HarmonyOS 保存图片的时候按照某个形状裁剪（非规则图形即异形图）

参考下这个demo：

import { photoAccessHelper } from '@kit.MediaLibraryKit'
import fs from '@ohos.file.fs';
import { util } from '@kit.ArkTS';
@Entry
@Component
struct GlobalCompositeOperation {
  private settings: RenderingContextSettings = new RenderingContextSettings(true)
  private context: CanvasRenderingContext2D = new CanvasRenderingContext2D(this.settings)
  private offCanvas: OffscreenCanvas = new OffscreenCanvas(600, 600)
  private img:ImageBitmap = new ImageBitmap("ic_public_refresh.png") //mask
  private img2:ImageBitmap = new ImageBitmap("background.png") //原图
  private offContext = this.offCanvas.getContext("2d", this.settings)
  private image?:ImageBitmap = undefined
  @State imageURL?:string = undefined
  @State canvasWidth:number = 0
  @State canvasHeight:number = 0


  aboutToAppear(): void {
    this.canvasWidth = this.img2.width
    this.canvasHeight = this.img2.height
    console.log(`${this.canvasWidth }`)
    console.log(`${this.canvasHeight }`)
  }

  build() {
    Flex({ direction: FlexDirection.Column, alignItems: ItemAlign.Center, justifyContent: FlexAlign.Center }) {
      Canvas(this.context)
        .backgroundColor(Color.Yellow)
        .width(this.canvasWidth)
        .height(this.canvasHeight)
        .onReady(() =>{
          this.offContext.drawImage(this.img2, 0, 0, this.canvasWidth, this.canvasHeight, 0, 0,this.canvasWidth, this.canvasHeight)
          this.image = this.offCanvas.transferToImageBitmap()
          this.context.transferFromImageBitmap(this.image)
        })


      Button('遮罩').onClick(() => {
        this.offContext.drawImage(this.img, 0, 0, this.img.width, this.img.height, 0, 0, this.img.width, this.img.height)
        this.image = this.offCanvas.transferToImageBitmap()
        this.context.transferFromImageBitmap(this.image)
      })
      Button('裁剪').onClick(() =>{
        this.context.reset()
        this.offContext.drawImage(this.img2, 0, 0, this.canvasWidth, this.canvasHeight, 0, 0,this.canvasWidth, this.canvasHeight)
        this.offContext.globalCompositeOperation = 'destination-in'
        this.offContext.drawImage(this.img, 0, 0, this.img.width, this.img.height, 0, 0, this.img.width, this.img.height)
        this.image = this.offCanvas.transferToImageBitmap()
        this.context.transferFromImageBitmap(this.image)
        this.canvasWidth = this.img.width
        this.canvasHeight = this.img.height
      })
      SaveButton().onClick(async () =>{
        try {
          this.imageURL = this.context.toDataURL()
          let base64Helper = new util.Base64Helper()
          let imageStr = this.imageURL.split(',')
          let arrayBuffer = base64Helper.decodeSync(imageStr[1], util.Type.MIME).buffer as ArrayBuffer
          const context = getContext(this)
          let helper = photoAccessHelper.getPhotoAccessHelper(context)
          let uri = await helper.createAsset(photoAccessHelper.PhotoType.IMAGE, 'jpg')
          let file = await fs.open(uri, fs.OpenMode.READ_WRITE | fs.OpenMode.CREATE)
          await fs.write(file.fd, arrayBuffer);
          // 关闭文件
          await fs.close(file.fd);
        } catch (error) {
          console.error("error is " + JSON.stringify(error))
        }
      })


      // Image(this.imageURL!).width(this.img.width).height(this.img.height)
    }
    .width('100%')
    .height('100%')
  }
}