android bitmap compress（圖片壓縮）

android bitmap compress

android的照相功能隨着手機硬件的發展，變得越來越強大，能夠找出很高分辨率的圖片。

有些場景中，需要照相併且上傳到服務，但是由於圖片的大小太大，那麼就上傳就會很慢(在有些網絡情況下)，而且很耗流量，要想速度快，那麼就需要減小圖片的大小。減少圖片的大小有兩種方法，1. 照小圖片； 2. 壓縮大圖片。 照相時獲取小圖片一般不太符合要求，因爲，圖片的清晰度會很差，但是這種情況有個好處就是應用速度會快些； 壓縮圖片，就是把大圖片壓縮小，降低圖片的質量，在一定範圍內，降低圖片的大小，並且滿足需求（圖片仍就清晰）。下面組要是介紹圖片的壓縮：

1. 照相請查看http://blog.csdn.net/luhuajcdd/article/details/8826587 ->想要保存圖片到制定目錄，啓動Camera應用時，需要指定文件

2. 壓縮過程：

    2.1 從圖片路徑中讀取圖片(圖片很大，不能全部加在到內存中處理，要是全部加載到內存中會內存溢出)

 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 處理圖片旋轉   

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壓縮圖片        

bm.compress(Bitmap.CompressFormat.JPEG, 30, baos);//30 是壓縮率，表示壓縮70%; 如果不壓縮是100，表示壓縮率爲0

完整的方法代碼：

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;
	}