_mapData(maskCtx, imageData, width, height) {
      const { data, height: imgHeight, width: imgWidth } = imageData;
      const sourceData = data;
      const len = imgWidth * imgHeight * 4;
      const maskData = maskCtx.getImageData(0, 0, width, height).data;

      for (let i = 0; i < len; i += 4) {
        sourceData[i + 3] = maskData[i]; // adjust value of alpha data
      }
    }
Map mask image data to source image data
@param {Object} maskCtx - Context of mask canvas
@param {Object} imageData - Data of source image
@param {number} width - Width of main canvas
@param {number} height - Height of main canvas
@private