From: "Earl F. Glynn"
Subject: D4 Arithmetic Bug or Feature when subtracting byte values?
Date: 12 Jul 1998 00:00:00 GMT
Message-ID: <6ob23e$dfc@bgtnsc02.worldnet.att.net>
Organization: AT&T WorldNet Services
X-MimeOLE: Produced By Microsoft MimeOLE V4.72.2106.4
Newsgroups: borland.public.delphi.objectpascal,alt.comp.lang.borland-delphi,comp.lang.pascal.delphi.misc

While converting the ShowImage program in my Computer Lab to Delphi 4, I
experienced a problem in converting from (R,G,B) coordinates to (H,S,V)
coordinates.

I traced the problem to the difference of two components of a TRGBTriple,
namely the difference of the two bytes, RGB.rgbtBlue - RGB.rgbtRed.

Below is as short example of the problem.  I'm guessing the difference of
two bytes, when negative, is being treated differently in D4 at least
sometimes.  (This is deja vu from UCSD to TP  conversions for me.)

The solution is an extra typecast that wasn't needed before.

procedure TForm1.Button1Click(Sender: TObject);
  VAR
    a,b  :  BYTE;
    Delta:  INTEGER;
    H    :  INTEGER;
begin
  Delta := 4;
  a := 40;
  b := 45;

  ShowMessage( IntToStr(a - b) );  // -5  OK (or should this be wrong with
unsigned Bytes?)

  H := (a - b) DIV Delta;
  ShowMessage( IntToSTr(H) ); // 1,073,741,822  was unexpected here

  H := Integer(a-b) DIV Delta;  // Or is Integer(a) - Integer(b) better
here?
  ShowMessage( IntToSTr(H) ); // -1  OK
end;


Any comments or insight?

efg
_________________________________________

Earl F. Glynn