Bitmap二次采样 有更新!

2017-09-05
0 评论 1,154 浏览

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;

	}

}

}

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