How can I increase the size of a CGRect by a certain percent value?

Question

How can I increase the size of a CGRect by a certain percent value? Should I use some form of CGRectInset to do it?

Example:

Assume I have a CGRect: {10, 10, 110, 110}

I want to increase it's size (retaining the same center point) by 20% to:

{0, 0, 120, 120}

Answer 1

You can use CGRectInset if you like:

double pct = 0.2;
CGRect newRect = CGRectInset(oldRect, -CGRectGetWidth(oldRect)*pct/2, -CGRectGetHeight(oldRect)*pct/2);

To decrease the size, remove the -s.

Side note: A CGRect that is 20% bigger than {10, 10, 100, 100} is {0, 0, 120, 120}.

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Edit: If the intention is to increase by area, then this'll do it (even for rectangles that aren't square):

CGFloat width = CGRectGetWidth(oldRect);
CGFloat height = CGRectGetHeight(oldRect);
double pct = 1.2; // 20% increase
double newWidth = sqrt(width * width * pct);
double newHeight = sqrt(height * height * pct);
CGRect newRect = CGRectInset(oldRect, (width-newWidth)/2, (height-newHeight)/2);