先举一个例子

public class Custom6view extends ImageView {
public int mWidth=0;
int mHigth=0;
private Matrix dragMatrix = new Matrix();
public static Path path;

public Custom6view(Context context) {
    super(context);
}

public Custom6view(Context context, AttributeSet attrs) {
    super(context, attrs, 0);
}

public Custom6view(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
}
@Override
protected void onDraw(Canvas canvas) {
    six(canvas);
}
//全本透明的遮罩
private void quan(Canvas canvas){
    //绘画半透明遮罩
    canvas.drawColor(Color.parseColor("#90000000"));
    Paint paint = new Paint();
    paint.setColor(Color.TRANSPARENT);
    paint.setStyle(Paint.Style.FILL);
    paint.setAntiAlias(true);
    int left = getWidth()/2;
    int top = getHeight()/2;
    //绘制透明预览框
    canvas.drawCircle(left, top, 300, paint);
}
//圆形透明的遮罩
private void yuan(Canvas canvas){
    //这里需要通过bitmap创建canvas才能对Xfermode生效果,具体原因这里也不大清楚
    Bitmap bitmap = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_4444);
    Canvas xFerCanvas = new Canvas(bitmap);
    //绘画半透明遮罩
    xFerCanvas.drawColor(Color.parseColor("#90000000"));
    Paint paint = new Paint();
    paint.setColor(Color.TRANSPARENT);
    paint.setStyle(Paint.Style.FILL);
    paint.setAntiAlias(true);
    //设置当前画笔的Xfermode模式,不同的模式效果可以参照Google提供的Demo-ApiDemos/Graphics/XferModes
    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
    int left = getWidth()/2;
    int top = getHeight()/2;
    //绘制透明预览框
    xFerCanvas.drawCircle(left, top, 300, paint);
    //最后将生成的bitmap绘制到我们的画布上
    canvas.drawBitmap(bitmap,0,0,null);
    bitmap.recycle();
    System.gc();
}
//圆形透明的遮罩
private void six(Canvas canvas){
    //这里需要通过bitmap创建canvas才能对Xfermode生效果,具体原因这里也不大清楚
    Bitmap bitmap = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_4444);
    Canvas xFerCanvas = new Canvas(bitmap);
    //绘画半透明遮罩
    xFerCanvas.drawColor(Color.parseColor("#90000000"));
    Paint paint = new Paint();
    paint.setColor(Color.TRANSPARENT);
    paint.setStyle(Paint.Style.FILL);
    paint.setAntiAlias(true);
    //设置当前画笔的Xfermode模式,不同的模式效果可以参照Google提供的Demo-ApiDemos/Graphics/XferModes
    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
    // 路径对象
    path = new Path();
    float d = (float) ((float)mWidth / 4 * (2 - Math.sqrt(3)));//六边形到边到内切圆的距离
    float h = (float) ((float)mWidth / 4 *(Math.sqrt(3)));//六边形到边到内切圆的距离
    float r = mWidth/4;
    float p0x = r;
    float p0y = mHigth/2-h;
    float p1x = r*3;
    float p1y = mHigth/2-h;
    float p2x = mWidth;
    float p2y = mHigth/2;
    float p3x = p1x;
    float p3y =  mHigth/2+h;
    float p4x = p0x;
    float p4y =  mHigth/2+h;
    float p5x = 0;
    float p5y =  mHigth/2;
    path.moveTo(p0x,p0y);
    path.lineTo(p1x,p1y);
    path.lineTo(p2x,p2y);
    path.lineTo(p3x,p3y);
    path.lineTo(p4x,p4y);
    path.lineTo(p5x,p5y);
    path.lineTo(p0x,p0y);
   //....  可以添加多个点。构成多边形
    path.close(); // 使终点和起点链接，构成封闭图形
    //绘制透明预览框
    xFerCanvas.drawPath(path, paint);
    //最后将生成的bitmap绘制到我们的画布上
    canvas.drawBitmap(bitmap,0,0,null);
    bitmap.recycle();
    System.gc();
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);//限制为正方形
    mWidth = Math.min(getMeasuredWidth(), getMeasuredHeight());
    mHigth=getMeasuredHeight();
}
public static Bitmap getBitmap(Bitmap bitmap) {
    Bitmap output = Bitmap.createBitmap(bitmap.getWidth(), bitmap.getHeight(),
            Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(output);

    final int color = 0xff424242;
    final Paint paint = new Paint();
    final Rect rect = new Rect(0, 0, bitmap.getWidth(), bitmap.getHeight());

    paint.setAntiAlias(true);
    paint.setFilterBitmap(true);
    paint.setDither(true);
    paint.setColor(color);
    //在画布上绘制一个圆 -1是为了去掉白色的边框
    canvas.drawPath(path, paint);
    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));
    canvas.drawBitmap(bitmap, rect, rect, paint);
    Log.i("生成图片", "getCircleBitmap()     output=" + output);
    return output;
}
}

其中最重要的就是

111.png

其中这里是在画透明的整体遮罩

这里是画你要的透明图形状，可以参考我的《自定义视图》就可以画出

222.png