Android的位图蒙版（Xfermode）留下不透明的黑色背景 [英] Android Bitmap Masking (Xfermode) leaves opaque black background behind

问题描述

我有一个自定义视图和的OnDraw（），我想执行的位图屏蔽。我有一个 squareBitmap （红色），它充满整个来看，我有一个 circleBitmap （蓝色）的充当掩模。我使用的模式： PorterDuff.Mode.DST_IN

I have a custom view and in the onDraw(), I am trying to perform bitmap masking. I have a squareBitmap (red color) which fills the whole view, and I have a circleBitmap (blue color) that acts as the mask. I am using the mode: PorterDuff.Mode.DST_IN.

结果，我希望是一个红色实心圆。我明白，但我也得到一个黑色不透明的背景。我不希望这种不透明的背景下，而应该是透明的。 图1 是我找来的，图2 是我期待的结果的结果。

The result I am expecting is a RED filled circle. I get that, but I also get a BLACK opaque background. I don't want this opaque background, rather it should be transparent. Figure 1 is the result I got, and Figure 2 is the result that I am looking for.

href="http://i.stack.imgur.com/TJZEO.jpg" rel="nofollow">

我的code：（我已搬到里面的一切的OnDraw（）对这个问题的目的）

My code: (I have moved everything inside onDraw() for the purpose of this question)

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);
}

我在哪里的问题呢？我怎样才能避免这种黑色不透明的背景？

Where am I going wrong? How can I avoid this black opaque background?

推荐答案

似乎得到了 Xfermode 做的，我需要绘制重定向到一个屏幕外的位图。因此，添加以下固定我的问题。

It seems that to get the Xfermode done, I need to redirect the drawing to an offscreen bitmap. So adding the following fixed my problem.

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

但话又说回来，的文档 saveLayer 说要避免使用此方法，因为它是昂贵的。因此建议使用这种方法的一个 harware层的代替。

But then again, the documentation of saveLayer says to avoid using this method since it is expensive. It is suggested to use a harware layer instead of this method.

避免使用这种方法，特别是如果提供的范围是大的，   或者如果CLIP_TO_LAYER_SAVE_FLAG从saveFlags省略   参数。建议在查看使用硬件层   施加xfermode，彩色滤光片，或α，因为它会执行许多   比这更好的方法。

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​​， saveLayer 法的这一翻译，我能够通过添加修复它下面的：

Hence, intead of saveLayer method, I was able to fix it by adding the following:

setLayerType(LAYER_TYPE_HARDWARE, q);

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