Detailed explanation of the sample code of html5 Canvas drawing (1)-H5 Tutorial-php.cn

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Detailed explanation of the sample code of html5 Canvas drawing (1)

黄舟

Mar 28, 2017 pm 03:34 PM

Only with a strong foundation in drawing, coloring, and transforming basic two-dimensional graphics can we use Canvas more effectively;

Below, I will briefly understand how canvas draws basic shapes (rectangle, straight line, arc, Bezier curve), etc.;

First post all the following related The basic code segment to implement operation:

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context
var 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>

For all the following example codes, just replace the above function drawScreen()!

Basic Rectangle Shape (rectangle)

In Canvas, there are three ways to draw a rectangle: filling (), stroking(), or clearing

The three methods correspond to the following three methods (the parameters are all the same, which are the coordinates [x, y] of the upper left corner of the rectangle, the Width, height of rectangle):

#fillRect(x,y,width,height): fill the rectangular area;
##strokeRect(x,y,width,height): draw A rectangular border;

Example

function drawScreen() {
context.fillStyle = &#39;#000000&#39;;//填充颜色
context.strokeStyle = &#39;#00ff00&#39;;//轮廓颜色
context.lineWidth = 2;//线宽
context.fillRect(10, 10, 40, 40);//填充矩形
context.strokeRect(7, 7, 46, 46);//画矩形轮廓
context.clearRect(20, 20, 20, 20);//清除矩形区域
}

上一篇有提到Current state；

当我们在绘画时，我们可以利用所谓的绘画状态的堆栈，

对于canvas context在任何一个时间的数据的每一个状态都会存储；

下面是对于每一个状态，存储在堆栈中的一个数据列表；

变换矩阵（旋转、移动、缩放等）；

剪切区域；

Canvas特征的当前值（部分）：

— globalAlpha

— globalCompositeOperation

— strokeStyle

— textAlign, textBaseline

— lineCap, lineJoin, lineWidth, miterLimit

— fillStyle

— font

— shadowBlur, shadowColor, shadowOffsetX, and shadowOffsetY

在绘图环境中，正在操作的当前path and 当前位置并不是状态的一部分；Importmant!!!

如何保存恢复当前的状态呢？

context.save()---push to stack;

context.restore()---pop form stack;

先有一个简单的印象，之后会更加详细的剖析；

Create Lines（直线）

利用path来创建线（line）

Path:用来画出一系列的相连的圆弧或者线条，可以称之为“轨迹”，使用它可以画出任意复杂的形状;

一个Canvas Context仅有一个current path ;

在调用context.save()时，current path并不做为当前的状态（current state）存储于stack中；

利用.beginPath()功能方法来启动一个Path；

利用.closePath()功能方法来关闭一个Path；

Example:画一条10px宽度的水平直线

function drawScreen() {
    context.strokeStyle = "#000000";//线的颜色                    
    context.lineWidth = 10;//线的宽度
    context.beginPath();//启动path
    context.moveTo(20, 20);
    context.lineTo(100, 20);
    context.stroke();//绘画
    context.closePath();//关闭path
}

线的属性：lineCap
直线lineCap属性：线帽，也就是线两端的样式，只有绘制较宽的线的，它才有效；

有三个有效值：butt\round\square

"butt":默认值,指定了线段应该没有线帽。

"round":线段应该带有一个半圆形的线帽，半圆的直径等于线段的宽度，并且线段在端点之外扩展了线段宽度的一半。

"square":线段应该带有一个矩形线帽。这个值和 "butt" 一样，但是线段扩展了自己的宽度的一半。

Example

function drawScreen() {
context.strokeStyle = "#000000";//线的颜色                    
context.lineWidth = 10;//线的宽度
context.lineCap="butt";//butt\round\square
context.beginPath();//启动path
context.moveTo(20, 20);
context.lineTo(100, 20);
context.stroke();//绘画
context.closePath();//关闭path
                    
context.lineCap="round";//butt\round\square
context.beginPath();//启动path
context.moveTo(20, 40);
context.lineTo(100, 40);
context.stroke();//绘画
context.closePath();//关闭path
                    
context.lineCap="square";//butt\round\square
context.beginPath();//启动path
context.moveTo(20, 60);
context.lineTo(100, 60);
context.stroke();//绘画
context.closePath();//关闭path
}

线的属性：lineJoin

lineJoin属性：表示两条线段如何连接；

当一个路径包含了线段或曲线相交的交点的时候，用lineJoin 属性来说明如何绘制这些交点；

该属性也有三个有效值：miter bevel round

"miter":默认值，两条线段的外边缘一直扩展到它们相交

"bevel":以一个斜边进行连接

"round":以一个圆弧边进行连接

    function drawScreen() {
    context.strokeStyle = "#000000";
    context.lineWidth = 10;    
    context.lineJoin = "miter";
    context.beginPath();
    context.moveTo(20, 20);
    context.lineTo(100, 20);
    context.lineTo(100, 40);
    context.stroke();    
    context.closePath();                    


    context.lineJoin = "bevel";
    context.beginPath();
    context.moveTo(20, 60);
    context.lineTo(100, 60);
    context.lineTo(100, 80);
    context.stroke();
    context.closePath();


    context.lineJoin = "round";
    context.beginPath();
    context.moveTo(20, 100);
    context.lineTo(100, 100);
    context.lineTo(100, 120);
    context.stroke();
    context.closePath();

    context.lineJoin = "miter";
    context.beginPath();
    context.moveTo(20, 140);
    context.lineTo(100, 140);
    context.lineTo(80, 180);
    context.stroke();    
    context.closePath();
}

Arcs（圆弧）

一段圆弧可以是一个完整的圆也可以圆的一部分；

生成圆弧：context.arc()

context.arc(x, y, radius, startAngle, endAngle, anticlockwise)

参数依次代表圆心,半径、起始角度、终止角度、圆弧的方向；角度都是以弧度来表示；

anticlockwise为布尔类型 ;true为顺时针、false为逆时针

function drawScreen() {
context.strokeStyle = "black";
context.lineWidth = 5;
context.beginPath();
context.arc(100, 100, 20, (Math.PI / 180) * 0, (Math.PI / 180) * 360, false);
context.stroke();
context.closePath();

context.beginPath();
context.arc(100, 200, 20, (Math.PI / 180) * 0, (Math.PI / 180) * 90, false);
context.stroke();
context.closePath();

context.beginPath();
context.arc(100, 300, 20, (Math.PI / 180) * 0, (Math.PI / 180) * 90, true);
context.stroke();
context.closePath();
}

Bezier Curves（贝赛尔曲线）

Canvas支持二次 and 三次贝塞尔曲线的绘画

此处的贝塞尔曲线是定义在二维空间里的，需要一个起始点、一个终止点，再加上一个或者两个控制点来创建曲线；

控制点来决定所构造曲线的走向；

三次贝塞尔曲线需要两个点；

二次贝塞尔曲线需要一个点即可；

主要通过以下两个方法来绘画：

context.bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)

context.quadraticCurveTo(cpx, cpy, x, y)

二次贝塞尔曲线:
function drawScreen()
{
context.strokeStyle = "black";
context.lineWidth = 5;
context.beginPath();
context.moveTo(0, 0);
context.quadraticCurveTo(500, 25, 0, 100);
context.stroke();
context.closePath();
}
曲线的起始点为（0，0），结束点为（0，100）
点（500，25）控制最终生成曲线的走向；

三次贝塞尔曲线：
 function drawScreen()
 {
     context.strokeStyle = "black";
     context.lineWidth = 5;
     context.beginPath();
     context.moveTo(0, 0);
     context.bezierCurveTo(0, 125, 300, 175, 150, 300);
     context.stroke();
     context.closePath();
 }
 曲线的起点（0，0），结束点（150，300）
 （0, 125）, （300, 175）这两个为控制点；
 大家可能自己运行一下代码，看看效果，此处就不贴图了。。

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