图像缩略图的坐标计算

这是一个代码片段，用于创建缩略图大小的图像（根据原始的大图像）并将其适当地放置在tableviewcell的顶部。当我学习代码时，我被卡在通过设置其横坐标和纵坐标为缩略图指定位置的部分。在方法-（void）setThumbDataFromImage：（UIImage *）image中，他们正在设置项目缩略图的尺寸和坐标，

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

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

   //    figure out a scaling ratio to make sure we maintain the same aspect ratio
         float ratio= MAX(newRect.size.width/origImageSize.width,   newRect.size.height/origImageSize.height);

  //       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 the subsequent drawing to clip to this rounded rectangle
       [path addClip];

    //    center the image in the thumbnail rectangle
      CGRect projectRect;
     projectRect.size.width=ratio * origImageSize.width;
     projectRect.size.height= ratio * origImageSize.height;
     projectRect.origin.x= (newRect.size.width- projectRect.size.width)/2;
     projectRect.origin.y= (newRect.size.height- projectRect.size.height)/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();
    }

我得到了宽度和高度的计算，其中我们将origImageSize与比例因子/比率相乘。但是，然后我们使用以下内容为缩略图指定一个位置：

     projectRect.origin.x= (newRect.size.width- projectRect.size.width)/2;
     projectRect.origin.y= (newRect.size.height- projectRect.size.height)/2;

这我不明白。我不能把头缠住它。：？这是定心过程的一部分。我的意思是，我们是在这里使用数学关系定位缩略图还是它是一些随机计算，即可能是什么？。我在这两行代码后面是否缺少一些基本知识？