Android画布:在图像上绘制透明圆圈 [英] Android canvas: draw transparent circle on image

问题描述

我正在创建一个像素狩猎游戏.所以我的活动显示了一个 ImageView.我想创建一个提示告诉我对象在哪里".为此，我需要模糊整个图像，除了围绕对象所在点的圆圈.我可以显示一个半透明的黑色背景，而不是模糊.在Canvas上画一个半透明的矩形是没有问题的.但我不知道如何从中裁剪一个透明的圆圈.结果应如下所示:

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:

请帮助我在 Android SDK 上实现相同的结果.

Please help me to achieve same result on Android SDK.

推荐答案

所以我终于做到了.

首先我在整个视图上绘制一个半透明的黑色矩形.之后使用 PorterDuff.Mode.CLEAR 我剪了一个透明的圆圈来显示猫的位置.

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.

我在使用 PorterDuff.Mode.CLEAR 时遇到问题:首先我得到的是一个黑色圆圈而不是透明圆圈.

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

感谢 Romain Guy 的评论:在这里评论 我知道我的窗口是不透明的，我应该在另一个位图上绘制.并且只有在 View 的画布上绘制之后.

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.

这是我的 onDraw 方法:

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

private void init(){
    Bitmap bitmap = Bitmap.createBitmap(canvas.getWidth(), canvas.getHeight(), Bitmap.Config.ARGB_8888);
    temp = new Canvas(bitmap);
    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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