代码如下，每次改变窗体大小的时候，绘出的图形就乱了，请大神指教怎么回事，
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, OPENGL;

type
  TfrmMain = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure FormPaint(Sender: TObject);
    procedure FormResize(Sender: TObject);
  private
    { Private declarations }
    hrc:HGLRC;
    w,h:glFloat;
    Sphere:GLUquadricObj;
  public
    { Public declarations }
    procedure MyDraw;
  end;

var
  frmMain: TfrmMain;

implementation

{$R *.dfm}

procedure TfrmMain.FormCreate(Sender: TObject);
var
  pfd:TPixelFormatDescriptor;   //设置描述表
  PixelFormat:Integer;
begin
  ControlStyle:=ControlStyle+[csOpaque];
  FillChar(pfd,sizeof(pfd),0);                //
  with pfd do
  begin
    nSize:=sizeof(TPixelFormatDescriptor);
    nVersion:=1;
    dwFlags:=PFD_DRAW_TO_WINDOW or PFD_SUPPORT_GDI or PFD_SUPPORT_OPENGL or PFD_DOUBLEBUFFER;
    iPixelType:=PFD_TYPE_RGBA;
    cColorBits:=24;
    cDepthBits:=32;
    iLayerType:=PFD_MAIN_PLANE;
  end;
  PixelFormat:=ChoosePixelFormat(Canvas.Handle,@pfd);
  SetPixelFormat(Canvas.Handle,PixelFormat,@pfd);
  hrc:=wglCreateContext(Canvas.Handle);
  w:=ClientWidth;
  h:=ClientHeight;

end;

procedure TfrmMain.FormDestroy(Sender: TObject);
begin
    wglMakeCurrent(Canvas.Handle,HRC);
    wglDeleteContext(hRc);
    ReleaseDC(Handle,Canvas.Handle);
end;

procedure TfrmMain.FormPaint(Sender: TObject);
begin
wglMakeCurrent(Canvas.Handle,hrc);
  MyDraw;
end;

procedure TfrmMain.FormResize(Sender: TObject);
begin

  glMatrixMode(GL_PROJECTION);
  glLoadIdentity;
  glFrustum(-1.0,1.0,-1.0,1.0,3.0,7.0);
  glViewport(0,0,ClientWidth,ClientHeight);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
  MyDraw;
end;

procedure TfrmMain.MyDraw;
begin
  glClearColor(0.8,1,1,1);
  glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT);
  glLineWidth(3.0);

  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
  glRotatef(45,0,0,1);
    glColor3f(1.0,0.0,0.0); //画红色的x轴
    glBegin(GL_LINES);
        glVertex3f(0.0, 0.0, 0.0);
        glVertex3f(1.0, 0.0, 0.0);
    glEnd();
    glColor3f(0.0,1.0,0.0); //画绿色的y轴
    glBegin(GL_LINES);
        glVertex3f(0.0, 0.0, 0.0);
        glVertex3f(0.0, 1.0, 0.0);
    glEnd();
    glColor3f(0.0,0.0,1.0); //画蓝色的z轴
    glBegin(GL_LINES);
        glVertex3f(0.0, 0.0, 0.0);
        glVertex3f(0.0, 0.0, 1.0);
    glEnd();

  glFlush;
  SwapBuffers(Canvas.handle);

end;

end.

肿么没有大神解答下