本篇主要讲html5 canvas中剪切区域(clip region)、分层、合成(compositing)、变换(Transformation)(旋转、缩放)功能如何应用。
先贴一个以下所有涉及到的实现运行的基本代码段:
Base Code <!DOCTYPE html> <html> <head> <meta charset="utf-8" /> <script type="text/javascript" src="modernizr-latest.js"></script> <script type="text/javascript"> window.addEventListener("load", eventWindowLoaded, false); var Debugger = function() {}; Debugger.log = function(message) { try { console.log(message); } catch (exception) { return; } } function eventWindowLoaded() { canvasApp(); } function canvasSupport() { return Modernizr.canvas; } function canvasApp() { //是否支持CANVAS判断 if(!canvasSupport()) { return; } //取Canvas var theCanvas = document.getElementById("canvasOne"); //获取绘图环境 contextvar context = theCanvas.getContext("2d"); //绘图方法的实现 function drawScreen() {} //绘图方法调用执行 drawScreen(); } </script> </head> <body> <div style="position: absolute; top: 50px; left: 50px; border:1px solid #0000ff"> <canvas id="canvasOne" width="550" height="400"> Your browser does not support HTML5 Canvas. </canvas> </div> </body> </html>
以下所有实例代码,只要把上面的function drawScreen()替换掉即可!
clip region 裁剪区域
在clip region中,我们利用.save 与 .restore来把当前绘图状态压入堆栈与从堆栈中恢复;
使用.save保证裁切前的绘图状态;
通过context.rect()来确定要裁切的区域在位置;
通过context.clip()来执行裁切动作;
在Canvas上画图,只有在裁切区域的图形才能显示出来;
使用context.restore()再恢复到裁切前的状态;
function drawScreen() { context.fillStyle = "black"; context.fillRect(0, 0, 200, 200); context.save(); //clip the canvas to a 50×50 square starting at 0,0 context.rect(0, 0, 100, 100); context.clip(); //red circle context.beginPath(); context.strokeStyle = "red"; context.lineWidth = 5; context.arc(100, 100, 100, (Math.PI/180)*0, (Math.PI/180)*360, false); //full circle context.stroke(); context.closePath(); context.restore(); //reclip to the entire canvas context.rect(0, 0, 300, 300); context.clip(); //draw a blue line that is not clipped context.beginPath(); context.strokeStyle = "blue"; //need list of available colors context.lineWidth = 5; context.arc(100, 100, 50, (Math.PI/180)*0, (Math.PI/180)*360, false); //full circle context.stroke(); context.closePath(); }
实例效果图:
Compositing 合成
在Canvas上图形合成,指更好的处理图形的透明度及在画布上的层效果;
compositing操作中有两个比较重要的属性我们先来认识一下: globalAlpha and globalCompositeOperation
globalAlpha:指Canvas的透明度,默认值为1.0,范围是[0.0-1.0];
globalCompositeOperation:应用了“alpha” 及"transformations"的多个图形,是如何绘制的Canvas上的。
共有11种类型: copy\ destination-atop\destination-in\destination-out\destination-over\ lighter\ source-atop\source-in\source-out\source-over\xor
这11种类型中有两个特殊的词"source" and "destination":
"source":将要在Canvas上绘制的图形(新图形);
"destination":Canvas上当前显示的图形(旧图形);
copy:指新图形会被保留,其它都被清除掉
destination-atop:指显示新图形与重叠部分的旧图形,旧图形(重叠部分)显示在上层
destination-in:指只显示旧图形的重叠部分的图形
destination-out:指只显示旧图形不重叠部分的图形
destination-over:指新旧图形都显示,把旧图形显示在上层
lighter:指新旧图形都显示,新旧图形重叠部分作加色处理
source-atop:指显示旧图表与重叠部分的新图形,新图形(重叠部分)显示在上层
source-in:指只显示新图形重叠部分的图形
source-out:指只显示新图形中不重叠部分的图形
source-over:指新旧图形都显示,新图形显示在上层
xor:指新旧图形都显示,新旧图形重叠的部分会变成透明。
<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8" /> <title>test</title> <script type="text/javascript" src="../script/modernizr-latest.js"></script> <script type="text/javascript"> window.addEventListener("load", eventWindowLoaded, ); Debugger = () { }; Debugger.log = (message) { { console.log(message); } (exception) { ; } } eventWindowLoaded() { canvasApp(); } canvasSupport() { Modernizr.canvas; } canvasApp() { (!canvasSupport()) { ; } drawScreen(compositing, num) { name = "c" + num; theCanvas = document.getElementById(name); theCanvas.width=100; theCanvas.height=100; context = theCanvas.getContext("2d"); context.fillStyle = "blue"; context.fillRect(10, 10, 50, 50); context.globalCompositeOperation = compositing; context.fillStyle = "red"; context.fillRect(30, 30, 50, 50); } drawScreen("copy", 1); drawScreen("destination-atop", 2); drawScreen("destination-in", 3); drawScreen("destination-out", 4); drawScreen("destination-over", 5); drawScreen("lighter", 6); drawScreen("source-atop", 7); drawScreen("source-in", 8); drawScreen("source-out", 9); drawScreen("source-over", 10); drawScreen("xor", 11); } </script> <style> td { border: 1px solid #000000; } </style> </head> <body> <p style="position: absolute; top: 50px; left: 50px; border:1px solid #0000ff"> <table> <tr> <td><canvas id="c1"></canvas></td> <td><canvas id="c2"></canvas></td> <td><canvas id="c3"></canvas></td> <td><canvas id="c4"></canvas></td> <td><canvas id="c5"></canvas></td> </tr> <tr> <td><canvas id="c6"></canvas></td> <td><canvas id="c7"></canvas></td> <td><canvas id="c8"></canvas></td> <td><canvas id="c9"></canvas></td> <td><canvas id="c10"></canvas></td> <td><canvas id="c11"></canvas></td> </tr> </table> </p> </body> </html>
实例效果图:从左到右、从上到下,分别表示1,2,……11,这11种类型生成的效果图
Transformations 变换
变换的本质就是指从数学(矩阵)的角度来调整图形的物理属性;当然,这是原理;我们在实现的时候,只需要调用方法即可;
移动translate、旋转rotation 、缩放scale 、变换transforms
Transformations are applied to shapes and paths drawn after the setTransform() or other transformation function is called
只有在应用context.setTransform()方法后,对图形的各种变换才能生效;
实例:
function drawScreen() { context.fillStyle = "blue"; context.fillRect(210,210,50,50); context.setTransform(1,0,0,1,0,0);//启动变换 var angleInRadians = 45 * Math.PI / 180; context.rotate(angleInRadians);//旋转(参数为弧度) context.fillStyle = "red"; context.fillRect(0,0,200,200); }
效果如图:
蓝色的box并没有发生旋转;红的box是以Canvas的原点为中心顺时针旋转了45度(为何没有以红色box中心为原点旋转呢?);
Canvas的旋转原点,默认为Canvas的坐标系的(0,0)点,若不进行原点平移就旋转,自然是像整个画布做了旋转;
We must “translate” the point of origin to the center of our shape to rotate it around its own center
我们必须通地context.translate()方法来平移原点,才能按绘制的图形中心来旋转图形自身;
平移原理:
实例:
translate function drawScreen() { context.fillStyle = "blue"; context.fillRect(210,210,50,50); context.setTransform(1,0,0,1,0,0); var angleInRadians = 45 * Math.PI / 180; var x = 0; var y = 0; var width = 200; var height = 200; context.translate(x+.5*width, y+.5*height);//平移 context.rotate(angleInRadians); context.fillStyle = "red"; context.fillRect(-.5*width,-.5*height , width, height); }
效果图:
context.scale()实现图形的缩放,
此方法有两个参数: 一个是对X轴方向的缩放,一个是对Y轴方向的缩放,正常的图形参数默认都为1;
例如,我们要对图形放大两倍,则使用context.scale(2,2);
以(25,25)为顶点,长宽为50的正方形,放大2倍 function drawScreen() { context.fillStyle = "blue"; context.fillRect(0, 0, 50, 50); context.fillRect(150, 50, 50, 50); context.fillRect(50, 150, 50, 50); context.setTransform(1, 0, 0, 1, 0, 0); context.scale(2, 2); context.fillStyle = "red"; context.fillRect(25, 25, 50, 50); }
效果图:
scale与ratation也存在相似问题,就是原点的问题;
如果我们在变换前不进行原点的平移的话,scale功能默认的原点也是canvas的原点;
相当于对整个画布进行scale 默认是延x轴正向(left),y轴正向进行缩放(down);
如果想图形依然在原来的位置,从图形的中心位置进行scale,就需要先进行原点的平移context.translate();
以(25,25)为顶点,长宽为50的正方形,为其中心为原点,放大二倍
以(25,25)为顶点,长宽为50的正方形,为其中心为原点,放大二倍 function drawScreen() { context.setTransform(1, 0, 0, 1, 0, 0); var x = 25; var y = 25; var width = 50; var height = 50; context.translate(x + .5 * width, y + .5 * height); context.scale(2, 2); context.fillStyle = "red"; context.fillRect(-.5 * width, -.5 * height, width, height); }
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