画布-在画布保存为图像后，用橡皮擦在画布上绘制黑线

Question

在画布上画画是非常好的。即使橡皮擦也能工作得很好。问题是当画布保存为图像时，它画的是黑线，而不是橡皮擦。

为了更好地理解，我添加了屏幕、镜头和代码。

1.在擦除图的同时-

erase(){
      this.ctx.globalCompositeOperation = 'destination-out';
    } 

handleMove(ev){

  // let ctx = this.canvasElement.getContext('2d');
  let currentX = ev.touches[0].pageX - this.offsetX;
  let currentY = ev.touches[0].pageY - this.offsetY;

  this.ctx.beginPath();
  this.ctx.lineJoin = "round";
  this.ctx.moveTo(this.lastX, this.lastY);
  this.ctx.lineTo(currentX, currentY);
  this.ctx.closePath();
  this.ctx.strokeStyle = this.currentColour;
  this.ctx.lineWidth = this.brushSize;
  this.ctx.stroke();      


  this.undoList.push({
    x_start: currentX,
    y_start: currentY,
    x_end: this.lastX,
    y_end: this.lastY,
    color: this.currentColour,
    size: this.brushSize,
    mode: this.ctx.globalCompositeOperation
  });

  this.lastX = currentX;
  this.lastY = currentY;
  
}

this.ctx.clearRect(0, 0, this.canvasElement.width, this.canvasElement.height);
    setTimeout(() => {
      
      // this.drawImg(this.newImg);
       for(let i=0; i<this.textAreasList.length; i++){
         let txt = this.textAreasList[i];
         this.ctx.font = this.textAreasList[i].bold + ' ' + this.textAreasList[i].italic + ' ' + this.textAreasList[i].fontSize + ' ' + 'Comic Sans MS';
         this.ctx.fillStyle = this.textAreasList[i].color;  
         if(this.textAreasList[i].left=="" || this.textAreasList[i].left==undefined) {
           this.textAreasList[i].left = 50;
         }
         if(this.textAreasList[i].top=="" || this.textAreasList[i].top==undefined) {
          this.textAreasList[i].top = 50;
        }
         this.ctx.fillText(this.textAreasList[i].value, this.textAreasList[i].left, this.textAreasList[i].top);
       }

       this.redrawCanvas(this.undoUseList);
       let imgPath = this.canvasElement.toDataURL('image/png');
       let message= "";
       this.base64ToGallery.base64ToGallery(imgPath).then(
         res => message = "Image saved to gallery!",
         err => message = "Something went wrong!!"
       );
       this.spinner.hide();
       let toast = this.toastCtrl.create({
         message: message,
         duration: 3000,
         position: 'bottom',
         cssClass: 'changeToast'
       });
       this.navCtrl.push(HomePage);
     }, 5000);
  }



redrawCanvas(arr){
      // this.ctx.globalCompositeOperation = 'source-over';
      for(let i=0; i<arr.length; i++){
        for(let j=0; j< arr[i].length; j++){
          let ctx = this.canvasElement.getContext('2d');
          ctx.globalCompositeOperation = arr[i][j].mode;
          console.log('x start', arr[i][j].x_start);
          console.log('y start', arr[i][j].y_start);
          console.log('x end', arr[i][j].x_end);
          console.log('y end', arr[i][j].y_end);
          ctx.beginPath();
          ctx.lineJoin = "round";
          ctx.moveTo(arr[i][j].x_start, arr[i][j].y_start);
          ctx.lineTo(arr[i][j].x_end, arr[i][j].y_end);
          ctx.closePath();
          ctx.strokeStyle = arr[i][j].color;
          ctx.lineWidth = arr[i][j].size;
          ctx.stroke();
        }
        
      }
      
    }

**b. Output -** 

我不明白为什么橡皮移动被替换为黑色，而保存画布作为一个图像。

Answer 1

最后我解决了这个问题。我做了以下改动。

redrawCanvas(arr){
      // this.ctx.globalCompositeOperation = 'source-over';
      for(let i=0; i<arr.length; i++){
        for(let j=0; j< arr[i].length; j++){
          let ctx = this.canvasElement.getContext('2d');
          // ctx.globalCompositeOperation = arr[i][j].mode;

          if(arr[i][j].mode== "destination-out"){
            // ctx.globalCompositeOperation = "destionation-out";
            // ctx.strokeStyle = "rgba(0,0,0,0.2)";
            let cImg = new Image();
            cImg.src = this.selectedImage;
            let pattern = ctx.createPattern(cImg, "no-repeat");
            ctx.strokeStyle = pattern;
          }else{
            ctx.strokeStyle = arr[i][j].color;
          }
          console.log('x start', arr[i][j].x_start);
          console.log('y start', arr[i][j].y_start);
          console.log('x end', arr[i][j].x_end);
          console.log('y end', arr[i][j].y_end);
          ctx.beginPath();
          ctx.lineJoin = "round";
          ctx.moveTo(arr[i][j].x_start, arr[i][j].y_start);
          ctx.lineTo(arr[i][j].x_end, arr[i][j].y_end);
          ctx.closePath();
          ctx.lineWidth = arr[i][j].size;
          ctx.stroke();
        }

      }

    }

正如你所看到的，我增加了一个检查是刷子还是橡皮擦。对于橡皮擦，我正在检查它是否是目的地输出。

如果是橡皮擦，我正在创建新的图像，使strokestyle。

所以你可以说我做了以下改变。

if(arr[i][j].mode== "destination-out"){
            // ctx.globalCompositeOperation = "destionation-out";
            // ctx.strokeStyle = "rgba(0,0,0,0.2)";
            let cImg = new Image();
            cImg.src = this.selectedImage;
            let pattern = ctx.createPattern(cImg, "no-repeat");
            ctx.strokeStyle = pattern;
          }else{
            ctx.strokeStyle = arr[i][j].color;
          }