Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!usc!snorkelwacker!bloom-beacon!eru!luth!sunic!tut!santra!kampi.hut.fi!jmunkki
From: jmunkki@kampi.hut.fi (Juri Munkki)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Line selection algorithims..
Message-ID: <1989Dec17.104221.23095@santra.uucp>
Date: 17 Dec 89 10:42:21 GMT
References: <8912160053.AA00720@cadman.nyu.edu>
Sender: news@santra.uucp (Cnews - USENET news system)
Reply-To: jmunkki@kampi.hut.fi (Juri Munkki)
Organization: Helsinki University of Technology, Finland
Lines: 31

In article <8912160053.AA00720@cadman.nyu.edu> deragon@CADMAN.NYU.EDU (John Deragon) writes:
>		I was wondering how the current applications like
>MacDraw, MacDraft...etc etc do the line selection. For example in 
>MacDraw you click on the line and it becomes selected? How does it 
>do this? What is it testing on mouseDown? 

I think I read that MacDraw uses QuickDraw to do this. You need a
grafport with a single pixel (or many pixels, if your pointer should
not be with single pixel resolution). Then draw all objects with a
black penpat. Check the pixel after every object and stop when it
is black. The last object you drew was hit.

You have to draw the objects from front to back. I also recommend that
you create a "select next object" menu item that resumes the last
selection check. You also have to set up the grafport so that the
single pixel (or group of pixels) have the coordinates of the mousedown.

I wrote a very simple Bezier curve program and used my own window as
the checking grafport. This causes a pixel to blink and you have to
update that pixel after this routine, so I recommend that you use
an offscreen bitmap. Offscreen bitmaps have the benefit that you
can just a look at the bitmap contents without having to call GetPixel.

This algorithm is surprisingly fast. I guess QuickDraw clips so well
that discards most objects without even considering them. Most of
the overhead probably still comes from trap dispatches.

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|     Juri Munkki jmunkki@hut.fi  jmunkki@fingate.bitnet        I Want   Ne   |
|     Helsinki University of Technology Computing Centre        My Own   XT   |
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