Re: [lazarus] Background Painting Problem in Linux

[Paul Michell]

On Thursday 15 June 2006 8:54 pm, Patrick Chevalley wrote:
> In a first time I set the background of the underlaying panel to black
> but this is not necessary and now it is clBtnface.
> The sky chart is first draw to a Tbitmap. In the drawing component
> onPaint  I just draw this bitmap to the canvas along with temporary
> drawing if any (selection mark, ...).
> Using a Timage there is a lot of flicker, when resizing, but also when
> drawing a temporary lines directly with pen.mode=pmXor.
> With the drawing component there is no problem.

You are right and it works on Linux even without the DoubleBuffered := True!
The presence of the control must stop (or clip) the GTK1 form background fill.
>
> This is unfortunately not the  most simple application to use as an
> example but if you want to look at the source code it is at:
> http://svn.sourceforge.net/viewcvs.cgi/skychart/trunk/skychart/
> The drawing component is in pu_chart.pas.
>
> Patrick
>
I tried to simplify the code to make it easier to see how the form and the 
control relate. In so doing I realised you could make direct use of 
TCustomControl if all you want to do is take over the client area painting:

unit Main;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, LResources, Forms, Controls, Graphics, Dialogs;

type
  TMainForm = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    { private declarations }
    PainterControl: TCustomControl;
  public
    { public declarations }
    Procedure ControlPaint(Sender: TObject);
  end; 

var
  MainForm: TMainForm;

implementation

procedure TMainForm.FormCreate(Sender: TObject);
begin
  PainterControl := TCustomControl.Create(Self);
  PainterControl.Align := alClient;
  PainterControl.OnPaint := @ControlPaint;
  PainterControl.Parent := Self;
  PainterControl.DoubleBuffered := True;
end;

procedure TMainForm.FormDestroy(Sender: TObject);
begin
  PainterControl.Free;
end;

Procedure TMainForm.ControlPaint(Sender: TObject);
Begin
  with PainterControl.Canvas do
    begin
      Brush.Color:=clRed;
      FillRect(ClientRect);
    end;
End;

initialization
  {$I Main.lrs}

end.

Of course you would want to create a descendent control if you want to do 
anything else, such as the mouse handling in SkyChart!

Thanks again for you help, I hope the above is useful to others.
Paul

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