android bitmap oom 优化

android使用位图显示图片，也就是像素点，jpg之类的压缩格式在android都会转成bitmap。

现在手机的分辨率也越来越高，480*800 大小的图片使用的内存大小：

480*800*32/8=1536000 =1.5M

32表示32位色，每个字节8位。

手机上有很多长图大小都是600*10000*32/8=24M，这样一来手机OOM是迟早的事。一些采用缩放和降低画质是解决不了问题的

例如下面这两种缩放还是会出现内存溢出的问题，

如何能让anroid获取网络图片时内存不OOM方法，使用BitmapFactory.decodeStream替代createBitmap方法，

原因是该方法直读取图片字节，调用JNI>>nativeDecodeAsset()来完成decode，无需再使用java层的createBitmap。

android使用Matrix实现bitmap缩放

[java]
Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.icon);

int width = bitmap.getWidth();
int height = bitmap.getHeight();
int newWidth = 640;
int newHeight = 480;

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

Matrix matrix = new Matrix();
matrix.postScale(scaleWidth, scaleHeight);

// create the new Bitmap object
Bitmap resizedBitmap = Bitmap.createBitmap(bitmap, 0, 0, width, height, matrix, true);
[/java]

android使用options.inJustDecodeBounds实现bitmap缩放

[java]
BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeFile(path, options);
if (options.mCancel || options.outWidth == -1
|| options.outHeight == -1) {
Log.d("OomDemo", "alert!!!" + String.valueOf(options.mCancel)
+ " " + options.outWidth + options.outHeight);
return null;
}
options.inSampleSize = Util.computeSampleSize(options, 600, (int) (1 * 1024 * 1024));
Log.d("OomDemo", "inSampleSize: " + options.inSampleSize);
options.inJustDecodeBounds = false;
options.inDither = false;
options.inPreferredConfig = Bitmap.Config.ARGB_8888; // 默认是Bitmap.Config.ARGB_8888
Bitmap bitmap = BitmapFactory.decodeFile(path, options);
[/java]

//四种构造Bitmap的使用的字节数
Bitmap.createBitmap(width, height,Bitmap.Config.ALPHA_8); 8bit
Bitmap.createBitmap(width, height,Bitmap.Config.ARGB_4444); 12bit
Bitmap.createBitmap(width, height,Bitmap.Config.ARGB_8888); 32bit
Bitmap.createBitmap(width, height,Bitmap.Config.RGB_565); 16bit

[java]
public class Util {

/*
* 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.
*/
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;
}

public 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;
}
}

}

[/java]

android上屏幕密度和像素转换
[java]
//转换dip为px

public static int convertDIP2PX(Context context, int dip) {
float scale = context.getResources().getDisplayMetrics().density;
return (int)(dip*scale + 0.5f*(dip>=0?1:-1));
}

//转换px为dip
public static int convertPX2DIP(Context context, int px) {
float scale = context.getResources().getDisplayMetrics().density;
return (int)(px/scale + 0.5f*(px>=0?1:-1));
}
[/java]
可以参考这里的实现方法
http://www.java2s.com/Open-Source/Android/android-platform-apps/Gallery3D/com/cooliris/media/UriTexture.java.htm