UIImage旋转引起图片像素降低

使用下面的代码旋转图片：

CGFloat DegreesToRads(CGFloat degrees) {
   return degrees * M_PI / 180;
}
- (UIImage *)scaleAndRotateImage:(UIImage *)image forAngle: (double) angle {

    float radians=DegreesToRads(angle);
    // calculate the size of the rotated view's containing box for our drawing space
    UIView *rotatedViewBox = [[UIView alloc] initWithFrame:CGRectMake(0,0, image.size.width, image.size.height)];
    CGAffineTransform t = CGAffineTransformMakeRotation(radians);
    rotatedViewBox.transform = t;
    CGSize rotatedSize = rotatedViewBox.frame.size;

    // Create the bitmap context
    UIGraphicsBeginImageContext(rotatedSize);
    CGContextRef bitmap = UIGraphicsGetCurrentContext();

    // Move the origin to the middle of the image so we will rotate and scale around the center.
    CGContextTranslateCTM(bitmap, rotatedSize.width/2, rotatedSize.height/2);

    //Rotate the image context
    CGContextRotateCTM(bitmap, radians);

    // Now, draw the rotated/scaled image into the context
    CGContextScaleCTM(bitmap, 1.0, -1.0);
    CGContextDrawImage(bitmap, CGRectMake(-image.size.width/2, -image.size.height/2 , image.size.width, image.size.height), image.CGImage );
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}

旋转实现了，但是旋转之后图片的像素就会下降。为什么？

你参考一下这个：
http://blog.sina.com.cn/s/blog_9cd1705d0101lo7a.html