Subject: Re: Exact definition of NSView/NSEvent coordinate
system origin?



On Aug 11, 2017, at 11:38 AM, Sean McBride <[email protected]> wrote:
>
> I'm tying to understand the exact definition of the Cocoa drawing and event
> coordinate system on macOS. In particular the exact location of 0,0 and
> subpixel accuracy.
>
> I know 0,0 is the bottom-left, but is it:
> - the centre of the bottom-left pixel?
> - the bottom-left corner of the bottom-left pixel?

It's the bottom-left corner of the bottom-left pixel.

> I made a custom NSView and implemented mouseDown:
>
> - (void)mouseDown:(NSEvent*)inEvent {
> NSPoint viewPoint = [self convertPoint:[inEvent locationInWindow]
> fromView:nil];
>
> I then use Pixie.app at max magnification and click the bottom-left pixel,
> yielding results like:
>
> (lldb) p viewPoint
> (NSPoint) $3 = (x = 0.15234375, y = 0.90625)
>
> So it's giving fractional values, but they are hard to interpret without
> knowing where 0,0 is exactly.

Depending on what you're doing, you should more or less ignore the fractional
part. At some point, Apple started supporting high-resolution mouse positions.
Since then, the mouse is almost never at an integral position.

> Then there's the 'locationInWindow' docs, that say "Note: The y coordinate in
> the returned point starts from a base of 1, not 0." That's quite odd. Why?

I believe for compatibility with what was originally a bug. When converting
from the Core Graphics coordinate space, where the origin is at the top left,
to the Cocoa coordinate space, where it's at the bottom left, somebody did pt.y
= primaryScreenHeight - pt.y. That's correct for an infinitely-small point.
However, fo...

r a pixel, it's not. On a single-screen system and with
integral-pixel positions, pt.y can range from 0 to primaryScreenHeight - 1,
inclusive. That means that (primaryScreenHeight - pt.y) can range from
primaryScreenHeight to 1, inclusive. The correct code would have been pt.y =
primaryScreenHeight - 1 - pt.y, but it's too late to fix it now.

Regards,
Ken

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