android 低版本sdk中，如何生成缩略图

本人在1.6版中做开发，要生产图片缩略图，怎么生成呢？搜索了下，感觉都不是很专业。无意发现android 2.2中新增了个缩略图ThumbnailUtils类。抓紧看代码，

很快就抽出了其中的代码段：

import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.PorterDuff.Mode;
import android.graphics.PorterDuffXfermode;
import android.graphics.Rect;
import android.graphics.RectF;

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

public class ThumbnailUtils {
	/** Options used internally. */
	private static final int OPTIONS_NONE = 0x0;
	private static final int OPTIONS_SCALE_UP = 0x1;
	private static final String mTag="ThumbnailUtils";

	/***
	 * 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.
	 */

	/***
	 * 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
	 */

	/***
	 * 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;
	}

	/***
	 * 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;
	}	
}

在需要生成缩略图的地方，调用ThumbnailUtils.extractThumbnail 方法，即可！该类的详细信息，请参阅：

http://www.oschina.net/code/explore/android-2.2-froyo/android/media/ThumbnailUtils.java