Draw Gradient Alpha Channel

type
  TTriVertex = packed record
    x: Longint;
    y: Longint;
    Red: Word;
    Green: Word;
    Blue: Word;
    Alpha: Word;
  end;
  TGradientFillProc = function(DC: HDC; Verteces: Pointer; NumVerteces: DWORD;
    Meshes: Pointer; NumMeshes: DWORD; Mode: DWORD): DWORD; stdcall;

var
  Form1: TForm1;
  MSImg32Lib: THandle;
  GradientFillProc: TGradientFillProc;
  
//

procedure TForm1.Button1Click(Sender: TObject);
var
  cr: Cardinal;
  Verteces: array[0..1] of TTriVertex;
  GradientRect: TGradientRect;
begin
  Verteces[0].x := 0;
  Verteces[0].y := 0;
  cr := GetSysColor(COLOR_ACTIVECAPTION);
  Verteces[0].Red := ((cr and $0F0F) or 8);
  Verteces[0].Green := (cr and $0ff00);
  Verteces[0].Blue := ((cr and $0ff0000) or 8);
  Verteces[0].Alpha := 0;
  Verteces[1].x := Width;
  Verteces[1].y := Height;
  cr := GetSysColor(COLOR_GRADIENTACTIVECAPTION);
  
  // Farben und Alpha Channel
  Verteces[1].Red := ((cr and $0FF) or 8);
  Verteces[1].Green := (cr and $0ff00);
  Verteces[1].Blue := ((cr and $0ffFF00) or 8);
  Verteces[1].Alpha := 255;
  
  GradientRect.UpperLeft := 0;
  GradientRect.LowerRight := 1;
  GradientFillProc(Canvas.Handle, @Verteces[0], 2, @GradientRect, 1, GRADIENT_FILL_RECT_H);
end;

initialization
  GradientFillProc := nil;
  MSImg32Lib       := LoadLibrary('msimg32.dll');
  if MSImg32Lib <> 0 then
  begin
    GradientFillProc := GetProcAddress(MSImg32Lib, 'GradientFill');
    if @GradientFillProc = nil then
    begin
      FreeLibrary(MSImg32Lib);
      MSImg32Lib := 0;
    end;
  end
  else
    ShowMessage('Could not load DLL');

finalization
  if @GradientFillProc <> nil then
    GradientFillProc := nil;
  if MSImg32Lib <> 0 then
  begin
    FreeLibrary(MSImg32Lib);
    MSImg32Lib := 0;
  end;