iOS function says "CGRectMinY" which should be bottom left corner

Question

According to the Apple documentation,

CGRectMinY(CGRect) rect returns the y coordinate of the top right corner of the specified rectangle

Shouldn't it be the bottom left corner of the rectangle? Because I think the Y-axis is downwards.

Answer 1

If this is for iOS I think your function is wrong.

http://developer.apple.com/library/ios/#documentation/GraphicsImaging/Reference/CGGeometry/Reference/reference.html

CGRectGetMinY

Returns the y-coordinate that establishes the bottom edge of a rectangle.

CGFloat CGRectGetMinY ( CGRect rect );

Parameters

rect

The rectangle to examine. 

Return Value

The y-coordinate of the bottom-left corner of the specified rectangle. Availability

Available in iOS 2.0 and later.

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Declared In CGGeometry.h

Edit: To clear the confussion. The "CGRectGetMinY" is a function to be used on CGRect, this means that it will return the result as if it was only considering a rectangle. for example:

// With a self created element

CGRect rect = CGRectMake(0, 429, 100, 44);

NSLog(@"rect.origin.y: %f",rect.origin.y);
NSLog(@"rect.size.height: %f",rect.size.height);
NSLog(@"CGRectGetMinY(rect): %f",CGRectGetMinY(rect));
NSLog(@"CGRectGetMaxY(rect): %f",CGRectGetMaxY(rect));

returns

2012-06-04 10:55:49.737 test[7613:707] rect.origin.y: 429.000000
2012-06-04 10:55:49.739 test[7613:707] rect.size.height: 44.000000
2012-06-04 10:55:49.741 test[7613:707] CGRectGetMinY(rect): 429.000000
2012-06-04 10:55:49.748 test[7613:707] CGRectGetMaxY(rect): 473.000000

The key is to JUST consider this, if you ask for the min you will get a lower value than if you ask for the max. Even if you think about this without using the ios coordinate system.

NOW the ios IS inverted, so you have to consider this, the previous function will work as well, but visually speaking the result is inverted because the system is inverted.

// With an element on the screen

NSLog(@"gpsButton.frame.origin.y: %f",gpsButton.frame.origin.y);
NSLog(@"gpsButton.frame.size.height: %f",gpsButton.frame.size.height);
NSLog(@"CGRectGetMinY(gpsButton.frame): %f",CGRectGetMinY(gpsButton.frame));
NSLog(@"CGRectGetMaxY(gpsButton.frame): %f",CGRectGetMaxY(gpsButton.frame));

returns

2012-06-04 10:55:49.725 test[7613:707] gpsButton.frame.origin.y: 429.000000
2012-06-04 10:55:49.727 test[7613:707] gpsButton.frame.size.height: 44.000000
2012-06-04 10:55:49.732 test[7613:707] CGRectGetMinY(gpsButton.frame): 429.000000
2012-06-04 10:55:49.735 test[7613:707] CGRectGetMaxY(gpsButton.frame): 473.000000

For a person who sees this, there is also nothing wrong, min is less than max.

So the confusion is in the ios inverted system, since you want to retrieve a visually less value from an inverted system.

This is why it seems weird, the description in CGGeometry is made for the "human world". Not for ios inverted system.