Android 位图遮罩 (Xfermode) 留下不透明的黑色背景

Question

我有一个自定义视图，在 onDraw() 中，我正在尝试执行位图遮罩。我有一个 squareBitmap（红色）填充整个视图，我有一个 circleBitmap（蓝色）作为遮罩。我正在使用模式：PorterDuff.Mode.DST_IN.

我期望的结果是一个红色实心圆圈。我明白了，但我也得到了黑色不透明背景。我不想要这个不透明的背景，而它应该是透明的。 Figure 1是我得到的结果，Figure 2是我要找的结果。

我的代码：（为了这个问题，我把所有东西都移到了onDraw()里面）

protected void onDraw(Canvas canvas) {

    final int width = canvas.getWidth();
    final int height = canvas.getHeight();

    Bitmap circleBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
    Canvas circleCanvas = new Canvas(circleBitmap);
    Paint p = new Paint(Paint.ANTI_ALIAS_FLAG);
    p.setStyle(Paint.Style.FILL_AND_STROKE);
    p.setColor(Color.BLUE);
    circleCanvas.drawCircle(width / 2, height / 2, width / 2, p);

    p.setColor(Color.RED);
    Bitmap squareBitmap = Bitmap.createBitmap(canvas.getWidth(), canvas.getHeight(), Bitmap.Config.ARGB_8888);
    Canvas squareCanvas = new Canvas(squareBitmap);
    final Rect squareRect = new Rect(0, 0, width, height);
    squareCanvas.drawRect(squareRect, p);

    Paint q = new Paint(Paint.ANTI_ALIAS_FLAG);
    canvas.drawBitmap(squareBitmap, 0, 0, q);
    q.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
    canvas.drawBitmap(circleBitmap, 0, 0, q);
    q.setXfermode(null);
}

我哪里错了？我怎样才能避免这种黑色不透明背景？

Answer 1

似乎要完成 Xfermode，我需要将绘图重定向到屏幕外位图。所以添加以下内容解决了我的问题。

canvas.saveLayer(0, 0, canvas.getWidth(), canvas.getHeight(), q);

但话又说回来，documentation of saveLayer says to avoid using this method since it is expensive. It is suggested to use a harware layer而不是这个方法。

Avoid using this method, especially if the bounds provided are large, or if the CLIP_TO_LAYER_SAVE_FLAG is omitted from the saveFlags parameter. It is recommended to use a hardware layer on a View to apply an xfermode, color filter, or alpha, as it will perform much better than this method.

因此，我可以通过添加以下内容来修复 saveLayer 方法：

setLayerType(LAYER_TYPE_HARDWARE, q);

Android 位图遮罩 (Xfermode) 留下不透明的黑色背景

Android Bitmap Masking (Xfermode) leaves opaque black background behind

android

android-canvas

android-view

android-bitmap