Why MagSetImageScalingCallback function fails when is executed in new desktop?

[flashcoder]

I'm trying get a screenshot of a new desktop created (CreateDesktop api + executing explorer.exe to new desktop) using Magnification api like showed in this tutorial.

I ported with success the C++ code from tutorial to Delphi and both examples (C++ and Delphi) works fine when executed from Win Vista ~ Win 10 and with Aero Theme enabled.

The trouble to this question is because if i create a new desktop (clone original desktop) and execute this example of screen capture, MagSetImageScalingCallback fails.

Someone know a possible solution to MagSetImageScalingCallback also work on new desktop created?

Thanks in advance by any help.

Main

Delphi-Quellcode:

type
  TForm1 = class(TForm)
    CAPTURE: TButton;
    SaveFileDialog1: TSaveDialog;
    procedure FormCreate(Sender: TObject);
    procedure CAPTUREClick(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

uses
  Magnification;

{$R *.dfm}

function HostWndProc(hWindow: HWND; Msg: UINT; wParam: wParam; lParam: lParam)
  : LRESULT; stdcall;
begin
  Result := DefWindowProc(hWindow, Msg, wParam, lParam);
end;

var
  MyBMP: TBitmap;
  abitmap: HBitmap;
  desktoprect: TRect;
  hWndMag: HWND;
  CallbackDone: Boolean = False;

function MagImageScalingCallback(HWND: HWND; srcdata: Pointer;
  srcheader: MAGIMAGEHEADER; destdata: Pointer; destheader: MAGIMAGEHEADER;
  unclipped: TRect; clipped: TRect; dirty: HRGN): BOOL; stdcall;
var
  lpbmih: TBitmapInfoHeader;
  lpbmi: TBitmapInfo;
  aDC: HDC;
begin

  aDC := GetWindowDC(HWND);

  Fillchar(lpbmih, sizeof(lpbmih), 0);
  lpbmih.biSize := sizeof(lpbmih);
                 // (-) Otherwise the image is upside down.
  lpbmih.biHeight := -srcheader.height { -GetDeviceCaps(aDC, VERTRES) };
  lpbmih.biWidth := srcheader.width { GetDeviceCaps(aDC, HORZRES) };
  lpbmih.biSizeImage := srcheader.cbSize;
  lpbmih.biPlanes := 1;
  lpbmih.biBitCount := 32;
  lpbmih.biCompression := BI_RGB;

  Fillchar(lpbmi, sizeof(lpbmi), 0);
  lpbmi.bmiHeader.biSize := sizeof(lpbmi.bmiHeader);
                          // (-) Otherwise the image is upside down.
  lpbmi.bmiHeader.biHeight := -srcheader.height { -GetDeviceCaps(aDC, VERTRES) };
  lpbmi.bmiHeader.biWidth := srcheader.width { GetDeviceCaps(aDC, HORZRES) };
  lpbmi.bmiHeader.biSizeImage := srcheader.cbSize;
  lpbmi.bmiHeader.biPlanes := 1;
  lpbmi.bmiHeader.biBitCount := 32;
  lpbmi.bmiHeader.biCompression := BI_RGB;

  MyBMP := TBitmap.Create;
  abitmap := 0;
  try
    abitmap := CreateDIBitmap(aDC, lpbmih, CBM_INIT, srcdata, lpbmi,
      DIB_RGB_COLORS);
    MyBMP.handle := abitmap;
    MyBMP.PixelFormat := pf32bit;

    CallbackDone := True;

  finally
    DeleteDC(aDC);
  end;

  Result := True;
end;

procedure TForm1.CAPTUREClick(Sender: TObject);
var
  filterList: THWNDArray;
  sourceRect: TRect;
begin
  filterList[0] := Form1.handle;

  If (MagSetWindowFilterList(hWndMag, MW_FILTERMODE_EXCLUDE, 1,
    @filterList[0])) Then
  begin

    sourceRect.left := 0;
    sourceRect.top := 0;
    sourceRect.right := desktoprect.width;
    sourceRect.bottom := desktoprect.height;

    CallbackDone := False;

    If (MagSetWindowSource(hWndMag, sourceRect)) Then
      Screen.Cursor := crHourGlass;

    repeat

    until CallbackDone;

    Screen.Cursor := crDefault;

    SaveFileDialog1.Title := 'Save Image File';
    SaveFileDialog1.Filter :=
      'JPeg Image|*.jpg|Bitmap Image|*.bmp|Gif Image|*.gif|Png Image|*.png';
    SaveFileDialog1.DefaultExt := 'bmp';
    SaveFileDialog1.FilterIndex := 2;
    SaveFileDialog1.InitialDir := GetCurrentDir;

    if SaveFileDialog1.Execute then
    begin
      MyBMP.SaveToFile(SaveFileDialog1.FileName);
      MessageDlg('File saved: ' + SaveFileDialog1.FileName, mtInformation,
        [mbOK], 0);
    end
    else
      MessageDlg('Save file was cancelled', mtWarning, [mbOK], 0);

    DeleteObject(abitmap);
    MyBMP.Free;
  end;

end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  if (MagUninitialize) then
    MessageDlg('Magnification api finished!', mtInformation, [mbOK], 0);
end;

procedure TForm1.FormCreate(Sender: TObject);
const
  HOST_CLASSNAME = 'MagnifierHost';
var
  wc: TWndClass;
  hWndHost, desktop: HWND;
begin

  hWndHost := 0;

  wc.lpszClassName := HOST_CLASSNAME;
  wc.lpfnWndProc := @HostWndProc;
  wc.Style := 0;
  wc.hInstance := 0;
  wc.hIcon := 0;
  wc.hCursor := 0;
  wc.hbrBackground := 0;
  wc.lpszMenuName := nil;
  wc.cbClsExtra := 0;
  wc.cbWndExtra := 0;

  desktop := GetDesktopWindow;
  GetWindowRect(desktop, desktoprect);

  if (Winapi.Windows.RegisterClass(wc) <> 0) then

    hWndHost := CreateWindowEx(WS_EX_TOPMOST Or WS_EX_LAYERED Or
      WS_EX_TRANSPARENT, HOST_CLASSNAME, 'Host Window',
      WS_POPUP Or WS_CLIPCHILDREN, 0, 0, desktoprect.width, desktoprect.height,
      0, 0, hInstance, nil);

  if (hWndHost <> 0) then
  begin
    SetWindowPos(hWndHost, 0, 0, 0, desktoprect.width, desktoprect.height,
      SWP_HIDEWINDOW);
    SetLayeredWindowAttributes(hWndHost, 0, 255, LWA_ALPHA);
  end;

  If (MagInitialize) Then
    hWndMag := CreateWindowEx(0, WC_MAGNIFIER, 'MagnifierWindow',
      WS_CHILD Or MS_SHOWMAGNIFIEDCURSOR Or WS_VISIBLE, 0, 0, desktoprect.width,
      desktoprect.height, hWndHost, 0, 0, nil);

  If (hWndMag = 0) Then
    ShowMessage(SysErrorMessage(GetLastError));

  if (MagSetImageScalingCallback(hWndMag, MagImageScalingCallback)) then
    ShowMessage('MagSetImageScalingCallback registred!');

  ShowMessage(SysErrorMessage(GetLastError));

  left := (GetSystemMetrics(SM_CXSCREEN) - width) div 2;
  top := (GetSystemMetrics(SM_CYSCREEN) - height) div 2;
end;

end.

Magnification.pas

Delphi-Quellcode:

unit Magnification;

{$ALIGN ON}
{$MINENUMSIZE 4}

interface

uses
  Windows;

const
  // Magnifier Class Name
  WC_MAGNIFIERA: AnsiString = 'Magnifier';
  WC_MAGNIFIERW: WideString = 'Magnifier';
  WC_MAGNIFIER = 'Magnifier';

  // Magnifier Window Styles
  MS_SHOWMAGNIFIEDCURSOR = $0001;
  MS_CLIPAROUNDCURSOR = $0002;
  MS_INVERTCOLORS = $0004;

  // Filter Modes
  MW_FILTERMODE_EXCLUDE = 0;
  MW_FILTERMODE_INCLUDE = 1;

type
  tagMAGTRANSFORM = record
    v: array[1..3, 1..3] of Single;
  end;
  MAGTRANSFORM = tagMAGTRANSFORM;
  TMagTransform = tagMAGTRANSFORM;
  PMagTransform = ^TMagTransform;

  tagMAGIMAGEHEADER = record
    width: UINT;
    height: UINT;
    format: TGUID;
    stride: UINT;
    offset: UINT;
    cbSize: UINT;
  end;
  MAGIMAGEHEADER = tagMAGIMAGEHEADER;
  TMagImageHeader = tagMAGIMAGEHEADER;
  PMagImageHeader = ^TMagImageHeader;

  tagMAGCOLOREFFECT = record
    transform: array[1..5, 1..5] of Single;
  end;
  MAGCOLOREFFECT = tagMAGCOLOREFFECT;
  TMagColorEffect = tagMAGCOLOREFFECT;
  PMagColorEffect = ^TMagColorEffect;

  TMagImageScalingCallback = function (hwnd: HWND; srcdata: Pointer;
    srcheader: MAGIMAGEHEADER; destdata: Pointer; destheader: MAGIMAGEHEADER;
    unclipped: TRect; clipped: TRect; dirty: HRGN): BOOL; stdcall;

  THWNDArray = array[0..0] of HWND;
  PHWNDArray = ^THWNDArray;

  // Public Functions
  function MagInitialize(): BOOL; stdcall;
  function MagUninitialize(): BOOL; stdcall;

  function MagSetWindowSource(hwnd: HWND; rect: TRect): BOOL; stdcall;
  function MagGetWindowSource(hwnd: HWND; var Rect: TRect): BOOL; stdcall;
  function MagSetWindowTransform(hwnd: HWND; var Transform: TMagTransform): BOOL; stdcall;
  function MagGetWindowTransform(hwnd: HWND; var Transform: TMagTransform): BOOL; stdcall;
  function MagSetWindowFilterList(hwnd: HWND; dwFilterMode: DWORD;
    count: Integer; pHWND: PHWNDArray): BOOL; stdcall;
  function MagGetWindowFilterList(hwnd: HWND; var dwFilterMode: DWORD;
    count: Integer; pHWND: PHWNDArray): Integer; stdcall;
  function MagSetImageScalingCallback(hwnd: HWND;
    MagImageScalingCallback: TMagImageScalingCallback): BOOL; stdcall;
//  MagImageScalingCallback WINAPI MagGetImageScalingCallback(HWND hwnd );
  function MagSetColorEffect(hwnd: HWND; var Effect: TMagColorEffect): BOOL; stdcall;
  function MagGetColorEffect(hwnd: HWND; var Effect: TMagColorEffect): BOOL; stdcall;

implementation

const
  MagnificationDll = 'Magnification.dll';

  function MagInitialize; external MagnificationDll name 'MagInitialize';
  function MagUninitialize; external MagnificationDll name 'MagUninitialize';
  function MagSetWindowSource; external MagnificationDll name 'MagSetWindowSource';
  function MagGetWindowSource; external MagnificationDll name 'MagGetWindowSource';
  function MagSetWindowTransform; external MagnificationDll name 'MagSetWindowTransform';
  function MagGetWindowTransform; external MagnificationDll name 'MagGetWindowTransform';
  function MagSetWindowFilterList; external MagnificationDll name 'MagSetWindowFilterList';
  function MagGetWindowFilterList; external MagnificationDll name 'MagGetWindowFilterList';
  function MagSetImageScalingCallback; external MagnificationDll name 'MagSetImageScalingCallback';
  function MagSetColorEffect; external MagnificationDll name 'MagSetColorEffect';
  function MagGetColorEffect; external MagnificationDll name 'MagGetColorEffect';

end.