http://blog.csdn.net/lmj623565791/article/details/49300989；
本文出自:【张鸿洋的博客】

对于加载图片，大家都不陌生，一般为了尽可能避免OOM都会按照如下做法：

1.对于图片显示：根据需要显示图片控件的大小对图片进行压缩显示。
2.如果图片数量非常多：则会使用LruCache等缓存机制，将所有图片占据的内容维持在一个范围内。

需求：不压缩，按照原图尺寸加载
那么屏幕肯定是不够大的，并且考虑到内存的情况，不可能一次性整图加载到内存中，所以肯定是局部加载，那么就需要用到一个类：

• BitmapRegionDecoder

BitmapRegionDecoder 主要用于显示图片的某一块矩形区域

• BitmapRegionDecoder提供了一系列的newInstance方法来构造对象，支持传入文件路径，文件描述符，文件的inputstrem等。

BitmapRegionDecoder decoder = BitmapRegionDecoder.newInstance(inputStream, false);

• 上述解决了传入我们需要处理的图片，那么接下来就是显示指定的区域。

//参数一很明显是一个rect，
//参数二是BitmapFactory.Options，可以控制图片的inSampleSize,inPreferredConfig等
bitmapRegionDecoder.decodeRegion(rect, options);

自定义显示大图控件

package com.example.largeimage.view;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.BitmapRegionDecoder;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

import com.example.largeimage.gesture.MoveGestureDetector;

import java.io.IOException;
import java.io.InputStream;

/**
 * Created by zhy on 15/5/16.
 */
public class LargeImageView extends View {
    private BitmapRegionDecoder mDecoder;
    /**
     * 图片的宽度和高度
     */
    private int mImageWidth;
    private int mImageHeight;
    /**
     * 绘制的区域
     */
    private volatile Rect mRect = new Rect();

    private MoveGestureDetector mDetector;
    
    private static final BitmapFactory.Options options = new BitmapFactory.Options();

    static {
        options.inPreferredConfig = Bitmap.Config.RGB_565;
    }
    
    public LargeImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }
    
    public void setInputStream(InputStream is) {
        try {
            //mDecoder = BitmapRegionDecoder.newInstance(is, false);
            //先获取decoder会出现 Options获取图片宽高异常 均为-1
            BitmapFactory.Options tmpOptions = new BitmapFactory.Options();
            tmpOptions.inJustDecodeBounds = true;
            BitmapFactory.decodeStream(is, null, tmpOptions);
            mDecoder = BitmapRegionDecoder.newInstance(is, false);
            mImageWidth = tmpOptions.outWidth;
            mImageHeight = tmpOptions.outHeight;

            requestLayout();
            invalidate();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (is != null) is.close();
            } catch (Exception e) {
            }
        }
    }
    
    public void init() {
        mDetector = new MoveGestureDetector(getContext(), new MoveGestureDetector.SimpleMoveGestureDetector() {
            @Override
            public boolean onMove(MoveGestureDetector detector) {
                int moveX = (int) detector.getMoveX();
                int moveY = (int) detector.getMoveY();

                if (mImageWidth > getWidth()) {
                    mRect.offset(-moveX, 0);
                    checkWidth();
                    invalidate();
                }
                if (mImageHeight > getHeight()) {
                    mRect.offset(0, -moveY);
                    checkHeight();
                    invalidate();
                }

                return true;
            }
        });
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mDetector.onToucEvent(event);
        return true;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        Bitmap bm = mDecoder.decodeRegion(mRect, options);
        canvas.drawBitmap(bm, 0, 0, null);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        int width = getMeasuredWidth();
        int height = getMeasuredHeight();

        int imageWidth = mImageWidth;
        int imageHeight = mImageHeight;

        mRect.left = imageWidth / 2 - width / 2;
        mRect.top = imageHeight / 2 - height / 2;
        mRect.right = mRect.left + width;
        mRect.bottom = mRect.top + height;

    }

    private void checkWidth() {
        Rect rect = mRect;
        int imageWidth = mImageWidth;

        if (rect.right > imageWidth) {
            rect.right = imageWidth;
            rect.left = imageWidth - getWidth();
        }

        if (rect.left < 0) {
            rect.left = 0;
            rect.right = getWidth();
        }
    }


    private void checkHeight() {

        Rect rect = mRect;
        int imageHeight = mImageHeight;

        if (rect.bottom > imageHeight) {
            rect.bottom = imageHeight;
            rect.top = imageHeight - getHeight();
        }

        if (rect.top < 0) {
            rect.top = 0;
            rect.bottom = getHeight();
        }
    }

}