Canvas入门（2）:图形渐变和图像形变换

Bertha 。 2022-08-13 04:58 101阅读 0赞

来源：[http://www.ido321.com/986.html][http_www.ido321.com_986.html]

一、图形渐变（均在最新版Google中测试）

1、绘制线性渐变

1: // 获取canvas 的ID

2:     var canvas = document.getElementById('canvas');

3:     if (canvas == null)

4:     {

5:         return false;

6:     }

7:     // 获取上下文

8:     var context = canvas.getContext('2d');

9:     // 获取渐变对象

10:     var g1 = context.createLinearGradient(0,0,0,300);

11:     // 添加渐变颜色

12:     g1.addColorStop(0,'rgb(255,255,0)');

13:     g1.addColorStop(1,'rgb(0,255,255)');

14:     context.fillStyle = g1;

15:     context.fillRect(0,0,400,300);

16:     var g2 = context.createLinearGradient(0,0,300,0);

17:     g2.addColorStop(0,'rgba(0,0,255,0.5)');

18:     g2.addColorStop(1,'rgba(255,0,0,0.5)');

19:     for(var i = 0; i<10;i++)

20:     {

21:         context.beginPath();

22:         context.fillStyle=g2;

23:         context.arc(i*25, i*25, i*10, 0, Math.PI * 2, true);

24:         context.closePath();

25:         context.fill();

26:     }

createLinearGradient(x1,y1,x2,y2):参数分别表示渐变起始位置和结束位置的横纵坐标

addColorStop(offset,color):offset表示设定的颜色离渐变起始位置的偏移量，取值范围是0~1的浮点值。渐变起始偏移量是0，渐变结束偏移量是1.color是渐变的颜色。

效果：

![SouthEast][]

2、绘制径向渐变

1: // 获取canvas 的ID

2:     var canvas = document.getElementById('canvas');

3:     if (canvas == null)

4:     {

5:         return false;

6:     }

7:     // 获取上下文

8:     var context = canvas.getContext('2d');

9:     // 获取渐变对象

10:     var g1 = context.createRadialGradient(400,0,0,400,0,400);

11:     // 添加渐变颜色

12:     g1.addColorStop(0.1,'rgb(255,255,0)');

13:     g1.addColorStop(0.3,'rgb(255,0,255)');

14:     g1.addColorStop(1,'rgb(0,255,255)');

15:     context.fillStyle = g1;

16:     context.fillRect(0,0,400,300);

17:     var g2 = context.createRadialGradient(250,250,0,250,250,300);

18:     g2.addColorStop(1,'rgba(0,0,255,0.5)');

19:     g2.addColorStop(0.7,'rgba(255,255,0,0.5)')

20:     g2.addColorStop(0.1,'rgba(255,0,0,0.5)');

21:     for(var i = 0; i<10;i++)

22:     {

23:         context.beginPath();

24:         context.fillStyle=g2;

25:         context.arc(i*25, i*25, i*10, 0, Math.PI * 2, true);

26:         context.closePath();

27:         context.fill();

28:     }

createRadialGradient(x1,y1,radius1,x2,y2,radius2):x1,y1,radius1分别是渐变开始圆的圆心横纵坐标和半径。x2,y2,radius2分别是渐变结束圆的圆心横纵坐标和半径。

效果

![SouthEast 1][]

二、图形变换

1、坐标变换：平移、缩放和旋转

1: // 获取canvas 的ID

2:     var canvas = document.getElementById('canvas');

3:     if (canvas == null)

4:     {

5:         return false;

6:     }

7:     // 获取上下文

8:     var context = canvas.getContext('2d');

9:     context.fillStyle = '#eeeeff';

10:     context.fillRect(0,0,400,300);

11:     // 平移坐标原点

12:     context.translate(200,50);

13:     context.fillStyle = 'rgba(255,0,0,0.25)';

14:     for(var i = 0; i<50;i++)

15:     {

16:         context.translate(25,25);

17:         // 图形缩放

18:         context.scale(0.95,0.95);

19:         // 图形旋转

20:         context.rotate(Math.PI / 10);

21:         context.fillRect(0,0,100,50);

22:     }

translate(x,y):平移原点，x,y表示向左和向下移动多少单位，默认单位是像素

scale(x,y):缩放，x,y表示水平和垂直方向的缩放大小。小于1缩小，大于1放大。

rotate(angle)：旋转，angle是旋转角度，单位是弧度。大于0表示顺时针旋转，反之逆时针。

效果：

![SouthEast 2][]

2、矩阵变换

1: // 获取canvas 的ID

2:     var canvas = document.getElementById('canvas');

3:     if (canvas == null)

4:     {

5:         return false;

6:     }

7:     // 获取上下文

8:     var context = canvas.getContext('2d');

9:     // 定义颜色

10:     var colors = ['red','orange','yellow','green','blue','navy','purple'];

11:     // 定义线宽

12:     context.lineWidth = 10;

13:     // 矩阵变换

14:     context.transform(1,0,0,1,100,0);

15:     // 循环绘制圆弧

16:     for (var i = 0; i < colors.length; i++)

17: \{ //原点每次下移10个px

18:          context.transform(1,0,0,1,0,10);

19:         context.strokeStyle = colors[i];

20:         context.beginPath();

21:         context.arc(50,100,100,0,Math.PI,true);

22:         context.stroke();

23:     }

transform(m11,m12,m21,m22,dx,dy):改方法使用一个新的变换矩阵与当前变换矩阵进行乘法运算。dx,dy表示原点坐标左移和下移的单位，默认是像素。

该变换矩阵格式如下

m11     m12   dx

m21      m22  dy

0            0         1

最终效果：

![SouthEast 3][]

总结:坐标变换的方法均可以用transform()代替，规则如下：

1、translate(x,y)  <=>  transform(1,0,0,1,dx,dy)或transform(0,1,1,0,dx,dy)，前四个参数表示不对图形进行缩放操作。

2、scale(x,y) <=> transform(x,0,0,y,0,0)或transform(0,y,x,0,0,0),后面两个参数表示不进行平移。

3、rotate(angle) <=> transform(Math.cos(angle\*Math.PI/180),Math.sin(angle\*Math.PI/180),-Math.sin(angle\*Math.PI/180),Math.cos(angle\*Math.PI/180),0,0)或

transform(-Math.sin(angle\*Math.PI/180),Math.cos(angle\*Math.PI/180),Math.cos(angle\*Math.PI/180),Math.sin(angle\*Math.PI/180),0,0)

下一篇：[Canvas入门（3）:图像处理和绘制文字][Canvas_3]

[http_www.ido321.com_986.html]: http://www.ido321.com/986.html
[SouthEast]: /images/20220810/c4153f9caeef4bd7b9697f7380028cc5.png
[SouthEast 1]: /images/20220810/6ac35956bd5a442c910abfbcb5f0d7ca.png
[SouthEast 2]: /images/20220810/6aea2645e7d04ad1b66257e7ea50701d.png
[SouthEast 3]: /images/20220810/3c78c1f7e72b43f1ac51e86fde392bec.png
[Canvas_3]: http://www.ido321.com/997.html