From: "Earl F. Glynn" <efg2@efg2.com>
To: "Thérèse Hanquet" <therese.hanquet@skynet.be>
Subject: Re: Computer Lab Feedback
Date: Monday, March 05, 2001 12:03 AM

Hi, Thérèse:
> I just looked at your lab report on Brush bitmaps and I
> wondered how you had colored squares when setting brush style
> to bsClear, because that's not what I get when I draw with this style.
...
>  Then I looked into the Print Demo it refers to, and I see that
> after setting the brush style to bsClear you set the brush color.

When I wrote Printer Demo #2 originally in D1 -- likely in 1995 or 1996 --
I wanted to compare the brush styles for the various colors on a variety of
printers.  I really didn't study why bsClear worked they way it did.  I
should have.  You have an eagle eye and mind to catch this problem.

>  Finally I looked into graphics.pas and saw that setting the color for
> the brush sets brush style to bsSolid if the style was previously bsClear.
> Then what you endly get is not bsClear but bsSolid, which explains that
> you get the same two first rows.

I didn't understand exactly what you were saying until I studied the code you described.

This short example shows the difference:

// Why does Brush.Style = bsClear behave differently if
// Brush.Color is set before or after?  This behavior
// appears to date back to D1.
procedure TFormBSenigma.ButtonDrawRectsClick(Sender: TObject);
  VAR
    Bitmap:  TBitmap;
    brush :  WORD;
    y     :  WORD;
begin

  Bitmap := TBitmap.Create;
  TRY
    Bitmap.Width  := Image1.Width;
    Bitmap.Height := Image1.Height;

    FOR brush := 0 TO 7 DO     // wrong bsClear behavior here?
    BEGIN
      Bitmap.Canvas.Brush.Style := TBrushStyle(brush);
      Bitmap.Canvas.Brush.Color := clRed;
      y := 5 + brush*20;
      Bitmap.Canvas.Rectangle (5, y, 5+20, y+20);
    END;

    FOR brush := 0 TO 7 DO    // expected bsClear behavior
    BEGIN
      Bitmap.Canvas.Brush.Color := clRed;
      Bitmap.Canvas.Brush.Style := TBrushStyle(brush);
      y := 5 + brush*20;
      Bitmap.Canvas.Rectangle (55, y, 55+20, y+20);
    END;

    Image1.Picture.Graphic := Bitmap;
  FINALLY
    Bitmap.Free
  END
end;

> This behaviour is not what is explained in the online help,
> which says that when you set the color after setting bsClear
> it has no effect and the color is lost. Do you think it's a bug?

My vote is "yes" this is a bug, but perhaps it's a feature I don't understand. <g>

...

Thanks for this interesting question.  Best wishes,
--
efg     efg2@efg2.com     Earl F. Glynn, Overland Park, KS  USA

efg's Computer Lab:  http://www.efg2.com/Lab
Mirror:  http://homepages.borland.com/efg2lab/Default.htm