Android系統處理圖片方面並不怎麼理想，稍不注意就會記憶體溢位。在處理大尺寸圖片時，有時候只需要顯示指定區域的圖片，Android在2.3中提供了BitmapRegionDecoder類來解決此問題。

public class DisplayImageRegionActivity extends Activity implements OnTouchListener {
    private final Rect mRect = new Rect();
    private BitmapRegionDecoder mDecoder;
    private ImageView mView;
    
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        mView = new ImageView(this);
        mView.setAdjustViewBounds(true);
        mView.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT));
        mView.setScaleType(ScaleType.CENTER);
        mView.setOnTouchListener(this);
        setContentView(mView);
        
        try {
            InputStream is = getResources().openRawResource(R.drawable.a);
            mDecoder = BitmapRegionDecoder.newInstance(is, true);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        final int action = event.getAction() & MotionEvent.ACTION_MASK;
        final int x = (int) event.getX();
        final int y = (int) event.getY();
        switch (action) {
        case MotionEvent.ACTION_DOWN:
        case MotionEvent.ACTION_MOVE:
            setImageRegion(x, y);
            break;
        }
        return true;
    }
    
    private void setImageRegion(int left, int top) {
//        BitmapFactory.Options opts = new BitmapFactory.Options();
        final int width = mView.getWidth();
        final int height = mView.getHeight();
        
        final int imgWidth = mDecoder.getWidth();
        final int imgHeight = mDecoder.getHeight();
        
        int right = left + width;
        int bottom = top + height;
        if(right > imgWidth) right = imgWidth;
        if(bottom > imgHeight) bottom = imgHeight;
        if(left < 0) left = 0;
        if(top < 0) top = 0;
        
        mRect.set(left, top, right, bottom);
        Bitmap bm = mDecoder.decodeRegion(mRect, null);
        mView.setImageBitmap(bm);
    }
}