type
  TPacket = record
    Image: TJPEGImage;
    student: string;
  end;

var
  Image: TBitmap;
  Desktop: TDesktop;
  by: TBytes;
  Packet: TPacket;

implementation

{$R *.dfm}

procedure TDesktop.Button1Click(Sender: TObject);
var
  can: TCanvas;
begin
  can := TCanvas.Create;
  can.Handle := GetWindowDC(GetDesktopWindow);
  ZeroMemory(by, 0);
  Packet.Image := TJPEGImage.Create;
  Image := TBitmap.Create;
  Image.Width := Screen.Width;
  Image.Height := Screen.Height;
  Image.Canvas.CopyRect(
    Rect(0, 0, Screen.Width, Screen.Height),
    can,
    Rect(0, 0, Screen.Width, Screen.Height)
  );
  Packet.Image.Assign(Image);
  Packet.Image.CompressionQuality := 50;
  Packet.student := 'student';
  IdUDPClient1.BufferSize := SizeOf(packet);
  SetLength(by, sizeof(packet));
  Move(packet, by[0], sizeof(packet));
  IdUDPClient1.SendBuffer('127.0.0.1', 5, by);
  Image.Free;
  Packet.Image.Free;
  ReleaseDC(0, can.Handle);
end;

procedure TDesktop.FormShow(Sender: TObject);
begin
  IdUDPServer1.BufferSize:=SizeOf(packet);
end;

procedure TDesktop.IdUDPServer1UDPRead(AThread: TIdUDPListenerThread;
  AData: array of Byte; ABinding: TIdSocketHandle);
begin
  Packet.Image := TJPEGImage.Create;
  Move(AData[0], packet, sizeof(AData));
  Caption := Packet.student;
  Packet.Image.SaveToFile('E:1.jpg');
  Packet.Image.Free;
end;