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From: sfleming@cs.hw.ac.uk (Stewart T. Fleming)
Newsgroups: comp.sys.mac.programmer
Subject: Re: An itsy bitsy question...
Message-ID: <1929@odin.cs.hw.ac.uk>
Date: 9 Jan 91 19:57:16 GMT
References: <La6cV1w164w@questor.wimsey.bc.ca> <27725.2787e3d0@kuhub.cc.ukans.edu> <hf"2h2."b2@smurf.sub.org>
Sender: news@cs.hw.ac.uk
Reply-To: sfleming@cs.hw.ac.uk (Stewart T. Fleming)
Organization: Dept. Of Computer Science, Heriot-Watt University, Edinburgh
Lines: 73

In article <hf"2h2."b2@smurf.sub.org>, urlichs@smurf.sub.org (Matthias Urlichs) writes:
|> In comp.sys.mac.programmer, article <27725.2787e3d0@kuhub.cc.ukans.edu>,
|>   mlab2@kuhub.cc.ukans.edu writes:
|> < 
|> < When in line-drawing-mode, a mouseDown event in the drawing window should 
|> Modify at will for drawing other shapes, of course...
|> 

If you simply extend this procedure to do other shapes, then you will likely get horrific flicker problems.  True/False ?  Drawing and erasing the whole shape is fairly obvious to the user.  Drawing lines with this method is fine, but for other shapes :

Procedure TrackShape(Var currPt, prevPt, startPt: Point);

Var
	currRect, prevRect: Rect;
	prevRgn, dRgn: RgnHandle;

Begin
{Draw if mouse moves or is outside the window frame (autoscroll)}
If NeedToScroll(currPt) Or Not EqualPt(currPt, prevPt) Then
	Begin
	GetWindowFrame(currRect);	{Pin point in frame}
	PinInRect(currRect, currPt);

	Pt2Rect(startPt, currPt, currRect);	{Make shapes}
	Pt2Rect(startPt, prevPt, prevRect);

	dRgn := OutlineRgn(currRect);		{Outline shapes}
	prevRgn := OutlineRgn(prevRect);

	XorRgn(dRgn, prevRgn, gUtilRgn);	{Find bits changed}

	DisposeRgn(prevRgn);
	DisposeRgn(dRgn);

	PenMode(patXor);			{Draw bits changed}
	PaintRgn(gUtilRgn);
	End;
End;

Function OutlineRgn (theRect: Rect): RgnHandle;

Var
outRgn, inRgn: RgnHandle;

Begin
outRgn := NewRgn;
inRgn := NewRgn;

RectRgn(outRgn, theRect);	{ Construct region consisting of a	}
CopyRgn(outRgn, inRgn);		{   one pixel thick outline of		}
InsetRgn(inRgn, 1, 1);		{   the previous selection rect	}
DiffRgn(outRgn, inRgn, outRgn);

OutlineRgn := outRgn;
DisposeRgn(inRgn);
End;

I wrote this as a mouse task for Think Pascal with OOP.  It works, but I don't think it's too pretty.  Feel free to take it apart :-)

Here, I just use rectangles, but any shape could be done in the same way by changing OutlineRgn.  NeedToScroll checks if the point is outside the frame; if so, it calls ScrollRect.

|> I'd suggest some enhancements:
> - invalidating the old line rather than simply erasing it;
|>   there might be other shapes behind it.

Do you need to do this after every move ?  Or not until the user releases the mouse ?

|> Matthias Urlichs -- urlichs@smurf.sub.org -- urlichs@smurf.ira.uka.de     

Stewart
-- 
sfleming@cs.hw.ac.uk                                ...ukc!cs.hw.ac.uk!sfleming
"Before starting any programming project, try explaining it to your cat."