android bitmap compress（图片压缩）

android bitmap compress

android的照相功能随着手机硬件的发展，变得越来越强大，能够找出很高分辨率的图片。

有些场景中，需要照相并且上传到服务，但是由于图片的大小太大，那么就上传就会很慢(在有些网络情况下)，而且很耗流量，要想速度快，那么就需要减小图片的大小。减少图片的大小有两种方法，1. 照小图片； 2. 压缩大图片。 照相时获取小图片一般不太符合要求，因为，图片的清晰度会很差，但是这种情况有个好处就是应用速度会快些； 压缩图片，就是把大图片压缩小，降低图片的质量，在一定范围内，降低图片的大小，并且满足需求（图片仍就清晰）。下面组要是介绍图片的压缩：

1. 照相请查看http://blog.csdn.net/luhuajcdd/article/details/8826587 ->想要保存图片到制定目录，启动Camera应用时，需要指定文件

2. 压缩过程：

    2.1 从图片路径中读取图片(图片很大，不能全部加在到内存中处理，要是全部加载到内存中会内存溢出)

 final BitmapFactory.Options options = new BitmapFactory.Options();
		options.inJustDecodeBounds = true;
		BitmapFactory.decodeFile(filePath, options);

		// Calculate inSampleSize
		options.inSampleSize = calculateInSampleSize(options, 480, 800);

		// Decode bitmap with inSampleSize set
		options.inJustDecodeBounds = false;
		
		Bitmap bm = BitmapFactory.decodeFile(filePath, options);

    2.2 处理图片旋转   

int degree = readPictureDegree(filePath);
		bm = rotateBitmap(bm,degree) ;

private static int readPictureDegree(String path) {  
	       int degree  = 0;  
	       try {  
	               ExifInterface exifInterface = new ExifInterface(path);  
	               int orientation = exifInterface.getAttributeInt(ExifInterface.TAG_ORIENTATION, ExifInterface.ORIENTATION_NORMAL);  
	               switch (orientation) {  
	               case ExifInterface.ORIENTATION_ROTATE_90:  
	                       degree = 90;  
	                       break;  
	               case ExifInterface.ORIENTATION_ROTATE_180:  
	                       degree = 180;  
	                       break;  
	               case ExifInterface.ORIENTATION_ROTATE_270:  
	                       degree = 270;  
	                       break;  
	               }  
	       } catch (IOException e) {  
	               e.printStackTrace();  
	       }  
	       return degree;  
	   } 

private static Bitmap rotateBitmap(Bitmap bitmap, int rotate){
		if(bitmap == null)
			return null ;
		
		int w = bitmap.getWidth();
		int h = bitmap.getHeight();

		// Setting post rotate to 90
		Matrix mtx = new Matrix();
		mtx.postRotate(rotate);
		return Bitmap.createBitmap(bitmap, 0, 0, w, h, mtx, true);
	}

    2.3压缩图片        

bm.compress(Bitmap.CompressFormat.JPEG, 30, baos);//30 是压缩率，表示压缩70%; 如果不压缩是100，表示压缩率为0

完整的方法代码：

public static Bitmap getSmallBitmap(String filePath) {
	    	
		final BitmapFactory.Options options = new BitmapFactory.Options();
		options.inJustDecodeBounds = true;
		BitmapFactory.decodeFile(filePath, options);

		// Calculate inSampleSize
		options.inSampleSize = calculateInSampleSize(options, 480, 800);

		// Decode bitmap with inSampleSize set
		options.inJustDecodeBounds = false;
		
		Bitmap bm = BitmapFactory.decodeFile(filePath, options);
		if(bm == null){
			return  null;
		}
		int degree = readPictureDegree(filePath);
		bm = rotateBitmap(bm,degree) ;
		ByteArrayOutputStream baos = null ;
		try{
			baos = new ByteArrayOutputStream();
			bm.compress(Bitmap.CompressFormat.JPEG, 30, baos);
			
		}finally{
			try {
				if(baos != null)
					baos.close() ;
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
		return bm ;

	}

private static int calculateInSampleSize(BitmapFactory.Options options,
			int reqWidth, int reqHeight) {
		// Raw height and width of image
		final int height = options.outHeight;
		final int width = options.outWidth;
		int inSampleSize = 1;

		if (height > reqHeight || width > reqWidth) {

			// Calculate ratios of height and width to requested height and
			// width
			final int heightRatio = Math.round((float) height
					/ (float) reqHeight);
			final int widthRatio = Math.round((float) width / (float) reqWidth);

			// Choose the smallest ratio as inSampleSize value, this will
			// guarantee
			// a final image with both dimensions larger than or equal to the
			// requested height and width.
			inSampleSize = heightRatio < widthRatio ? widthRatio : heightRatio;
		}

		return inSampleSize;
	}