This time I will bring you a detailed explanation of the use case of H5 Canvas. What are the precautions when using H5 Canvas. The following is a practical case, let’s take a look.
<canvas id='canvas'></canvas>
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Get canvas object context
var convas=document.getElementById('canvas');
var context=canvas.getContext('2d');Copy after login
canvas is a state-based drawing environment
Draw a straight line
context.moveTo(x,y); 挪到笔头
context.lineTo(x,y);按着画笔移动 （可以结合使用moveTo画不相连的线段）moveTo lineTo +moveTo lineTo
context.lineWith=10; canvas是基于状态的，它不会创建一个直线对象。我们是对canvas上下文进行其他设置。读完所以设置后再绘图
context.strokeStyle='red'; 这里还有其他的样式，不仅仅只有颜色。
context.stroke();
context.beginPath(); 开始一个全新的canvas状态。之前对context的设置不会因为beginPath()而改变。新的设置会覆盖之前的设置
context.closePath(); 使用线段绘制封闭的图形。可以去掉最后一个lineTo。closePath()会完成封闭这个动作。
context.fillStyle='green';
context.fill(); 建议在进行完所有的设置之后再fill和stroke。不然fill会覆盖一部分的线段宽度，会由10变成5;
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You can encapsulate various graphics functions through the above methods. Of course, canvas also provides a lot of graphics API
context.rect(x,y,width,height);绘制矩形的路径
context.fillRect(x,y,width,height);使用当前fillStyle绘制矩形 ( 颜色值可以使用css认同的所有颜色值包括rgb、rgba、hsl、hsla)
context.strokeRect(x,y,width,height);使用当前的strokeStyle绘制矩形
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If the graphics drawn later overlap with the previous graphics, the latter will overwrite the former.
But if the color value of fillStyle or strokeStyle has a certain degree of transparency, part of the covered part will still be displayed. Of course, there are other settings for the overlapping parts
Line attributes:
lineWidth=10;
lineCap=''; 线段的2段的样式。butt(default)、round圆头、square方头。round和squqre比默认的长线条宽度的一半（5）的的长度。
lineJoin;线段与线段之间连接的样式。miter(default)、bevel(斜截)、round(圆头)
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Graphic transformation
context.translate(x,y);移动坐标系
context.rotate(deg);旋转坐标系弧度制
context.scale(x,y);缩放坐标系--->需要注意的是，它也会对坐标数值，边框宽度属性等进行缩放，使用时需要谨慎选择。
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Tip: After a transformation, the coordinate system has changed. If the transformation is used, it will be different from what was expected before. Then we need to perform the previous transformation in reverse, such as context.translate(100,200). If we still need to perform a rotate transformation, we need to perform another context.translate(-100,-200) operation. This operation will be very troublesome. Canvas provides a simple method to save the canvas state before transformation. As follows
context.save();
context.translate(x,y);
context.restore();
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Perform the save operation before the transformation, and then the restore operation after the transformation is completed to restore the previous canvas state to avoid the first transformation affecting the second transformation.
Transformation matrix---canvas provides a more advanced method for transformation.
contex.transform(a,b,c,d,e,f); a、d-水平垂直缩放；b、c水平垂直倾斜；e、f水平垂直位移。
context.transform(1,0,0,1,0,0);基本矩阵，图形不发生任何变化。
当我们进行多次transform矩阵之后，标准系以及很混乱了，就需要下面这个方法了。
context.setTransform(a,b,e,d,e,f)。覆盖之前的transform设置，使用当前的transform设置，这样我们就很清楚当前具体transform变换。
context.fillStyle ---填充样式 之 渐变
linearGradient:
var grd=context.creatLinearGradient(xStart,yStrart,xEnd,yEnd); 定义渐变色。在（xEnd,yEnd）点之后，填充的是最后的颜色值。
grd.addColorStop(stop,color);(stop的值，0-1的浮点数) 一般多个颜色断点配合使用，从而实现多颜色的渐变
context.fillStyle=grd;
context.fill();
radialGradient;
var grd=context.createRadialGradient(x0,y0,r0,x1,y1,r1); 定义2个同心圆，（x0,y0）位置的半径r0
gra.addColorStop(stop,color);
context.fillStyle=grd;
context.fill();
createPattern
var pattern=context.createPattern(img || canvas || video , repeat-style);
Img的值可以使一张图片，也可以使一个canvas画布，甚至是一段video
repeat-style的值：no-repeat、repeat-x、repeat-y、repeat
context.fillStyle=pattern;
context.fill();
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Note: The above explains the value of fillStyle: color, gradient, img, canvas, video. Of course, the above style values also apply to strokeStyle.
Draw an arc---arc
context.arc(centerX , centerY , radius , startAngle , endAngle , anticlockwise=false)，
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These parameters represent the center coordinates, radius, starting and ending radians, and whether it is counterclockwise (Default false)
You can encapsulate some methods to draw circles, round lines, rounded rectangles, etc.
contex.arcTo(x1, y1, x2, y2, radius);
The current point (x0, y0) and (x1, The three points y1) and (x2, y2) combine to form an angle, and the arcs tangent to the two sides are the effect of arcTo drawing. The starting point is the current point and the end point (x2, y2). Of course, this is the effect of a straight line and an arc, and radius is the radius of the arc. As shown below.
Applicable scenarios: Generally, drawing an arc requires the center coordinates of the circle. Using arcTo we can draw an arc without knowing the coordinates of the center of the circle.
Insert a paragraph:
When we use canvas to encapsulate some graphics drawing functions, the general steps are to first use ctx.save() and ctx.restore() wraps the beginning and end of the function. Make some style settings, transformation settings, and a function for drawing graphics paths. This path function starts and ends with ctx.beginPath() and ctx.closePath(), and uses some moveTo, lineTo, arc, arcTo and other methods.
Quadratic Bezier curve quadraticCurveTo(x1, y1, x2, y2);------http://tinyurl.com/html5quadratic
ctx.moveTo(x0,y0);
ctx.quadraticCurveTo(x1，y1，x2，y2);
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In this way, an arc is drawn with (x0, y0) as the starting point and (x2, y2) as the end point. Then such an arc is not necessarily a circular arc.
cubic Bezier curve bezierCurveTo(x1, y1, x2, y2, x3, y3)-----http://tinyurl.com/ html5bezier
ctx.moveTo(x0,y0);
ctx.bezierCurveTo(x1，y1，x2，y2，x3，y3)
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这样就绘制一段以（x0,y0）为起点，（x3,y3）为终点的一段弧。 这么一段弧度可以绘制二次曲线不能完成的波浪弧线。
canvas中文字的绘制
ctx.font = "bold 40px Arial";
ctx.fillStyle=;
ctx.fillText(string ，x ，y，[maxlen]);在（x,y）位置绘制string 这个字符串
ctx.strokeStyle=;
ctx.strokeText(string，x，y，[maxlen]);在（x，y）位置绘制string 这个字符串，这个文字只有文字的边框，并没有填充
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string 这个字符串如果设置了maxlen这个可选参数，那么这段文字就会强制压缩在maxlen这个宽度中。
当然，我们也可以设置fillStyle为一段纹理背景，在绘制文字，那么就会一段带纹理背景的文字。
文字细节部分
ctx.font="20px sans-serif"(默认)，如果需要设置，只是这2个值
font：font-style、font-varient、font-weight、font-size、font-family（一个5个属性，和css基本相似）
font-style:normal、italic(斜体)、oblique（倾斜字体）。一般的web页面中italic和oblique是一样的，除非使用自己导入的倾斜字体。
font-varient：normal、small-caps(使用小型大写字母 替代 小写字母，大小和小写一样，只是他是大写的)
font-weight：lighter、normal、bold、bolder。一般浏览器，前2者一样，后2者一样。
font-size：px、em、%、large等
font-family：支持设置多种字体备用、支持@font-face、支持web安全字体
ctx.textAlign=left、center、right。
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分别是以fillText(string，x，y);中x坐标为左边界、中间线、右边界绘制文字。（注意：left、right分别是指的边界）
ctx.textBaseLine=top、middle、bottom;
分别是以fillText(string，x，y);中x坐标为上边界、中间线、下边界绘制文字。同样也是边界。
另外还有alphabetic（默认，英文等）、ideographic（汉字，日语）、hanging（印度）三种值分别对应三类语言设置基准线
ctx.measureText(string).width-------根据对font设置之后，返回一个一段文字渲染的宽度
暂时不支持其他的属性如height等
阴影
ctx.shadowColor --->CSS接受的颜色值均可
ctx.shadowOffsetX --->x、y方向上的偏移 可为负值
ctx.shadowOffsetY
ctx.shadowBlur --->模糊 0--> 越大越模糊
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只需要设置shadowColor 和下面的任何一个属性，阴影就会出现，一般需要全部设置。
canvas绘制 均可设置shadow阴影
global属性 对全局设置
ctx.globalAlpha=1(default) 对全局设置透明度
ctx.globalCompositeOperation= 前后绘制图形重叠部分的样式 共有11种样式
"source-over"(default) : 后绘制的图形覆盖前绘制的图形（默认）
"destination-over":前绘制的图形覆盖后绘制的图形
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11种样式如下:
source-oversource-atopsource-insource-out
destination-over destination-atopdestination-indestination-out
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lighter(颜色计算)copy（只保留后者）xor(清除重叠部分)
剪辑区域clip-----》使用刚刚规划的路径 为剪辑区域，那么我们看到，也只是这个剪辑区域的图形
ctx.beginPath();
ctx.arc(400,400,200,0,Math.PI*2);
ctx.clip(); --->有绘制了一个圆形的剪辑区域
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>canvas之globalCompositeOperation</title>
<style type="text/css">
body{
background-color: #ccc;
}
</style>
</head>
<body>
<canvas id="canvas" style="display: block;margin: 50px auto;border:1px solid #ccc; background-color: #FFF"></canvas>
</body>
<script type="text/javascript">
var canvas=document.getElementById('canvas');
var WIDTH=1500;
var HEIGHT=800;
canvas.width=WIDTH;
canvas.height=HEIGHT;
var CIRCLES=[];
var COLORS=["0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f"];
var NUMS=220;
var SPEEDX=16;
var SPEEDY=8;
var RADIUS=25;
window.onload=function(){
if(canvas){
var context=canvas.getContext('2d');
gloablCompositeOperation(context);
}else{
console.log('浏览器不支持canvas，请更新浏览器！')
}
}
function gloablCompositeOperation(context){
context.clearRect(0, 0, WIDTH, HEIGHT);
context.globalCompositeOperation="xor";
context.globalAlpha=1;
addCircles(CIRCLES,NUMS);
paintCircles(context,CIRCLES);
//setInterval(run(context),90); //setInterval、setTimeout不能直接传入带参数的方法调用。 一般有2中方法，1、使用匿名方法包装下。
var interval=setInterval(function(){
go(context);
},90);
}
/*function run(context){ //2、定义一个带参数,返回一个无参方法，在返回的无参方法中调用之前带参方法。
return function(){
go(context);
}
}*/
function go(context){
updateCirCles(CIRCLES);
paintCircles(context,CIRCLES);
}
/*绘制一个圆对象*/
function paintCircles(ctx ,CIRCLES){
ctx.clearRect(0, 0, WIDTH, HEIGHT);
for(var i=0 ; i<CIRCLES.length ; i++){
ctx.beginPath();
ctx.arc(CIRCLES[i].x,CIRCLES[i].y,CIRCLES[i].r,0,Math.PI*2);
ctx.closePath();
ctx.fillStyle=CIRCLES[i].color;
ctx.fill();
}
}
/*随机生成新的圆对象，加入到数组*/
function addCircles(CIRCLES,num){
for(var i=0 ; i<num ;i++){
var circle={};
var radius=(1+Math.random())*RADIUS;
var cX=Math.random()*WIDTH;
var cY=Math.random()*HEIGHT;
cX=cX<radius?radius:cX;
cY=cY<radius?radius:cY;
cX=cX>(WIDTH-radius)?(WIDTH-radius):cX;
cY=cY>(HEIGHT-radius)?(HEIGHT-radius):cY;
circle.x=cX;
circle.y=cY;
circle.r=radius;
circle.color=createRandomRGBColor();
circle.vX=(Math.floor(Math.random()*10)%2)==0?SPEEDX:(-1)*SPEEDX;
circle.vY=(Math.floor(Math.random()*10)%2)==0?SPEEDY:(-1)*SPEEDY;
circle.vX=circle.vX+Math.random()*6;
circle.vY=circle.vY+Math.random()*3;
CIRCLES[i]=circle;
}
}
function updateCirCles(CIRCLES){
for(var i=0 ; i<CIRCLES.length ; i++){
var width=WIDTH-CIRCLES[i].r;
var height=HEIGHT-CIRCLES[i].r;
if(CIRCLES[i].x + CIRCLES[i].vX>width){
CIRCLES[i].x=CIRCLES[i].x;
CIRCLES[i].vX=CIRCLES[i].vX*(-1);
}else if(CIRCLES[i].y + CIRCLES[i].vY>height){
CIRCLES[i].y=CIRCLES[i].y;
CIRCLES[i].vY=CIRCLES[i].vY*(-1);
}else if(CIRCLES[i].x + CIRCLES[i].vX<CIRCLES[i].r){
CIRCLES[i].x=CIRCLES[i].x;
CIRCLES[i].vX=CIRCLES[i].vX*(-1);
}else if(CIRCLES[i].y + CIRCLES[i].vY<CIRCLES[i].r){
CIRCLES[i].y=CIRCLES[i].y;
CIRCLES[i].vY=CIRCLES[i].vY*(-1);
}else{
CIRCLES[i].x=CIRCLES[i].x + CIRCLES[i].vX;
CIRCLES[i].y=CIRCLES[i].y + CIRCLES[i].vY;
}
}
}
//随机生成rgb颜色值"#123abc"
function createRandomRGBColor(){
var color="";
for(var i=0 ; i<6 ; i++){
color+=COLORS[Math.floor(Math.random()*COLORS.length)];
}
return "#"+color;
}
//随机生成rgba颜色值"#123abc"
function createRandomRGBAColor(){
var color="";
for(var i=0 ; i<3 ; i++){
color+=Math.floor(Math.random()*255)+",";
}
return "rgba("+color+Math.random()+")"; //"rgba("+color+Math.random()+")"
}
</script>
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