Connection between UIGraphicsGetImageFromCurrentImageContext and drawInrect - iphone

Question

I have found the following code to resize an UIImage:
CGSize newSize = CGSizeMake(self.image.size.width*0.25, self.image.size.height*0.25);
UIGraphicsBeginImageContextWithOptions(newSize, NO, 0.0);
[self.image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
self.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
but there are some couple of things I don't understand.
First I'm trying to resize the original image to 25% of the original size - but this method resizes it to 50% of the original size. Why?
What is the connection between drawInRect and UIGraphicsGetImageFromCurrentImageContext. As I see it, the UIGraphicsGetImageFromCurrentImageContext is overwriting the current image making the call to drawInRect redundant.
I would be grateful if someone could help me understand what's going on in details.
Thanks in advance.

Answer 1

first , because it's retina screen, you should set the scale to 1.0 , or it just x2
UIGraphicsBeginImageContextWithOptions(newSize, NO, 1.0);
if you call UIGraphicsBeginImageContext , any paint work will result in the Context you specific
the drawInRect in your code paint the image to the context, it's not redundant
or you can remove it, you get a empty image
finally, you can get a merged UIImage from the context
UIGraphicsGetImageFromCurrentImageContext();
just get a UIImage From What you have done(on context)
if you don't set the image back , it's won't change anything
self.image = UIGraphicsGetImageFromCurrentImageContext();