iOS, Thumbnail from image, Big Nerd Ranch, chapter 19 - ios

Question

I am going through 19th chapter of Big Nerd Ranch, iOS textbook and can not understand several parts of the function that takes in a big image and creates a thumbnail out of it. Have a look:
- (void)setThumbnailFromImage:(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
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 subsequent drawing 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) / 2.0;
projectRect.origin.y = (newRect.size.height - projectRect.size.height) / 2.0;
[image drawInRect:projectRect];
// Get the image from the image context; keep it as our thumbnail
UIImage *smallImage = UIGraphicsGetImageFromCurrentImageContext();
self.thumbnail = smallImage;
// Cleanup image context resources; we're done
UIGraphicsEndImageContext();
}
From my understanding we are getting the MAX of the two ratios and then we put a smaller edge of the original image equal to newRect's edge (which is 40 in our case), the other edge seemingly should stick out of the newRect since the edge would be larger than the edge of newRect, when we UIGraphicsGetImageFromCurrentImageContext(). That's my vague 'understanding'.
Could anyone please explain what is this whole code doing in a detailed way, especially, the centering part? If you know some tutorials that might be relevant, it would also be great.

Answer 1

I just took or added to the previous comments and tried to explain each part more clearly. You seemed to get the basic idea, so I hope this helps solidify everything.
- (void)setThumbnailFromImage:(UIImage *)image
{
CGSize origImageSize = image.size;
//Create new rectangle of your desired size
CGRect newRect = CGRectMake(0, 0, 40, 40);
//Divide both the width and the height by the width and height of the original image to get the proper ratio.
//Take whichever one is greater so that the converted image isn't distorted through incorrect scaling.
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
// Basically everything within this builds the image
UIGraphicsBeginImageContextWithOptions(newRect.size, NO, 0.0);
// Create a path that is a rounded rectangle -- essentially a frame for the new image
UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:newRect
cornerRadius:5.0];
// Applying path
[path addClip];
// Center the image in the thumbnail rectangle
CGRect projectRect;
// Scale the image with previously determined ratio
projectRect.size.width = ratio * origImageSize.width;
projectRect.size.height = ratio * origImageSize.height;
// I believe the anchor point of the new image is (0.5, 0.5), so here he is setting the position to be in the middle
// Half of the width and height added to whatever origin you have (in this case 0) will give the proper coordinates
projectRect.origin.x = (newRect.size.width - projectRect.size.width) / 2.0;
projectRect.origin.y = (newRect.size.height - projectRect.size.height) / 2.0;
// Add the scaled image
[image drawInRect:projectRect];
// Retrieving the image that has been created and saving it in memory
UIImage *smallImage = UIGraphicsGetImageFromCurrentImageContext();
self.thumbnail = smallImage;
// Cleanup image context resources; we're done
UIGraphicsEndImageContext();
}