For the More Curious: isFlipped

Both PDF and PostScript use the standard Cartesian coordinate system, whereby y increases as you move up the page. Quartz follows this model by default. The origin is usually at the lower-left corner of the view.

For some types of drawing, the math becomes easier if the upper-left corner is the origin and y increases as you move down the page. We say that such a view is flipped.

To flip a view, you override isFlipped in your view class to return YES:

- (BOOL)isFlipped
{
    return YES;
}

While we are discussing the coordinate system, note that x- and y-coordinates are measured in points. A point is typically defined as 72.0 points = 1 inch. In reality, by default 1.0 point = 1 pixel on your screen. You can, however, change the size of a point by changing the coordinate system:

// Make everything in the view twice as large
NSSize newScale;
newScale.width = 2.0;
newScale.height = 2.0;
[myView scaleUnitSquareToSize:newScale];
[myView setNeedsDisplay:YES];