这个createRoundDrawable里面是生成一个圆形透明图？里面的算法不太懂

private RoundedBitmapDrawable createRoundDrawable(Bitmap bitmap, final int color) {

        int thumbWidth = mThumbnailView.getLayoutParams().width;

        int thumbHeight = mThumbnailView.getLayoutParams().height;

        //setContentDescription is added for testing

        mThumbnailView.setContentDescription("Has Content");

        // crop a square bitmap according minimum edge.

        if (bitmap == null) {

            bitmap = Bitmap.createBitmap(thumbWidth, thumbHeight,

                    Bitmap.Config.ARGB_8888);

            mThumbnailView.setContentDescription("No Content");

        }

        int bmpWidth = bitmap.getWidth();

        int bmpHeight = bitmap.getHeight();

        Bitmap squareBitmap;

        if (bmpHeight > bmpWidth) {

            squareBitmap = Bitmap.createBitmap(bitmap, 0,

                    (bmpHeight - bmpWidth) / 2, bmpWidth, bmpWidth);

        } else if (bmpHeight < bmpWidth) {

            squareBitmap = Bitmap.createBitmap(bitmap,

                    (bmpWidth - bmpHeight) / 2, 0, bmpHeight, bmpHeight);

        } else {

            squareBitmap = bitmap;

        }

        // center scale to the size of thumbnail view.

        Bitmap scaledBitmap = Bitmap.createScaledBitmap(squareBitmap,

                thumbWidth, thumbHeight, true);

        // Calculate the bitmap radius

        int borderWidthHalf = 2;

        int bitmapWidth = scaledBitmap.getWidth();

        int bitmapHeight = scaledBitmap.getHeight();

        int bitmapRadius = Math.min(bitmapWidth, bitmapHeight) / 2;

        int bitmapSquareWidth = Math.min(bitmapWidth, bitmapHeight);

        int newBitmapSquareWidth = bitmapSquareWidth - borderWidthHalf * 2;

        /*

         * Initializing a new empty bitmap. Set the bitmap size from source

         * bitmap Also add the border space to new bitmap

         */

        //if (mRoundedBitmap != null) {

        //    mRoundedBitmap.recycle();

        //    mRoundedBitmap = null;

        //}

        mRoundedBitmap = Bitmap.createBitmap(newBitmapSquareWidth,

                newBitmapSquareWidth, Bitmap.Config.ARGB_8888);

        // Initialize a new Canvas to draw empty bitmap

        Canvas canvas = new Canvas(mRoundedBitmap);

        // Draw a solid color to canvas

        canvas.drawColor(color);

        // Calculation to draw bitmap at the circular bitmap center position

        int centerX = newBitmapSquareWidth - bitmapWidth;

        int centerY = newBitmapSquareWidth - bitmapHeight;

        /*

         * Now draw the bitmap to canvas. Bitmap will draw its center to

         * circular bitmap center by keeping border spaces

         */

        canvas.drawBitmap(scaledBitmap, centerX, centerY, null);

        // Initializing a new Paint instance to draw circular border

        Paint borderPaint = new Paint();

        borderPaint.setStyle(Paint.Style.STROKE);

        borderPaint.setStrokeWidth(borderWidthHalf * 2);

        borderPaint.setColor(BORDER_COLOR);

        /*

         * Draw the circular border to bitmap. Draw the circle at the center of

         * canvas.

         */

        canvas.drawCircle(canvas.getWidth() / 2, canvas.getWidth() / 2,

                newBitmapSquareWidth / 2, borderPaint);

        // Create a new RoundedBitmapDrawable

        RoundedBitmapDrawable roundedBitmapDrawable = RoundedBitmapDrawableFactory

                .create(mApp.getActivity().getResources(), mRoundedBitmap);

        // Set the corner radius of the bitmap drawable

        roundedBitmapDrawable.setCornerRadius(bitmapRadius);

        roundedBitmapDrawable.setAntiAlias(true);

        bitmap.recycle();

        squareBitmap.recycle();

        scaledBitmap.recycle();

        bitmap = null;

        squareBitmap = null;

        scaledBitmap = null;

        return roundedBitmapDrawable;

    }