Bitmap转Drawble
Resources res=getResources();
Bitmap bm=BitmapFactory.decodeResource(res, R.drawable.header);
BitmapDrawable bitmapDrawable = new BitmapDrawable(bm);
Drawable drawable = (Drawable)bitmapDrawable;
Drawable 转Bitmap:
Bitmap bitmap = ((BitmapDrawable)drawable).getBitmap();