Android的画布：利用图像透明圈 [英] Android canvas: draw transparent circle on image

问题描述

我创建一个像素的狩猎游戏。因此，我的活动显示了一个ImageView的。我想创建一个提示告诉我在哪里对象。为此，我需要模糊图像整体除围绕点对象所位于的圆。模糊的而不是我可以给一个公正的半透明的黑色背景。 有没有什么问题画一个半透明的矩形画布上。 但我不知道如何从它裁剪透明圈。 结果应该像这样的：

I am creating a pixel-hunting game. So my activity shows an ImageView. And I want to create a hint "show me where is the object". For this I need to blur whole image except a circle around a point where the object is located. Instead of blur I can show a just semitransparent black background. There is there is no problem to draw a semitransparent rectangle on the Canvas. But I don't know how to crop a transparent circle from it. The result should look like like this:

请帮我实现同样的结果在Android SDK。

Please help me to achieve same result on Android SDK.

推荐答案

所以最后我成功地做到这一点。

So finally I managed to do this.

首先，我借鉴全景半透明的黑色矩形。 在使用PorterDuff.Mode.CLEAR之后我剪一个透明圈，以显示猫的地位。

Firstly I draw a semitransparent black rectangle on whole view. After that using PorterDuff.Mode.CLEAR I cut a transparent circle to show cat's position.

我有问题PorterDuff.Mode.CLEAR：首先我得到了一个透明的黑圈代替

I had problem with PorterDuff.Mode.CLEAR: firstly I was getting a black circle instead of a transparent one.

由于罗曼盖伊的跟贴：​​这里评论我明白，我的窗户是不透明的，我应该利用另一个位图。只有经过借鉴视图的画布。

Thanks to Romain Guy's comments here: comment here I understood that my window is opaque and I should draw on another bitmap. And only after draw on view's canvas.

下面是我的OnDraw方法：

Here is my onDraw method:

private Canvas temp;
private Paint paint;
private Paint p = new Paint();
private Paint transparentPaint;

private void init(){
    temp = new Canvas(bitmap);
    Bitmap bitmap = Bitmap.createBitmap(canvas.getWidth(), canvas.getHeight(), Bitmap.Config.ARGB_8888);
    paint = new Paint();
    paint.setColor(0xcc000000);
    transparentPaint = new Paint();
    transparentPaint.setColor(getResources().getColor(android.R.color.transparent));
    transparentPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
}


protected void onDraw(Canvas canvas) {
    temp.drawRect(0, 0, temp.getWidth(), temp.getHeight(), paint);
    temp.drawCircle(catPosition.x + radius / 2, catPosition.y + radius / 2, radius, transparentPaint);
    canvas.drawBitmap(bitmap, 0, 0, p);
}

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