android 合成两张图片，上层图片设置透明度

    private Bitmap mergeThumbnailBitmap(Bitmap firstBitmap, Bitmap secondBitmap) {
        Bitmap bitmap = Bitmap.createBitmap(firstBitmap.getWidth(), firstBitmap
                .getHeight(), firstBitmap.getConfig());
        Canvas canvas = new Canvas(bitmap);
        Paint paint = new Paint();
        paint.setAlpha(150);
        canvas.drawBitmap(firstBitmap, new Matrix(), null);
        canvas.drawBitmap(secondBitmap, 0, 0, paint);
        return bitmap;
    }