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From: hawley@adobe.COM (Steve Hawley)
Newsgroups: comp.sys.mac.programmer
Subject: Re: An itsy bitsy question...
Message-ID: <9700@adobe.UUCP>
Date: 7 Jan 91 21:47:44 GMT
References: <La6cV1w164w@questor.wimsey.bc.ca> <27725.2787e3d0@kuhub.cc.ukans.edu>
Reply-To: hawley@adobe.UUCP (Steve Hawley)
Organization: Adobe Systems Incorporated, Mountain View
Lines: 101

In article <27725.2787e3d0@kuhub.cc.ukans.edu> mlab2@kuhub.cc.ukans.edu writes:
>In article <La6cV1w164w@questor.wimsey.bc.ca>, aberno@questor.wimsey.bc.ca (Anthony Berno) writes:
|> How is the "rubberbanding" done in programs like Superpaint when you are 
|> drawing a line on the screen?
|> Anthony Berno.
|
|When in line-drawing-mode, a mouseDown event in the drawing window should call
|a procedure similar to the below:
|procedure RubberBand;
|  var
|    originalPt, lastPt, newPt : Point;
|begin
|  GetMouse(originalPoint);
|  PenMode(patXOr);
|  GetMouse(lastPt);
|  MoveTo(originalPt.h, originalPt.v);
|  LineTo(lastPt.h, lastPt.v);
|  repeat
|    MoveTo(originalPt.h, originalPt.v);
|    LineTo(lastPt.h, lastPt);
|    GetMouse(newPt);
|    MoveTo(originalPt.h,originalPt.v);
|    LineTo(newPt.h, newPt.v);
|    lastPt:=newPt;
|  until (not Button);
|  PenNormal;
|end; 

Problem:
1) If the mouse doesn't move, the line will be flickering very badly.
2) The MoveTo/LineTo pairs are separated by a GetMouse() which will increase
   the flicker.
3) Erasing the old line first increases the "apparent" flicker
4) The loop should be terminated by the StillDown() function, not Button()

Solution:

  repeat
    GetMouse(newPt);
    MoveTo(originalPt.h, originalPt.v);    { Do the new point first }
    LineTo(newPt.h, newPt.v);              { This reduces apparent flicker }
    MoveTo(originalPt.h, originalPt.v);
    LineTo(lastPt.h, lastPt.v);
    lastPt := newPt;
  until (not StillDown(...I forget the args...));

Problem:
1) In such a tight loop, the overhead of the trap dispatcher is significant
   and contributes to the flicker in a bad way.

Solution:
Get the real function address from the GrafPorts QD procs in variables and
execute them directly

Problem:
1) When the new point differs from the old point by only a small amount, the
   line will still flicker because of the large amount of common area between
   the two lines, ie, drawing the new line will XOR over a large part of the
   old line leaving a big blank spot until the old line is re-drawn.

Solution:
   Copy window contents into offscreen bitmap #1 (a paint program would have
     this as the actual image data)
   SetPort to your own GrafPort that you have already setup with offscreen
     bitmap #2 as its portBits
   Hide the cursor

   repeat
      Get the new mouse coordinates
      CopyBits the smallest rectangle that encloses the old line, the new line
        and the cursor from bitmap #1 to bitmap #2
      Draw the new line in your port
      CopyBits the cursor mask onto bitmap #2
      CopyBits the cursor data onto bitmap #2
      CopyBits the smallest rectangle that encloses the old line, the new line
        and the cursor from bitmap #2 to the window
   until (not StillDown(...));

   Show the cursor

Can you say "Yuck"?  I knew you could...

The final method will give a minimal amount of flicker.  The problem is that
moving all that data around gets to be expensive in terms of response,
escpecially on bit lines.  For bonus points, you'd want the bitmaps to be
word or long word aligned to speed up the transfer, and you'd want the last
CopyBits (to the screen) to be in sync with the VBL rate.

The method that uses the pre-fetched addresses for the functions is probably
suitable for most needs, although probably not in a professional product.

Good luck...

Steve Hawley
hawley@adobe.com
Disclaimer: Adobe pays me to hack printers, not Macs.  I hack Macs in my spare
time
-- 
"I'm sick and tired of being told that ordinary decent people are fed up with
being sick and tired.  I know I'm certainly not, and I'm sick and tired of
begin told that I am." -Monty Python