二次采样:

    (一)、意义或目的:

    1、用BitmapFactory解码一张图片时,有时会遇到该错误。这往往是由于图片过大造成的。要想正常使用,则需要分配更少的内存空间来存储。BitmapFactory.decodeFile(imageFile);

    2、BitmapFactory.Options.inSampleSize:设置恰当的inSampleSize可以使BitmapFactory分配更少的空间以消除该错误。

    3、BitmapFactory.Options提供了另一个成员inJustDecodeBounds。设置inJustDecodeBounds为true后,decodeFile并不分配空间,但可计算出原始图片的长度和宽度,即opts.width和opts.height。有了这两个参数,再通过一定的算法,即可得到一个恰当的inSampleSize。

    例如:

    BitmapFactory.Options opts = new BitmapFactory.Options();

    opts.inSampleSize = 4;

    Bitmap bitmap = BitmapFactory.decodeFile(imageFile, opts);

    (二)、获取inSampleSize:

    设置恰当的inSampleSize是解决该问题的关键之一。

    查看Android源码,我们得知,为了得到恰当的inSampleSize,Android提供了一种动态计算的方法。

    封装的工具类:

    package com.hht.android17_httpclient;

    import android.graphics.Bitmap;
    import android.graphics.BitmapFactory;

    public class MyBitmapUtil {

    /**
     * 根据图片存放路径,对图片可能进行二次采样,不致于加载过大图片出现内存溢出
     * 
     * @param String
     * @return
     */
    public static Bitmap createImageThumbnail(String filePath) {
    
        Bitmap bitmap = null;
    
        BitmapFactory.Options opts = new BitmapFactory.Options();
    
        opts.inJustDecodeBounds = true;
    
        BitmapFactory.decodeFile(filePath, opts);
    
        opts.inSampleSize = computeSampleSize(opts, -1, 128 * 128);
    
        opts.inJustDecodeBounds = false;
    
        try {
    
            bitmap = BitmapFactory.decodeFile(filePath, opts);
    
        } catch (Exception e) {
    
            // TODO: handle exception
    
        }
    
        return bitmap;
    
    }
    
    /**
     * 根据图片字节数组,对图片可能进行二次采样,不致于加载过大图片出现内存溢出
     * 
     * @param String
     * @return
     */
    public static Bitmap createImageThumbnail(byte[] data) {
    
        Bitmap bitmap = null;
    
        BitmapFactory.Options opts = new BitmapFactory.Options();
    
        opts.inJustDecodeBounds = true;
    
        BitmapFactory.decodeByteArray(data, 0, data.length, opts);
    
        opts.inSampleSize = computeSampleSize(opts, -1, 128 * 128);
    
        opts.inJustDecodeBounds = false;
    
        try {
    
            BitmapFactory.decodeByteArray(data, 0, data.length, opts);
    
        } catch (Exception e) {
    
            // TODO: handle exception
    
        }
    
        return bitmap;
    
    }
    
    public static int computeSampleSize(BitmapFactory.Options options,
    
    int minSideLength, int maxNumOfPixels) {
    
        int initialSize = computeInitialSampleSize(options, minSideLength,
    
        maxNumOfPixels);
    
        int roundedSize;
    
        if (initialSize <= 8) {
    
            roundedSize = 1;
    
            while (roundedSize < initialSize) {
    
                roundedSize <<= 1;
    
            }
    
        } else {
    
            roundedSize = (initialSize + 7) / 8 * 8;
    
        }
    
        return roundedSize;
    
    }
    
    private static int computeInitialSampleSize(BitmapFactory.Options options,
    
    int minSideLength, int maxNumOfPixels) {
    
        double w = options.outWidth;
    
        double h = options.outHeight;
    
        int lowerBound = (maxNumOfPixels == -1) ? 1 : (int) Math.ceil(Math
    
        .sqrt(w * h / maxNumOfPixels));
    
        int upperBound = (minSideLength == -1) ? 128 : (int) Math.min(
    
        Math.floor(w / minSideLength), Math.floor(h / minSideLength));
    
        if (upperBound < lowerBound) {
    
            // return the larger one when there is no overlapping zone.
    
            return lowerBound;
    
        }
    
        if ((maxNumOfPixels == -1) && (minSideLength == -1)) {
    
            return 1;
    
        } else if (minSideLength == -1) {
    
            return lowerBound;
    
        } else {
    
            return upperBound;
    
        }
    
    }
    

    }