android webView怎么设置截取图片大小

android webView如何设置截取图片大小
现应用webview编了个小程序，准备截取网页图片
可是使用Picture pic = webView.capturePicture(); 每次截取的图片是整个网页的，而我希望截取的是屏幕显示的区域，请问如何调整截取区域的大小呢？

Java code

cutButton.setOnClickListener(new View.OnClickListener()         {            @Override            public void onClick(View v)            {                Picture pic = webView.capturePicture();                                int width = pic.getWidth();                int height = pic.getHeight();                if(width > 0 && height > 0)                {                    Bitmap bmp = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);                    Canvas canvas = new Canvas(bmp);                    pic.draw(canvas);                    try                    {                        String fileName = "sdcard/" + System.currentTimeMillis() + ".png";                        FileOutputStream fos = new FileOutputStream(fileName);                        if(fos != null)                        {                            bmp.compress(Bitmap.CompressFormat.PNG, 90, fos);                            fos.close();                        }                        Toast.makeText(getApplicationContext(), "截图成功，文件名是：" + fileName, Toast.LENGTH_SHORT).show();                    }                    catch (Exception e)                    {                        e.printStackTrace();                    }                }            }        });