景图片，后面主要是初始化画笔事件，像click、mouseup、mouseleave等鼠标事件。

下面是draw的主要方法：

function redraw()
{
	// Make sure required resources are loaded before redrawing
	if(curLoadResNum < totalLoadResources){ return; }

	clearCanvas();

	var locX;
	var locY;
	if(curTool == "crayon")
	{
		// Draw the crayon tool background
		context.drawImage(crayonBackgroundImage, 0, 0, canvasWidth, canvasHeight);

		// Purple
		locX = (curColor == colorPurple) ? 18 : 52;
		locY = 19;

		context.beginPath();
		context.moveTo(locX + 41, locY + 11);
		context.lineTo(locX + 41, locY + 35);
		context.lineTo(locX + 29, locY + 35);
		context.lineTo(locX + 29, locY + 33);
		context.lineTo(locX + 11, locY + 27);
		context.lineTo(locX + 11, locY + 19);
		context.lineTo(locX + 29, locY + 13);
		context.lineTo(locX + 29, locY + 11);
		context.lineTo(locX + 41, locY + 11);
		context.closePath();
		context.fillStyle = colorPurple;
		context.fill();	

		if(curColor == colorPurple){
			context.drawImage(crayonImage, locX, locY, mediumImageWidth, mediumImageHeight);
		}else{
			context.drawImage(crayonImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
		}

		// Green
		locX = (curColor == colorGreen) ? 18 : 52;
		locY += 46;

		context.beginPath();
		context.moveTo(locX + 41, locY + 11);
		context.lineTo(locX + 41, locY + 35);
		context.lineTo(locX + 29, locY + 35);
		context.lineTo(locX + 29, locY + 33);
		context.lineTo(locX + 11, locY + 27);
		context.lineTo(locX + 11, locY + 19);
		context.lineTo(locX + 29, locY + 13);
		context.lineTo(locX + 29, locY + 11);
		context.lineTo(locX + 41, locY + 11);
		context.closePath();
		context.fillStyle = colorGreen;
		context.fill();	

		if(curColor == colorGreen){
			context.drawImage(crayonImage, locX, locY, mediumImageWidth, mediumImageHeight);
		}else{
			context.drawImage(crayonImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
		}

		// Yellow
		locX = (curColor == colorYellow) ? 18 : 52;
		locY += 46;

		context.beginPath();
		context.moveTo(locX + 41, locY + 11);
		context.lineTo(locX + 41, locY + 35);
		context.lineTo(locX + 29, locY + 35);
		context.lineTo(locX + 29, locY + 33);
		context.lineTo(locX + 11, locY + 27);
		context.lineTo(locX + 11, locY + 19);
		context.lineTo(locX + 29, locY + 13);
		context.lineTo(locX + 29, locY + 11);
		context.lineTo(locX + 41, locY + 11);
		context.closePath();
		context.fillStyle = colorYellow;
		context.fill();	

		if(curColor == colorYellow){
			context.drawImage(crayonImage, locX, locY, mediumImageWidth, mediumImageHeight);
		}else{
			context.drawImage(crayonImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
		}

		// Yellow
		locX = (curColor == colorBrown) ? 18 : 52;
		locY += 46;

		context.beginPath();
		context.moveTo(locX + 41, locY + 11);
		context.lineTo(locX + 41, locY + 35);
		context.lineTo(locX + 29, locY + 35);
		context.lineTo(locX + 29, locY + 33);
		context.lineTo(locX + 11, locY + 27);
		context.lineTo(locX + 11, locY + 19);
		context.lineTo(locX + 29, locY + 13);
		context.lineTo(locX + 29, locY + 11);
		context.lineTo(locX + 41, locY + 11);
		context.closePath();
		context.fillStyle = colorBrown;
		context.fill();	

		if(curColor == colorBrown){
			context.drawImage(crayonImage, locX, locY, mediumImageWidth, mediumImageHeight);
		}else{
			context.drawImage(crayonImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
		}
	}
	else if(curTool == "marker")
	{
		// Draw the marker tool background
		context.drawImage(markerBackgroundImage, 0, 0, canvasWidth, canvasHeight);

		// Purple
		locX = (curColor == colorPurple) ? 18 : 52;
		locY = 19;

		context.beginPath();
		context.moveTo(locX + 10, locY + 24);
		context.lineTo(locX + 10, locY + 24);
		context.lineTo(locX + 22, locY + 16);
		context.lineTo(locX + 22, locY + 31);
		context.closePath();
		context.fillStyle = colorPurple;
		context.fill();	

		if(curColor == colorPurple){
			context.drawImage(markerImage, locX, locY, mediumImageWidth, mediumImageHeight);
		}else{
			context.drawImage(markerImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
		}

		// Green
		locX = (curColor == colorGreen) ? 18 : 52;
		locY += 46;

		context.beginPath();
		context.moveTo(locX + 10, locY + 24);
		context.lineTo(locX + 10, locY + 24);
		context.lineTo(locX + 22, locY + 16);
		context.lineTo(locX + 22, locY + 31);
		context.closePath();
		context.fillStyle = colorGreen;
		context.fill();	

		if(curColor == colorGreen){
			context.drawImage(markerImage, locX, locY, mediumImageWidth, mediumImageHeight);
		}else{
			context.drawImage(markerImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
		}

		// Yellow
		locX = (curColor == colorYellow) ? 18 : 52;
		locY += 46;

		context.beginPath();
		context.moveTo(locX + 10, locY + 24);
		context.lineTo(locX + 10, locY + 24);
		context.lineTo(locX + 22, locY + 16);
		context.lineTo(locX + 22, locY + 31);
		context.closePath();
		context.fillStyle = colorYellow;
		context.fill();	

		if(curColor == colorYellow){
			context.drawImage(markerImage, locX, locY, mediumImageWidth, mediumImageHeight);
		}else{
			context.drawImage(markerImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
		}

		// Yellow
		locX = (curColor == colorBrown) ? 18 : 52;
		locY += 46;

		context.beginPath();
		context.moveTo(locX + 10, locY + 24);
		context.lineTo(locX + 10, locY + 24);
		context.lineTo(locX + 22, locY + 16);
		context.lineTo(locX + 22, locY + 31);
		context.closePath();
		context.fillStyle = colorBrown;
		context.fill();	

		if(curColor == colorBrown){
			context.drawImage(markerImage, locX, locY, mediumImageWidth, mediumImageHeight);
		}else{
			context.drawImage(markerImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
		}
	}
	else if(curTool == "eraser")
	{
		context.drawImage(eraserBackgroundImage, 0, 0, canvasWidth, canvasHeight);
		context.drawImage(eraserImage, 18, 19, mediumImageWidth, mediumImageHeight);	
	}else{
		alert("Error: Current Tool is undefined");
	}

	if(curSize == "small"){
		locX = 467;
	}else if(curSize == "normal"){
		locX = 450;
	}else if(curSize == "large"){
		locX = 428;
	}else if(curSize == "huge"){
		locX = 399;
	}
	locY = 189;
	context.beginPath();
	context.rect(locX, locY, 2, 12);
	context.closePath();
	context.fillStyle = '#333333';
	context.fill();	

	// Keep the drawing in the drawing area
	context.save();
	context.beginPath();
	context.rect(drawingAreaX, drawingAreaY, drawingAreaWidth, drawingAreaHeight);
	context.clip();

	var radius;
	var i = 0;
	for(; i < clickX.length; i++)
	{		
		if(clickSize[i] == "small"){
			radius = 2;
		}else if(clickSize[i] == "normal"){
			radius = 5;
		}else if(clickSize[i] == "large"){
			radius = 10;
		}else if(clickSize[i] == "huge"){
			radius = 20;
		}else{
			alert("Error: Radius is zero for click " + i);
			radius = 0;	
		}

		context.beginPath();
		if(clickDrag[i] && i){
			context.moveTo(clickX[i-1], clickY[i-1]);
		}else{
			context.moveTo(clickX[i], clickY[i]);
		}
		context.lineTo(clickX[i], clickY[i]);
		context.closePath();

		if(clickTool[i] == "eraser"){
			//context.globalCompositeOperation = "destination-out"; // To erase instead of draw over with white
			context.strokeStyle = 'white';
		}else{
			//context.globalCompositeOperation = "source-over";	// To erase instead of draw over with white
			context.strokeStyle = clickColor[i];
		}
		context.lineJoin = "round";
		context.lineWidth = radius;
		context.stroke();

	}
	//context.globalCompositeOperation = "source-over";// To erase instead of draw over with white
	context.restore();

	// Overlay a crayon texture (if the current tool is crayon)
	if(curTool == "crayon"){
		context.globalAlpha = 0.4; // No IE support
		context.drawImage(crayonTextureImage, 0, 0, canvasWidth, canvasHeight);
	}
	context.globalAlpha = 1; // No IE support

	// Draw the outline image
	context.drawImage(outlineImage, drawingAreaX, drawingAreaY, drawingAreaWidth, drawingAreaHeight);
}

其实HTML5说白了还是需要很多Javascript支持，不过Canvas非常不错，可以让你在上面自由地绘制图形和动画。这款基于HTML5 Canvas的网页画板就是一个很好的例子。

在线演示源码下载

HTML5 Canvas画板画图工具可定义笔刷和画布

HTML代码：

Javascript代码：

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