const
  KNOB_HORZ = -1;
  KNOB_VERT = 0;
  KNOB_NONE = 1;

  KNOB_GAP = 45 * pi / 180;
  KNOB_ANGLE_RANGE = 2 * pi {360°} - KNOB_GAP;
  KNOB_OFFSET_ANGLE = 0.5 * pi {90°, also "unten"} + 0.5 * KNOB_GAP;

  KNOB_IMAGE = 1;
  KNOB_MINVAL = 2;
  KNOB_MAXVAL = 3;
  KNOB_VALUE = 4;
  KNOB_MOVING = 5;
  KNOB_WAS_VALUE = 6;
  KNOB_DEFAULT_VALUE = 7;

function ValueFromAngle(Angle: single): single;
var
  RelativeAngle: single;
begin
  RelativeAngle := Angle - KNOB_OFFSET_ANGLE;
  if RelativeAngle < 0 then
    RelativeAngle := RelativeAngle + 2 * pi;

  Result := (RelativeAngle) / KNOB_ANGLE_RANGE;
  if Result > 1.0 then
    if RelativeAngle < 0.5 * KNOB_ANGLE_RANGE + pi then
      Result := 1
    else
      Result := 0
end;

procedure UpdateFramePosition(WinHandle: HWND);
var
  Img, imgW, imgH: cardinal;
begin
  //with SkinEngine do
  //begin
    Img := SkinEngine.GetProperty(WinHandle, KNOB_IMAGE);
    SkinEngine.GetImageSize(Img, imgW, imgH);

    XOffset := FFrame mod tmax;
    YOffset := FFrame div tmax;
    dXOffset := integer(imgW) div tmax;
    dYOffset := 0;
  //end;

end;

procedure SetKnobLocation(WinHandle: HWND; x, y: single);
var
  rc: TRect;
  Img, imgW, imgH: cardinal;

begin

  //with SkinEngine do
  //begin

    GetClientRect(WinHandle, rc);

    Img := SkinEngine.GetProperty(WinHandle, KNOB_IMAGE);
    SkinEngine.GetImageSize(Img, imgW, imgH);

    tMin := SkinEngine.GetProperty(WinHandle, KNOB_MINVAL);
    tMax := SkinEngine.GetProperty(WinHandle, KNOB_MAXVAL);

    Angle := arctan2(y - rc.Bottom div 2, x - rc.Right div 2);
    Value := ValueFromAngle(Angle) * tMax ;

    FFrame := round(Value) mod tMax;
    UpdateFramePosition(WinHandle);


    SkinEngine.SetProperty(WinHandle, KNOB_VALUE, round(Value));
    if Value <> SkinEngine.GetProperty(WinHandle, KNOB_WAS_VALUE) then
    begin
      SkinEngine.FUpdateWindow(WinHandle, False);
      SetToolTipText(WinHandle, PWideChar(IntToStr(round(Value))));
    end;

    SkinEngine.SetProperty(WinHandle, KNOB_WAS_VALUE, round(Value));
  //end;

end;

// Berechne die X, Y koordinaten abhängig von der aktuellen tVal
procedure GetKnobLocation(WinHandle: HWND);
var
  Img, imgW, imgH: cardinal;
begin

  //with SkinEngine do
  //begin
    Img := SkinEngine.GetProperty(WinHandle, KNOB_IMAGE);
    SkinEngine.GetImageSize(Img, imgW, imgH);
    tMax := SkinEngine.GetProperty(WinHandle, KNOB_MAXVAL);
    Value := SkinEngine.GetProperty(WinHandle, KNOB_VALUE);
    if Value <= (tMax - 1) then
    begin
      FFrame := round(Value) mod tMax;
      UpdateFramePosition(WinHandle);
    end else
    begin
       FFrame := round(Value - 1) mod tMax;
       UpdateFramePosition(WinHandle);
    end;
  //end;

end;

// Default Value setzen
procedure SetKnobDefaultValue(WinHandle: HWND; tVal: Integer);

begin

  //with SkinEngine do
  //begin
    tMin := SkinEngine.GetProperty(WinHandle, KNOB_MINVAL);
    tMax := SkinEngine.GetProperty(WinHandle, KNOB_MAXVAL);
    if tMin > tMax then
      SWAP(tMin, tMax);

    tVal := MIN(MAX(tMin, tVAL), tMax);
    if tVal <> SkinEngine.GetProperty(WinHandle, KNOB_VALUE) then
    begin
      SkinEngine.SetProperty(WinHandle, KNOB_VALUE, tVal);
      SkinEngine.SetProperty(WinHandle, KNOB_WAS_VALUE, tVal);

      SkinEngine.FUpdateWindow(WinHandle, False);
    end;
  //end;

end;