android读取sd卡图片并进行缩放操作

在Android中对大图片进行缩放真的很不尽如人意，不知道是不是我的方法不对。下面我列出3种对图片缩放的方法，并给出相应速度。请高人指教。

第一种是BitmapFactory和BitmapFactory.Options。
首先，BitmapFactory.Options有几个Fields很有用：
inJustDecodeBounds：If set to true, the decoder will return null (no bitmap), but the out...
也就是说，当inJustDecodeBounds设成true时，bitmap并不加载到内存，这样效率很高哦。而这时，你可以获得bitmap的高、宽等信息。
outHeight：The resulting height of the bitmap, set independent of the state of inJustDecodeBounds.
outWidth：The resulting width of the bitmap, set independent of the state of inJustDecodeBounds. 
看到了吧，上面3个变量是相关联的哦。
inSampleSize ： If set to a value > 1, requests the decoder to subsample the original image, returning a smaller image to save memory.
这就是用来做缩放比的。这里有个技巧：
inSampleSize=（outHeight/Height+outWidth/Width）/2
实践证明，这样缩放出来的图片还是很好的。
最后用BitmapFactory.decodeFile(path, options)生成。
由于只是对bitmap加载到内存一次，所以效率比较高。解析速度快。

第二种是使用Bitmap加Matrix来缩放。

首先要获得原bitmap，再从原bitmap的基础上生成新图片。这样效率很低。

第三种是用2.2新加的类ThumbnailUtils来做。
让我们新看看这个类，从API中来看，此类就三个静态方法：createVideoThumbnail、extractThumbnail(Bitmap source, int width, int height, int options)、extractThumbnail(Bitmap source, int width, int height)。
我这里使用了第三个方法。再看看它的源码，下面会附上。是上面我们用到的BitmapFactory.Options和Matrix等经过人家一阵加工而成。
效率好像比第二种方法高一点点。

下面是我的例子：

xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >

<ImageView
    android:id="@+id/imageShow"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
/>
<ImageView
    android:id="@+id/image2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
/>
<TextView
    android:id="@+id/text"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/hello"
    />
LinearLayout>

package com.linc.ResolvePicture;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Matrix;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.media.ThumbnailUtils;
import android.os.Bundle;
import android.util.Log;
import android.widget.ImageView;
import android.widget.TextView;

public class ResolvePicture extends Activity {
    private static String tag="ResolvePicture";
    Drawable bmImg;
    ImageView imView;
    ImageView imView2;
    TextView text;
    String theTime;
    long start, stop;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        text=(TextView)findViewById(R.id.text);

        imView=(ImageView) findViewById(R.id.imageShow);
        imView2=(ImageView) findViewById(R.id.image2);

        Bitmap bitmap = BitmapFactory.decodeResource(getResources(),
                R.drawable.pic);

        start=System.currentTimeMillis();

//        imView.setImageDrawable(resizeImage(bitmap, 300, 100));

        imView2.setImageDrawable(resizeImage2("/sdcard/2.jpeg", 200, 100));

        stop=System.currentTimeMillis();

        String theTime= String.format("\n1 iterative: (%d msec)",
                stop - start);

        start=System.currentTimeMillis();
        imView.setImageBitmap(ThumbnailUtils.extractThumbnail(bitmap,200,100));//2.2才加进来的新类，简单易用
//        imView.setImageDrawable(resizeImage(bitmap, 30, 30));
        stop=System.currentTimeMillis();

         theTime+= String.format("\n2 iterative: (%d msec)",
                stop - start);

        text.setText(theTime);
    }

    //使用Bitmap加Matrix来缩放
    public static Drawable resizeImage(Bitmap bitmap, int w, int h)
    {
        Bitmap BitmapOrg = bitmap;
        int width = BitmapOrg.getWidth();
        int height = BitmapOrg.getHeight();
        int newWidth = w;
        int newHeight = h;

        float scaleWidth = ((float) newWidth) / width;
        float scaleHeight = ((float) newHeight) / height;

        Matrix matrix = new Matrix();
        matrix.postScale(scaleWidth, scaleHeight);
        // if you want to rotate the Bitmap
        // matrix.postRotate(45);
        Bitmap resizedBitmap = Bitmap.createBitmap(BitmapOrg, 0, 0, width,
                        height, matrix, true);
        return new BitmapDrawable(resizedBitmap);
    }

    //使用BitmapFactory.Options的inSampleSize参数来缩放
    public static Drawable resizeImage2(String path,
            int width,int height)
    {
        BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;//不加载bitmap到内存中
        BitmapFactory.decodeFile(path,options);
        int outWidth = options.outWidth;
        int outHeight = options.outHeight;
        options.inDither = false;
        options.inPreferredConfig = Bitmap.Config.ARGB_8888;
        options.inSampleSize = 1;

        if (outWidth != 0 && outHeight != 0 && width != 0 && height != 0)
        {
            int sampleSize=(outWidth/width+outHeight/height)/2;
            Log.d(tag, "sampleSize = " + sampleSize);
            options.inSampleSize = sampleSize;
        }

        options.inJustDecodeBounds = false;
        return new BitmapDrawable(BitmapFactory.decodeFile(path, options));
    }

    //图片保存
    private void saveThePicture(Bitmap bitmap)
    {
        File file=new File("/sdcard/2.jpeg");
        try
        {
            FileOutputStream fos=new FileOutputStream(file);
            if(bitmap.compress(Bitmap.CompressFormat.JPEG, 100, fos))
            {
                fos.flush();
                fos.close();
            }
        }
        catch(FileNotFoundException e1)
        {
            e1.printStackTrace();
        }
        catch(IOException e2)
        {
            e2.printStackTrace();
        }
    }
}

ThumbnailUtils源码：

/*
 * Copyright (C) 2009 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.media;

import android.content.ContentResolver;
import android.content.ContentUris;
import android.content.ContentValues;
import android.database.Cursor;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.graphics.Rect;
import android.media.MediaMetadataRetriever;
import android.media.MediaFile.MediaFileType;
import android.net.Uri;
import android.os.ParcelFileDescriptor;
import android.provider.BaseColumns;
import android.provider.MediaStore.Images;
import android.provider.MediaStore.Images.Thumbnails;
import android.util.Log;

import java.io.FileInputStream;
import java.io.FileDescriptor;
import java.io.IOException;
import java.io.OutputStream;

/**
 * Thumbnail generation routines for media provider.
 */

public class ThumbnailUtils {
    private static final String TAG = "ThumbnailUtils";

    /* Maximum pixels size for created bitmap. */
    private static final int MAX_NUM_PIXELS_THUMBNAIL = 512 * 384;
    private static final int MAX_NUM_PIXELS_MICRO_THUMBNAIL = 128 * 128;
    private static final int UNCONSTRAINED = -1;

    /* Options used internally. */
    private static final int OPTIONS_NONE = 0x0;
    private static final int OPTIONS_SCALE_UP = 0x1;

    /**
     * Constant used to indicate we should recycle the input in
     * {@link #extractThumbnail(Bitmap, int, int, int)} unless the output is the input.
     */
    public static final int OPTIONS_RECYCLE_INPUT = 0x2;

    /**
     * Constant used to indicate the dimension of mini thumbnail.
     * @hide Only used by media framework and media provider internally.
     */
    public static final int TARGET_SIZE_MINI_THUMBNAIL = 320;

    /**
     * Constant used to indicate the dimension of micro thumbnail.
     * @hide Only used by media framework and media provider internally.
     */
    public static final int TARGET_SIZE_MICRO_THUMBNAIL = 96;

    /**
     * This method first examines if the thumbnail embedded in EXIF is bigger than our target
     * size. If not, then it'll create a thumbnail from original image. Due to efficiency
     * consideration, we want to let MediaThumbRequest avoid calling this method twice for
     * both kinds, so it only requests for MICRO_KIND and set saveImage to true.
     *
     * This method always returns a "square thumbnail" for MICRO_KIND thumbnail.
     *
     * @param filePath the path of image file
     * @param kind could be MINI_KIND or MICRO_KIND
     * @return Bitmap
     *
     * @hide This method is only used by media framework and media provider internally.
     */
    public static Bitmap createImageThumbnail(String filePath, int kind) {
        boolean wantMini = (kind == Images.Thumbnails.MINI_KIND);
        int targetSize = wantMini
                ? TARGET_SIZE_MINI_THUMBNAIL
                : TARGET_SIZE_MICRO_THUMBNAIL;
        int maxPixels = wantMini
                ? MAX_NUM_PIXELS_THUMBNAIL
                : MAX_NUM_PIXELS_MICRO_THUMBNAIL;
        SizedThumbnailBitmap sizedThumbnailBitmap = new SizedThumbnailBitmap();
        Bitmap bitmap = null;
        MediaFileType fileType = MediaFile.getFileType(filePath);
        if (fileType != null && fileType.fileType == MediaFile.FILE_TYPE_JPEG) {
            createThumbnailFromEXIF(filePath, targetSize, maxPixels, sizedThumbnailBitmap);
            bitmap = sizedThumbnailBitmap.mBitmap;
        }

        if (bitmap == null) {
            try {
                FileDescriptor fd = new FileInputStream(filePath).getFD();
                BitmapFactory.Options options = new BitmapFactory.Options();
                options.inSampleSize = 1;
                options.inJustDecodeBounds = true;
                BitmapFactory.decodeFileDescriptor(fd, null, options);
                if (options.mCancel || options.outWidth == -1
                        || options.outHeight == -1) {
                    return null;
                }
                options.inSampleSize = computeSampleSize(
                        options, targetSize, maxPixels);
                options.inJustDecodeBounds = false;

                options.inDither = false;
                options.inPreferredConfig = Bitmap.Config.ARGB_8888;
                bitmap = BitmapFactory.decodeFileDescriptor(fd, null, options);
            } catch (IOException ex) {
                Log.e(TAG, "", ex);
            }
        }

        if (kind == Images.Thumbnails.MICRO_KIND) {
            // now we make it a "square thumbnail" for MICRO_KIND thumbnail
            bitmap = extractThumbnail(bitmap,
                    TARGET_SIZE_MICRO_THUMBNAIL,
                    TARGET_SIZE_MICRO_THUMBNAIL, OPTIONS_RECYCLE_INPUT);
        }
        return bitmap;
    }

    /**
     * Create a video thumbnail for a video. May return null if the video is
     * corrupt or the format is not supported.
     *
     * @param filePath the path of video file
     * @param kind could be MINI_KIND or MICRO_KIND
     */
    public static Bitmap createVideoThumbnail(String filePath, int kind) {
        Bitmap bitmap = null;
        MediaMetadataRetriever retriever = new MediaMetadataRetriever();
        try {
            retriever.setMode(MediaMetadataRetriever.MODE_CAPTURE_FRAME_ONLY);
            retriever.setDataSource(filePath);
            bitmap = retriever.captureFrame();
        } catch (IllegalArgumentException ex) {
            // Assume this is a corrupt video file
        } catch (RuntimeException ex) {
            // Assume this is a corrupt video file.
        } finally {
            try {
                retriever.release();
            } catch (RuntimeException ex) {
                // Ignore failures while cleaning up.
            }
        }
        if (kind == Images.Thumbnails.MICRO_KIND && bitmap != null) {
            bitmap = extractThumbnail(bitmap,
                    TARGET_SIZE_MICRO_THUMBNAIL,
                    TARGET_SIZE_MICRO_THUMBNAIL,
                    OPTIONS_RECYCLE_INPUT);
        }
        return bitmap;
    }

    /**
     * Creates a centered bitmap of the desired size.
     *
     * @param source original bitmap source
     * @param width targeted width
     * @param height targeted height
     */
    public static Bitmap extractThumbnail(
            Bitmap source, int width, int height) {
        return extractThumbnail(source, width, height, OPTIONS_NONE);
    }

    /**
     * Creates a centered bitmap of the desired size.
     *
     * @param source original bitmap source
     * @param width targeted width
     * @param height targeted height
     * @param options options used during thumbnail extraction
     */
    public static Bitmap extractThumbnail(
            Bitmap source, int width, int height, int options) {
        if (source == null) {
            return null;
        }

        float scale;
        if (source.getWidth() < source.getHeight()) {
            scale = width / (float) source.getWidth();
        } else {
            scale = height / (float) source.getHeight();
        }
        Matrix matrix = new Matrix();
        matrix.setScale(scale, scale);
        Bitmap thumbnail = transform(matrix, source, width, height,
                OPTIONS_SCALE_UP | options);
        return thumbnail;
    }

    /*
     * Compute the sample size as a function of minSideLength
     * and maxNumOfPixels.
     * minSideLength is used to specify that minimal width or height of a
     * bitmap.
     * maxNumOfPixels is used to specify the maximal size in pixels that is
     * tolerable in terms of memory usage.
     *
     * The function returns a sample size based on the constraints.
     * Both size and minSideLength can be passed in as IImage.UNCONSTRAINED,
     * which indicates no care of the corresponding constraint.
     * The functions prefers returning a sample size that
     * generates a smaller bitmap, unless minSideLength = IImage.UNCONSTRAINED.
     *
     * Also, the function rounds up the sample size to a power of 2 or multiple
     * of 8 because BitmapFactory only honors sample size this way.
     * For example, BitmapFactory downsamples an image by 2 even though the
     * request is 3. So we round up the sample size to avoid OOM.
     */
    private 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 == UNCONSTRAINED) ? 1 :
                (int) Math.ceil(Math.sqrt(w * h / maxNumOfPixels));
        int upperBound = (minSideLength == UNCONSTRAINED) ? 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 == UNCONSTRAINED) &&
                (minSideLength == UNCONSTRAINED)) {
            return 1;
        } else if (minSideLength == UNCONSTRAINED) {
            return lowerBound;
        } else {
            return upperBound;
        }
    }

    /**
     * Make a bitmap from a given Uri, minimal side length, and maximum number of pixels.
     * The image data will be read from specified pfd if it's not null, otherwise
     * a new input stream will be created using specified ContentResolver.
     *
     * Clients are allowed to pass their own BitmapFactory.Options used for bitmap decoding. A
     * new BitmapFactory.Options will be created if options is null.
     */
    private static Bitmap makeBitmap(int minSideLength, int maxNumOfPixels,
            Uri uri, ContentResolver cr, ParcelFileDescriptor pfd,
            BitmapFactory.Options options) {
            Bitmap b = null;
        try {
            if (pfd == null) pfd = makeInputStream(uri, cr);
            if (pfd == null) return null;
            if (options == null) options = new BitmapFactory.Options();

            FileDescriptor fd = pfd.getFileDescriptor();
            options.inSampleSize = 1;
            options.inJustDecodeBounds = true;
            BitmapFactory.decodeFileDescriptor(fd, null, options);
            if (options.mCancel || options.outWidth == -1
                    || options.outHeight == -1) {
                return null;
            }
            options.inSampleSize = computeSampleSize(
                    options, minSideLength, maxNumOfPixels);
            options.inJustDecodeBounds = false;

            options.inDither = false;
            options.inPreferredConfig = Bitmap.Config.ARGB_8888;
            b = BitmapFactory.decodeFileDescriptor(fd, null, options);
        } catch (OutOfMemoryError ex) {
            Log.e(TAG, "Got oom exception ", ex);
            return null;
        } finally {
            closeSilently(pfd);
        }
        return b;
    }

    private static void closeSilently(ParcelFileDescriptor c) {
      if (c == null) return;
      try {
          c.close();
      } catch (Throwable t) {
          // do nothing
      }
    }

    private static ParcelFileDescriptor makeInputStream(
            Uri uri, ContentResolver cr) {
        try {
            return cr.openFileDescriptor(uri, "r");
        } catch (IOException ex) {
            return null;
        }
    }

    /**
     * Transform source Bitmap to targeted width and height.
     */
    private static Bitmap transform(Matrix scaler,
            Bitmap source,
            int targetWidth,
            int targetHeight,
            int options) {
        boolean scaleUp = (options & OPTIONS_SCALE_UP) != 0;
        boolean recycle = (options & OPTIONS_RECYCLE_INPUT) != 0;

        int deltaX = source.getWidth() - targetWidth;
        int deltaY = source.getHeight() - targetHeight;
        if (!scaleUp && (deltaX < 0 || deltaY < 0)) {
            /*
            * In this case the bitmap is smaller, at least in one dimension,
            * than the target.  Transform it by placing as much of the image
            * as possible into the target and leaving the top/bottom or
            * left/right (or both) black.
            */
            Bitmap b2 = Bitmap.createBitmap(targetWidth, targetHeight,
            Bitmap.Config.ARGB_8888);
            Canvas c = new Canvas(b2);

            int deltaXHalf = Math.max(0, deltaX / 2);
            int deltaYHalf = Math.max(0, deltaY / 2);
            Rect src = new Rect(
            deltaXHalf,
            deltaYHalf,
            deltaXHalf + Math.min(targetWidth, source.getWidth()),
            deltaYHalf + Math.min(targetHeight, source.getHeight()));
            int dstX = (targetWidth  - src.width())  / 2;
            int dstY = (targetHeight - src.height()) / 2;
            Rect dst = new Rect(
                    dstX,
                    dstY,
                    targetWidth - dstX,
                    targetHeight - dstY);
            c.drawBitmap(source, src, dst, null);
            if (recycle) {
                source.recycle();
            }
            return b2;
        }
        float bitmapWidthF = source.getWidth();
        float bitmapHeightF = source.getHeight();

        float bitmapAspect = bitmapWidthF / bitmapHeightF;
        float viewAspect   = (float) targetWidth / targetHeight;

        if (bitmapAspect > viewAspect) {
            float scale = targetHeight / bitmapHeightF;
            if (scale < .9F || scale > 1F) {
                scaler.setScale(scale, scale);
            } else {
                scaler = null;
            }
        } else {
            float scale = targetWidth / bitmapWidthF;
            if (scale < .9F || scale > 1F) {
                scaler.setScale(scale, scale);
            } else {
                scaler = null;
            }
        }

        Bitmap b1;
        if (scaler != null) {
            // this is used for minithumb and crop, so we want to filter here.
            b1 = Bitmap.createBitmap(source, 0, 0,
            source.getWidth(), source.getHeight(), scaler, true);
        } else {
            b1 = source;
        }

        if (recycle && b1 != source) {
            source.recycle();
        }

        int dx1 = Math.max(0, b1.getWidth() - targetWidth);
        int dy1 = Math.max(0, b1.getHeight() - targetHeight);

        Bitmap b2 = Bitmap.createBitmap(
                b1,
                dx1 / 2,
                dy1 / 2,
                targetWidth,
                targetHeight);

        if (b2 != b1) {
            if (recycle || b1 != source) {
                b1.recycle();
            }
        }

        return b2;
    }

    /**
     * SizedThumbnailBitmap contains the bitmap, which is downsampled either from
     * the thumbnail in exif or the full image.
     * mThumbnailData, mThumbnailWidth and mThumbnailHeight are set together only if mThumbnail
     * is not null.
     *
     * The width/height of the sized bitmap may be different from mThumbnailWidth/mThumbnailHeight.
     */
    private static class SizedThumbnailBitmap {
        public byte[] mThumbnailData;
        public Bitmap mBitmap;
        public int mThumbnailWidth;
        public int mThumbnailHeight;
    }

    /**
     * Creates a bitmap by either downsampling from the thumbnail in EXIF or the full image.
     * The functions returns a SizedThumbnailBitmap,
     * which contains a downsampled bitmap and the thumbnail data in EXIF if exists.
     */
    private static void createThumbnailFromEXIF(String filePath, int targetSize,
            int maxPixels, SizedThumbnailBitmap sizedThumbBitmap) {
        if (filePath == null) return;

        ExifInterface exif = null;
        byte [] thumbData = null;
        try {
            exif = new ExifInterface(filePath);
            if (exif != null) {
                thumbData = exif.getThumbnail();
            }
        } catch (IOException ex) {
            Log.w(TAG, ex);
        }

        BitmapFactory.Options fullOptions = new BitmapFactory.Options();
        BitmapFactory.Options exifOptions = new BitmapFactory.Options();
        int exifThumbWidth = 0;
        int fullThumbWidth = 0;

        // Compute exifThumbWidth.
        if (thumbData != null) {
            exifOptions.inJustDecodeBounds = true;
            BitmapFactory.decodeByteArray(thumbData, 0, thumbData.length, exifOptions);
            exifOptions.inSampleSize = computeSampleSize(exifOptions, targetSize, maxPixels);
            exifThumbWidth = exifOptions.outWidth / exifOptions.inSampleSize;
        }

        // Compute fullThumbWidth.
        fullOptions.inJustDecodeBounds = true;
        BitmapFactory.decodeFile(filePath, fullOptions);
        fullOptions.inSampleSize = computeSampleSize(fullOptions, targetSize, maxPixels);
        fullThumbWidth = fullOptions.outWidth / fullOptions.inSampleSize;

        // Choose the larger thumbnail as the returning sizedThumbBitmap.
        if (thumbData != null && exifThumbWidth >= fullThumbWidth) {
            int width = exifOptions.outWidth;
            int height = exifOptions.outHeight;
            exifOptions.inJustDecodeBounds = false;
            sizedThumbBitmap.mBitmap = BitmapFactory.decodeByteArray(thumbData, 0,
                    thumbData.length, exifOptions);
            if (sizedThumbBitmap.mBitmap != null) {
                sizedThumbBitmap.mThumbnailData = thumbData;
                sizedThumbBitmap.mThumbnailWidth = width;
                sizedThumbBitmap.mThumbnailHeight = height;
            }
        } else {
            fullOptions.inJustDecodeBounds = false;
            sizedThumbBitmap.mBitmap = BitmapFactory.decodeFile(filePath, fullOptions);
        }
    }
}