Re-learning gdi+[17]: FillRectangle, Colorreftoargb, Tgpsolidbrush

This example effect chart:

Code files:Unit Unit1;
Interface
Uses
Windows, Messages, sysutils, variants, Classes, Graphics, Controls, Forms,
Dialogs, Comctrls, Stdctrls, Extctrls;
Type
TForm1 = Class (Tform)
Colorlistbox1:tcolorlistbox;
Trackbar1:ttrackbar;
Procedure Formcreate (Sender:tobject);
Procedure Formpaint (Sender:tobject);
Procedure Colorlistbox1click (Sender:tobject);
Procedure Trackbar1change (Sender:tobject);
End
Var
Form1:tform1;
Implementation
{$R *.DFM}
Uses Gdipobj,gdipapi;
Procedure Tform1.formcreate (Sender:tobject);
Begin
Self.position: = Podesktopcenter;
Colorlistbox1.align: = Alright;
Trackbar1.showselrange: = False;
Trackbar1.pagesize: = 20;
Trackbar1.min: = 0;
Trackbar1.max: = 255;
Trackbar1.position: = Trackbar1.max;
End
Procedure Tform1.formpaint (Sender:tobject);
Var
G:tgpgraphics;
R:tgprect;
Sb:tgpsolidbrush;
Clr:tgpcolor;
Alpha:byte;
Begin
G: = Tgpgraphics.create (Canvas.handle);
G.clear (Makecolor (255,255,255));
r: = Makerect (20, 20, 200, 150);
CLR: = Colorreftoargb (colorlistbox1.selected);
Alpha: = trackbar1.position;
CLR: = Makecolor (Alpha, getred (CLR), Getgreen (CLR), Getblue (CLR));
SB: = Tgpsolidbrush.create (CLR);
G.fillrectangle (SB, R);
Sb. Free;
G.free;
End
Procedure Tform1.colorlistbox1click (Sender:tobject);
Begin
Repaint;
End
Procedure Tform1.trackbar1change (Sender:tobject);
Begin
Repaint;
Text: = Format (' Transparency:%d ', [trackbar1.position]);
End
End.