实例如下所示：
<!DOCTYPE html><html> <head> <meta charset="utf-8" /> <title></title> </head> <body> <div> <div> <!--<input type="button" id="open" value="open"></input> <input type="button" id= "save" value = "save"></input> <input type="button" id= "color" value="color"></input> --> <input type="button" value="size"></input> <input type="text" id="size" onchange="sizeChange()"></input> <input type="button" id="clear" value="clear"></input> <input type="button" id="eraser" value="eraser" onclick="doEraser()"></input> <select id = "shape" onchange="shapeChange()"> <option value = "99">shape</option> <option value = "1">rectangle</option> <option value = "0">circle</option> <option value = "2">line</option> </select> <input id="color" type="color"/> </div> <canvas id="myCanvas" style=" border:1px solid;" width="800" height="500"></canvas> </div> </body> <script language="JavaScript"> var shap = 99; //0 is circle; 1 is rectangle var orignalX, orignalY;//the coordinate of mouse down var lastX, lastY;//the coordinate of last mouse position var isMouseDown = false; // flag of mouse pressing down var myCanvas = document.getElementById("myCanvas"); var context = myCanvas.getContext('2d'); var width = myCanvas.width, height = myCanvas.height; var data;//storing last canvas' content context.strokeStyle = "black"; context.strokeWidth=1; context.lineWidth = 1; document.getElementById('color').onchange = function(){ context.strokeStyle = this.value }; function doEraser(){ context.strokeStyle = "white"; shap = 2; } function sizeChange(){ context.lineWidth = parseInt(document.getElementById('size').value); } function shapeChange(){ context.strokeStyle = "black"; var myselect = document.getElementById("shape"); var index=myselect.selectedIndex ; var myvalue = myselect.options[index].value; var mytext=myselect.options[index].text; shap = parseInt(myvalue); } function myCanvasMouseDown(event) { //event.preventDefault(); if(event.button == 0) { orignalX = event.offsetX; orignalY = event.offsetY; context.moveTo(orignalX,orignalY); data = context.getImageData(0, 0, width, height); isMouseDown = true; } } function myCanvasMouseMove(event) { if (isMouseDown){ //event.preventDefault(); switch(shap){ case 0: context.clearRect(0,0,width,height); context.putImageData(data,0,0); lastX = event.offsetX; lastY = event.offsetY; context.beginPath(); context.arc(orignalX+(lastX-orignalX)/2,orignalY+(lastY-orignalY)/2,Math.abs(lastX-orignalX)/2,0,Math.PI * 2,true); context.stroke(); context.closePath(); break; case 1: context.clearRect(0,0,width,height); context.putImageData(data,0,0); lastX = event.offsetX; lastY = event.offsetY; context.strokeRect(orignalX, orignalY, lastX-orignalX, lastY-orignalY); break; case 2: lastX = event.offsetX; lastY = event.offsetY; context.lineTo(lastX, lastY); //根据鼠标路径绘画 context.stroke(); //立即渲染 break; } } } function myCanvasMouseUp(event) { if (isMouseDown){ //event.preventDefault(); context.clearRect(0,0,width,height); context.putImageData(data,0,0); lastX = event.offsetX; lastY = event.offsetY; switch(shap){ case 0: context.beginPath(); context.arc(orignalX+(lastX-orignalX)/2,orignalY+(lastY-orignalY)/2,Math.abs(lastX-orignalX)/2,0,Math.PI * 2,true); context.stroke(); context.closePath(); break; case 1: context.beginPath(); context.strokeRect(orignalX, orignalY, lastX-orignalX, lastY-orignalY); context.closePath(); break; case 2: context.lineTo(lastX, lastY); //根据鼠标路径绘画 context.stroke(); //立即渲染 break; } isMouseDown = false; lastX = null; lastY = null; orignalX = null; orignalY = null; data = context.getImageData(0, 0, width, height); context.beginPath(); context.clearRect(0,0,width,height); context.putImageData(data,0,0); context.closePath(); } } myCanvas.addEventListener("mousedown", myCanvasMouseDown, false); myCanvas.addEventListener("mousemove", myCanvasMouseMove, false); myCanvas.addEventListener("mouseup", myCanvasMouseUp, false); </script> </html>
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