# OffscreenCanvasRenderingContext2D对象

> ![icon-note.gif](public_sys-resources/icon-note.gif) **说明：**
> 从API version 7开始支持。后续版本如有新增内容，则采用上角标单独标记该内容的起始版本。


使用OffscreenCanvasRenderingContext2D在offscreen canvas上进行绘制，绘制对象可以是矩形、文本、图片等


## 属性

除支持与CanvasRenderingContext2D对象相同的属性外，还支持如下属性：

| 属性 | 类型 | 描述 |
| -------- | -------- | -------- |
| filter | string | 设置图像的滤镜。<br/>支持的滤镜效果如下：<br/>-&nbsp;blur：给图像设置高斯模糊<br/>-&nbsp;brightness：给图片应用一种线性乘法，使其看起来更亮或更暗<br/>-&nbsp;contrast：调整图像的对比度<br/>-&nbsp;drop-shadow：给图像设置一个阴影效果<br/>-&nbsp;grayscale：将图像转换为灰度图像<br/>-&nbsp;hue-rotate：给图像应用色相旋转<br/>-&nbsp;invert：反转输入图像<br/>-&nbsp;opacity：转化图像的透明程度<br/>-&nbsp;saturate：转换图像饱和度<br/>-&nbsp;sepia：将图像转换为深褐色 |

- 示例
  ```
  <!-- xxx.hml -->
  <div style="width: 180px; height: 60px;">
    <canvas ref="canvasId" style="width: 500px; height: 500px; background-color: #ffff00;"></canvas>
  </div>
  ```

  ```
  //xxx.js
  export default {
    onShow(){
      var ctx = this.$refs.canvasId.getContext('2d');
      var offscreen = new OffscreenCanvas(360, 500);
      var offCanvas2 = offscreen.getContext("2d");
      var img = new Image();
      img.src = 'common/images/flower.jpg';
      offCanvas2.drawImage(img, 0, 0, 100, 100);
      offCanvas2.filter = 'blur(5px)';
      offCanvas2.drawImage(img, 100, 0, 100, 100);
  
      offCanvas2.filter = 'grayscale(50%)';
      offCanvas2.drawImage(img, 200, 0, 100, 100);
  
      offCanvas2.filter = 'hue-rotate(90deg)';
      offCanvas2.drawImage(img, 0, 100, 100, 100);
  
      offCanvas2.filter = 'invert(100%)';
      offCanvas2.drawImage(img, 100, 100, 100, 100);
  
      offCanvas2.filter = 'drop-shadow(8px 8px 10px green)';
      offCanvas2.drawImage(img, 200, 100, 100, 100);
  
      offCanvas2.filter = 'brightness(0.4)';
      offCanvas2.drawImage(img, 0, 200, 100, 100);
  
      offCanvas2.filter = 'opacity(25%)';
      offCanvas2.drawImage(img, 100, 200, 100, 100);
  
      offCanvas2.filter = 'saturate(30%)';
      offCanvas2.drawImage(img, 200, 200, 100, 100);
  
      offCanvas2.filter = 'sepia(60%)';
      offCanvas2.drawImage(img, 0, 300, 100, 100);
  
      offCanvas2.filter = 'contrast(200%)';
      offCanvas2.drawImage(img, 100, 300, 100, 100);
      var bitmap = offscreen.transferToImageBitmap();
      ctx.transferFromImageBitmap(bitmap);
    }
  }
  ```

  ![zh-cn_image_0000001152773860](figures/zh-cn_image_0000001152773860.png)


## 方法

除支持与CanvasRenderingContext2D对象相同的方法外，还支持如下方法：


### isPointInPath

isPointInPath(path?: Path2D, x: number, y: number): boolean

判断指定点是否在路径的区域内。

- 参数
  | 参数名 | 参数类型 | 必填 | 描述 |
  | -------- | -------- | -------- | -------- |
  | path | Path2D | 否 | 可选对象，指定用来判断的路径。若没有设置，则使用当前路径。 |
  | x | number | 是 | 待判断点的x轴坐标。 |
  | y | number | 是 | 待判断点的y轴坐标。 |

- 返回值
  | 类型 | 说明 |
  | -------- | -------- |
  | boolean | 指定点是否在路径的区域内。 |

- 示例
  ```
  <!-- xxx.hml -->
  <div style="width: 180px; height: 60px;">
    <text>In path:{{textValue}}</text>
    <canvas ref="canvas" style="width: 500px; height: 500px; background-color: #ffff00;"></canvas>
  </div>
  ```

  ```
  // xxx.js
  export default {
    data: {
      textValue: 0
    },
    onShow(){
      var canvas = this.$refs.canvas.getContext('2d');
      var offscreen = new OffscreenCanvas(500,500);
      var offscreenCanvasCtx = offscreen.getContext("2d");
  
      offscreenCanvasCtx.rect(10, 10, 100, 100);
      offscreenCanvasCtx.fill();
      this.textValue = offscreenCanvasCtx.isPointInPath(30, 70);
  
      var bitmap = offscreen.transferToImageBitmap();
      canvas.transferFromImageBitmap(bitmap);
    }
  }
  ```

  ![zh-cn_image_0000001224354967](figures/zh-cn_image_0000001224354967.png)


### isPointInStroke

isPointInStroke(path?: Path2D, x: number, y: number): boolean

判断指定点是否在路径的边缘线上。


- 参数
  | 参数名 | 参数类型 | 必填 | 描述 |
  | -------- | -------- | -------- | -------- |
  | path | Path2D | 否 | 可选对象，指定用来判断的路径。若没有设置，则使用当前路径。 |
  | x | number | 是 | 待判断点的x轴坐标。 |
  | y | number | 是 | 待判断点的y轴坐标。 |

- 返回值
  | 类型 | 说明 |
  | -------- | -------- |
  | boolean | 指定点是否在路径的区域内。 |

- 示例
  ```
  <!-- xxx.hml -->
  <div style="width: 180px; height: 60px;">
    <text>In path:{{textValue}}</text>
    <canvas ref="canvas" style="width: 500px; height: 500px; background-color: #ffff00;"></canvas>
  </div>
  ```

  ```
  // xxx.js
  export default {
    data: {
      textValue: 0
    },
    onShow(){
      var canvas = this.$refs.canvas.getContext('2d');
      var offscreen = new OffscreenCanvas(500,500);
      var offscreenCanvasCtx = offscreen.getContext("2d");
  
      offscreenCanvasCtx.rect(10, 10, 100, 100);
      offscreenCanvasCtx.stroke();
      this.textValue = offscreenCanvasCtx.isPointInStroke(50, 10);
  
      var bitmap = offscreen.transferToImageBitmap();
      canvas.transferFromImageBitmap(bitmap);
    }
  }
  ```

  ![zh-cn_image_0000001178875308](figures/zh-cn_image_0000001178875308.png)


### resetTransform

resetTransform(): void

- 示例
  ```
  <!-- xxx.hml -->
  <div style="width: 180px; height: 60px;">
    <text>In path:{{textValue}}</text>
    <canvas ref="canvas" style="width: 500px; height: 500px; background-color: #ffff00;"></canvas>
  </div>
  ```

  ```
  //xxx.js
  export default {
    data:{
     textValue:0
    },
    onShow(){
     var canvas = this.$refs.canvas.getContext('2d');
     var offscreen = new OffscreenCanvas(500,500);
     var offscreenCanvasCtx = offscreen.getContext("2d");
  
     offscreenCanvasCtx.transform(1, 0, 1.7, 1, 0, 0);
     offscreenCanvasCtx.fillStyle = 'gray';
     offscreenCanvasCtx.fillRect(40, 40, 50, 20);
     offscreenCanvasCtx.fillRect(40, 90, 50, 20);
  
     // Non-skewed rectangles
     offscreenCanvasCtx.resetTransform();
     offscreenCanvasCtx.fillStyle = 'red';
     offscreenCanvasCtx.fillRect(40, 40, 50, 20);
     offscreenCanvasCtx.fillRect(40, 90, 50, 20);
  
     var bitmap = offscreen.transferToImageBitmap();
     canvas.transferFromImageBitmap(bitmap);
    } 
  }
  ```

  ![zh-cn_image_0000001179035242](figures/zh-cn_image_0000001179035242.png)