Detailed explanation of the use of canvas's drawing api
This time I will bring you a detailed explanation of the use of canvas's drawing api. What are the things to note when using canvas's drawing api? The following is a practical case, let's take a look. Canvas can draw many wonderful styles and beautiful effects. Through a few simple APIs, you can present ever-changing effects on the canvas. You can also create web games. Next, I will summarize what is related to drawing. API.
When painting, canvas is equivalent to the canvas, and context is equivalent to the brush.
1. Draw lines moveTo(x0,y0): Move the current brush (ictx) to the position (x0,y0) .
lineTo(x1,y1): Draw a straight line from the current position (x0, y0) to (x1, y1).
beginPath(): Open a path or reset the current path.
closePath(): Return to the starting point of the path from the current point, which is the position, avoidance and path of the previous beginPath.
stroke(): draw. This
function must be added before the picture can be drawn, so this must be placed at the end. var icanvas=document.getElementById("iCanvas");
var ictx=icanvas.getContext("2d");
ictx.beginPath();
ictx.moveTo(0,0);
ictx.lineTo(300,150);
ictx.lineTo(3,150);
ictx.closePath();
ictx.stroke();
It should be noted here that if closepath is placed after the stroke function, it will not be drawn into a closed line, because before closing , has been drawn, so the straight line on the left will not be drawn.
2. Line stylelineCap: Line endpoint style, butt, round, square. through miterLimet can also be used to set the maximum length of the inflection point junction.
miterLimet: If the miter length exceeds the value of miterLimit, the corners will be displayed with the "bevel" type of lineJoin.
lineWidth: line width
strokeStyle: line color, gradient (defined gradient
object
var iCanvas=document.getElementById("iCanvas");
var ictx=iCanvas.getContext("2d");
ictx.beginPath();
ictx.strokeStyle="#0000ff";
ictx.lineWidth=20;
ictx.lineCap="round";
ictx.moveTo(10,10);
ictx.lineTo(80,80);
ictx.stroke();
ictx.beginPath();//在这里必须beginPath，不然一直会以第一个为基础会话，在最后的stroke的时候，会再次画一条黑色的斜线，一共3条线。
ictx.strokeStyle="#000000";
ictx.lineCap="butt";
ictx.lineWidth=10;
ictx.moveTo(80,10);
ictx.lineTo(10,80);
ictx.stroke();arcTo(x1,y1,x2,y2,radius): draw two The curve before the tangent. ictx.beginPath();
ictx.moveTo(20,20); // 创建开始点
ictx.lineTo(100,20); // 创建水平线
ictx.arcTo(150,20,150,70,50); // 创建弧
ictx.lineTo(150,120); // 创建垂直线
ictx.stroke();
quadraticCurveTo(x1,y1,x2,y2): quadratic Bezier curve. (x1, y1) The coordinates of the control point, (x2, y2) the coordinates of the end point
bezierCurveTo(x1,y1,x2,y2,x,y): cubic Bezier curve. (x1, y1) The coordinates of control point 1, (x2, y2) The coordinates of control point 2 (x, y) The coordinates of the end point.
Bezier curves are very useful when drawing some very smooth curves. 
fillRect(x,y,width ,height): Draw a filled rectangle: (x, y) starting point, width, height rectangle width and height
strokeRect(): Draw a rectangular wireframe clearRect(): Clear the rectangle.ictx.fillStyle="#0000ff";//设定填充颜色 ictx.fillRect(20,20,150,100); ictx.strokeRect(180,20,100,100);
fillStyle:设置填充的颜色，渐变或模式（patten）；
strokeStyle：画笔的颜色，渐变或者模式
6.绘制阴影
shadowColor：阴影yanse
shadowBlur:模糊级别
shadowOffsetX：阴影的水平距离
shadowOffsetY：阴影的垂直距离
ictx.shadowBlur=20; ictx.shadowColor="#456"; ictx.shadowOffsetX=-10; ictx.shadowOffsetY=30;//先设置阴影再画矩形 ictx.fillStyle="#108997"; ictx.fillRect(20,20,100,80); ictx.stroke();
7.绘制渐变
createLinearGradient(x1,y1,x2,y2):绘制线性渐变，(x1,y1)是渐变的起始点，（x2,y2）是渐变的终点，位置不同可以制作出垂直或者水平渐变。
createRadialGradient(x1,y1,r1,x2,y2,r2)：径向渐变：，(x1,y1)是渐变的起始点圆心，r1是半径，（x2,y2）是渐变的终点，r2是结束点半径;
两种渐变都需要使用
addColorStop(stop,color)来设置渐变过程，stop是0.0到1.0的值。
var grd=ictx.createLinearGradient(0,0,170,0); grd.addColorStop(0,"#000"); grd.addColorStop(0.5,"#378923"); grd.addColorStop(1,"#ddd"); ictx.fillStyle=grd;//这里渐变是一个对象，用来向fillstyle传值 ictx.fillRect(20,20,150,100); var grd=ictx.createRadialGradient(300,225,15,250,225,100); grd.addColorStop(0,"#345"); grd.addColorStop(1,"#fff"); ictx.fillStyle=grd; ictx.fillRect(200,150,150,100);
8.填充背景
createPattern(image,"repeat|repeat-x|repeat-y|no-repeat"):image是一个图片对象，后面的参数是用来设定图片的重复方式。
9.其他相关API
fill():填充当前路径。
isPointInPath():ictx.isPointInPath(x,y);判断这个点是否位于当前路径
清除画布方法：获取画布的宽高，icanvas.height,icanvas.width;然后使用clearRect();
修改画布的宽高：icanvas.width='200'；icanvas.width='300'的方法。
globalAlpha:设置透明度，只能是0~1的数字，如果透明度不一样，在画第二幅之前重新设置即可。
toDataURL:icanvas.toDataURL(type,encoderOptions)，这个函数返回一个image的base64的URI，参数都是可选的，type可以设置图片类型如image/jpeg,image/webp，默认是image/png；encoderOptions是一个0~1的数字，用来设置image/jpeg,image/webp的图片质量，其他格式的type设置这个参数无效。
10.剪裁
clip():从画布中剪裁任意形状和尺寸的画布，之后所有的绘图都会被限制在剪裁的区域内。这个方法通常和绘制矩形，圆形等路径一起使用，在这些方法后面，剪切这个图像，后来画的就必须在这个剪切后的画布上了。
ictx.arc(100,100,50,(Math.PI/180)*0,(Math.PI/180)*360,true); ictx.stroke(); ictx.clip(); ictx.fillStyle="green"; ictx.fillRect(0,0,150,100);
如果还想操作外部的画布，在剪切前使用save()函数保存，剪切后使用restore()函数恢复到之前保存的状态，但是中间做的操作不会消失哈。
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