用特殊的位图形式清除画布区域

Question

我开发了一个绘图应用程序，我需要实现橡皮擦工具来清除图片的特殊区域。橡皮擦必须不均匀地擦除一些圆形区域。这意味着 - 只有圆心应该被完全擦除，圆的边缘应该被部分擦除，带有 alpha。

为了实现这种行为，我创建了代表橡皮擦图形的图像

然后，为了擦除某些区域，我将橡皮擦的图形画在一些图片上方，如下所示：

private val picture: Bitmap
  private val bgPaint = Paint()
  private var eraser: Bitmap

  private val eraserPaint: Paint = Paint().apply {
    val mode: PorterDuff.Mode = PorterDuff.Mode.SRC_OUT
    xfermode = PorterDuffXfermode(mode)
    color = Color.TRANSPARENT
    alpha = 0
  }

  init {
    val options = BitmapFactory.Options()
    options.inMutable = true
    picture = BitmapFactory.decodeResource(resources, R.drawable.petushara, options)
    eraser = BitmapFactory.decodeResource(resources, R.drawable.eraser, options)
    setLayerType(View.LAYER_TYPE_SOFTWARE, bgPaint)
  }

  override fun draw(canvas: Canvas) {
    super.draw(canvas)
    canvas.drawBitmap(picture, 0f, 0f, bgPaint)
    canvas.drawBitmap(eraser, 100f, 300f, eraserPaint)
  }

这是我的整个视图类，不包括构造函数。我希望图片的区域会被橡皮擦擦除，但它会像这样被正方形擦除：

Answer 1

好的，我刚刚找到了解决方案。编写如下代码：

  private val picture: Bitmap
  private val bgPaint = Paint()
  private var eraser: Bitmap

  private val eraserPaint: Paint = Paint().apply {
    val mode: PorterDuff.Mode = PorterDuff.Mode.XOR
    xfermode = PorterDuffXfermode(mode)
  }

  init {
    val options = BitmapFactory.Options()
    options.inMutable = true
    picture = BitmapFactory.decodeResource(resources, R.drawable.petushara, options)
    eraser = BitmapFactory.decodeResource(resources, R.drawable.eraser, options)
    setLayerType(View.LAYER_TYPE_SOFTWARE, bgPaint)
  }

  override fun draw(canvas: Canvas) {
    super.draw(canvas)
    canvas.drawBitmap(picture, 0f, 0f, bgPaint)
    canvas.drawBitmap(eraser, 100f, 300f, eraserPaint)
  }

然后结果图片看起来像我的预期：