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



On Fri, 11 Aug 2017 11:53:10 -0500, Ken Thomases said:

>> 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.

Playing around with NSRectFill() that indeed seems to be the case, thanks.

I assume it's the same for screen, window, and layer coordinates, surely?

>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.

Ah, found this in the 10.1 (sic) AppKit Release Notes: "-[NSEvent
locationInWindow] may now return NSPoints with non-integral coordinates to
represent sub-pixel precision generated by some input devices..."

Presumably these are also corner-pixel based?

>> 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, for 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.

Right, classic off-by-1 error. :)

Doing the following in mouseDown: is idiomatic:

NSPoint viewPoint = [self convertPoint:[inEvent locationInWindow]
fromView:nil];

but it's not clear to me if convertPoint:fromView: expects the off-by-1 value
from locationInWindow or not.

It seems to work correctly the idiomatic way, that is, if I use Pixie to zoom
and carefully click the corner pixel in my NSView, the 'viewPoint' is always
some fraction between 0 and 1 in both x and y. I would have expected to be
off-by-1 in y....

Cheers,

Sean


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