CODEWITHSUNDEEP
delphi
CyprUS
CyprUS

Reputation: 4239

How to make a Drawn Line on a Form Invisible in Delphi 7

I am trying to make an analog clock, wherein I would like to make my 'seconds' Line Invisible when the Seconds Changes. I have tried to set the Pen mode to pmNotCopy but it only gives Inverse of Pen Color. What Property must be set in this Form1.Canvas.Pen.Mode:=<Blank> so that My Line Disappears?

Any other Ideas are also appreciated.

Thanks

Upvotes: 2

Views: 1462

Answers (3)

Ken White
Ken White

Reputation: 125669

You were close. You need to use pmXOR.

Try this:

Create a new Delphi VCL Forms application. Drop a TButton on the bottom of the form (Button1). Add the code below to the Button1Click event.

Run the application. Click the button, and three parallel lines will be drawn across the top. Click the button again, and the three lines will disappear. 

procedure TForm1.Button1Click(Sender: TObject);
var
  i: Integer;
begin
  Canvas.Pen.Mode := pmXor;
  Canvas.Pen.Color := clGreen;
  for i := 1 to 3 do
  begin
    Canvas.MoveTo(50, i * 20);
    Canvas.LineTo(Width - 50, i * 20);
  end;
end;

All drawing should normally be done in the OnPaint event; I'm intentionally drawing in the Button1Click event for demonstration reasons only.

Just as a note: You should never use Form1 inside an event handler for that form. Referencing the Form1 variable prevents you from creating more than one instance of the form. Use Self instead, and that will automatically refer to the instance the code is running in.

Upvotes: 2

Karel-Jan Misseghers
Karel-Jan Misseghers

Reputation: 807

I don't know anything about delphi but just some out if the box thinking: couldn't you change the color of your line to the background color, making it 'invisible'

Upvotes: 2

Andreas Rejbrand
Andreas Rejbrand

Reputation: 108948

Modern computers are very fast. If I were you, I'd definitely draw the entire clock from scratch every second. Problem solved. In addition, if you need anti-aliasing, then a simple pen mode trick will never work.

(If you are not using a remote desktop, you might want to employ double-buffering.)

Upvotes: 9

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