- (void)setThumbnailDataFromImage:(UIImage *)image
{
    CGSize origImageSize = [image size];

    // The rectangle of the thumbnail
    CGRect newRect = CGRectMake(0, 0, 40, 40);

    // Figure out a scaling ratio to make sure we maintain the same aspect ratio
    // --------------------
    // [START] 这里看不懂 1
    float ratio = MAX(newRect.size.width / origImageSize.width, 
                      newRect.size.height / origImageSize.height);
    // [END] 这里看不懂 1
    // --------------------

    // Create a transparent bitmap context with a scaling factor 
    // equal to that of the screen
    UIGraphicsBeginImageContextWithOptions(newRect.size, NO, 0.0);

    // Create a path that is a rounded rectangle
    UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:newRect
                                                    cornerRadius:5.0];
    // Make all subsequent drawing clip to this rounded rectangle
    [path addClip];

    // Center the image in the thumbnail rectangle
    // --------------------
    // [START] 这里看不懂 2
    CGRect projectRect;
    projectRect.size.width = ratio * origImageSize.width;
    projectRect.size.height = ratio * origImageSize.height;
    projectRect.origin.x = (newRect.size.width - projectRect.size.width) / 2.0;
    projectRect.origin.y = (newRect.size.height - projectRect.size.height) / 2.0;
    // [END] 这里看不懂 2
    // --------------------

    // Draw the image on it
    [image drawInRect:projectRect];

    // Get the image from the image context, keep it as our thumbnail
    UIImage *smallImage = UIGraphicsGetImageFromCurrentImageContext();
    [self setThumbnail:smallImage];

    // Get the PNG representation of the image and set it as our archivable data
    NSData *data = UIImagePNGRepresentation(smallImage);
    [self setThumbnailData:data];

    // Cleanup image context resources, we're done
    UIGraphicsEndImageContext();
}

关于那两段代码，

1. 第一段那个比例为什么要取大的？
2. 第二段代码是计算图片的居中显示位置，为什么要这么计算，为什么这么计算就居中了？

看了半会没看懂，求大神解释