====== zoom centered on point ======
<code objc>
- (void)zoom:(id)sender mode:(int)mode
{

	// find the base value of the currently viewed centerpoint (no scaleFactor)
	NSClipView *cv = (NSClipView *)[self superview];
	NSRect docVisRect = [cv documentVisibleRect];
	NSPoint viewCenterPoint;
	viewCenterPoint.x = ( docVisRect.origin.x + (docVisRect.size.width / 2.) ) * 1/scaleFactor;
	viewCenterPoint.y = ( docVisRect.origin.y + (docVisRect.size.height / 2.) ) * 1/scaleFactor;

	if(mode == ZOOM_IN){
		// the NSSlider won't allow values outside of our min/max, so this is safe
		[sfSlider setFloatValue:scaleFactor + KEY_ZOOM_INCREMENT];
	}else if(mode == ZOOM_OUT){
		// the NSSlider won't allow values outside of our min/max, so this is safe
		[sfSlider setFloatValue:scaleFactor - KEY_ZOOM_INCREMENT];
	}
	
	[self changeScale:self];  // this also updates the display (frame, etc...)
	
	// transform the point using the new scaleFactor
	NSPoint scaledCenterPoint;
	scaledCenterPoint.x = viewCenterPoint.x * scaleFactor;
	scaledCenterPoint.y = viewCenterPoint.y * scaleFactor;
	
	// calculate the new origin - we must scroll to a new origin, not to a new center point
	NSPoint newOrigin;
	docVisRect = [cv documentVisibleRect];
	newOrigin.x = scaledCenterPoint.x - (docVisRect.size.width / 2.);
	newOrigin.y = scaledCenterPoint.y - (docVisRect.size.height / 2.);
	
	// force the NSScrollView to scroll to the new point
	[cv scrollToPoint:[cv constrainScrollPoint:newOrigin]];
	[self setNeedsDisplay:YES];
	
	// make sure the scroll bars are updated
	[[[self superview] superview] reflectScrolledClipView:cv];
	
}
</code>