Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!purdue!mentor.cc.purdue.edu!miyakekm
From: miyakekm@mentor.cc.purdue.edu (Keith Miyake)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Mathematical problems with QuickDraw
Message-ID: <15474@mentor.cc.purdue.edu>
Date: 22 Oct 90 08:30:38 GMT
References: <1990Oct20.193947.16700@agate.berkeley.edu>
Reply-To: miyakekm@mentor.cc.purdue.edu (Keith Miyake)
Organization: Purdue University
Lines: 36

In article <1990Oct20.193947.16700@agate.berkeley.edu> deadman@garnet.berkeley.edu (Ben Haller) writes:
>Well.  Ragging on QuickDraw being one of my favorite pastimes...
>[ gaps in ovals ]

I agree that this is not the way it should be done.  I don't know if I'd call
it "mathematically incorrect."  It was just a stupid algorithm for them to
have chosen.  I don't remember where I heard this, but Quickdraw's
oval frames are supposed to be the set of points between the ideal oval
enclosed by the given rectangle and the ideal oval given by the rectangle
inset by the pen width and pen height.  Sometimes, this means that there are
gaps in the oval.  Supposedly, this is a feature, not a bug.

>[ doesn't draw lines quite right ]
Agreed.

>  QuickDraw has a strange habit of drawing extra pixels in its circles.  
Same reason as above.

>Why doesn't QuickDraw use them [Bresenham's etc.]?

Quickdraw's oval algorithm is based on a different mathematical concept
of ideal oval.

>  IMHO, the whole thing with QuickDraw using "pixel grid intersections"
>instead of pixel centers is ridiculous.  

There are times where it makes more sense than the more common model, and
there are times when it makes less sense.

>Apple just messed up for no good reason.  That what *I* think.

Well, I think it's a *little* more complicated than that.  But let's just
assume you were joking.


-Sho