DevBytes: Bitmap Scaling

图片的Scaling:

下面这个example展示了怎样使用BitmapOptions，影响图片的加载的效果。Sub-sampling 能够加快图片加载的时间和 减少图片所占用的内存。它不能得到图片的大小，而只是在2的幂下的图片的大小  1/2 1/4 1/8。

源码：

*
 * This example shows how the use of BitmapOptions affects the resulting size of a loaded
 * bitmap. Sub-sampling can speed up load times and reduce the need for large bitmaps
 * in memory if your target bitmap size is much smaller, although it's good to understand
 * that you can't get specific Bitmap sizes, but rather power-of-two reductions in sizes.
 * 
 * 这个example展示了怎样使用BitmapOptions，影响图片的加载的效果。Sub-sampling 能够加快图片加载的时间和
 * 减少图片所占用的内存。它不能得到图片的大小，而只是在2的幂下的图片的大小  1/2 1/4 1/8
 * 
 * Watch the associated video for this demo on the DevBytes channel of developer.android.com
 * or on YouTube at https://www.youtube.com/watch?v=12cB7gnL6po.
 */
public class BitmapScaling extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_bitmap_scaling);

        LinearLayout container = (LinearLayout) findViewById(R.id.scaledImageContainer);
        ImageView originalImageView = (ImageView) findViewById(R.id.originalImageHolder);

        Bitmap bitmap = BitmapFactory.decodeResource(getResources(),
                R.drawable.jellybean_statue);
        
        originalImageView.setImageBitmap(bitmap);

        for (int i = 2; i < 10; ++i) {
            addScaledImageView(bitmap, i, container);
        }
    }

    private void addScaledImageView(Bitmap original, int sampleSize, LinearLayout container) {

        // inSampleSize tells the loader how much to scale the final image, which it does at
        // load time by simply reading less pixels for every pixel value in the final bitmap.
        // Note that it only scales by powers of two, so a value of two results in a bitmap
        // 1/2 the size of the original and a value of four results in a bitmap 1/4 the original
        // size. Intermediate values are rounded down, so a value of three results in a bitmap 1/2
        // the original size.

        BitmapFactory.Options bitmapOptions = new BitmapFactory.Options();
        bitmapOptions.inSampleSize = sampleSize;
        
        Bitmap scaledBitmap = BitmapFactory.decodeResource(getResources(),
                R.drawable.jellybean_statue, bitmapOptions);
        
        ImageView scaledImageView = new ImageView(this);
        scaledImageView.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT,
                LayoutParams.WRAP_CONTENT));
        
        scaledImageView.setImageBitmap(scaledBitmap);
        container.addView(scaledImageView);
    }
}

截图：

ps:我们可以看到图片的大小的尺寸都是1/2 1/4 1/8...的比例渐变的

图片的尺寸：

我们知道用 BitmapFactory.decodeFile之类的直接加载图片的方法，当图片过大的时候会发生OOM。怎么避免它呢？
这就用到了我们上面提到的BitmapFactory.Options这个类。
BitmapFactory.Options这个类，有一个字段叫做 inJustDecodeBounds 。SDK中对这个成员的说明是这样的：
If set to true, the decoder will return null (no bitmap), but the out…
也就是说，如果我们把它设为true，那么BitmapFactory.decodeFile(String path, Options opt)并不会真的返回一个Bitmap给你，它仅仅会把它的宽，高取回来给你，这样就不会占用太多的内存，也就不会那么频繁的发生OOM了。
示例代码如下：

	BitmapFactory.Options bitmapOptions = new BitmapFactory.Options();
    	 bitmapOptions.inJustDecodeBounds=true;
        	Bitmap scaledBitmap = BitmapFactory.decodeResource(getResources(),
                R.drawable.jellybean_statue, bitmapOptions);
	int outHeight = bitmapOptions.outHeight;
        	int outWidth = bitmapOptions.outWidth;

这段代码之后，options.outWidth 和 options.outHeight就是我们想要的宽和高了


参考：

http://blog.csdn.net/hjm4702192/article/details/7821519

http://my.eoe.cn/isnull/archive/564.html

http://www.youtube.com/watch?v=12cB7gnL6po&list=PLWz5rJ2EKKc_XOgcRukSoKKjewFJZrKV0&index=51

 android BitmapFactory.Options 翻译