{******************************************************************************}
{                                                                              }
{                              Offscreen-Bitmap                                }
{                                                                              }
{                  Copyright (c) 2004 Michael .:Luckie:. Puff                  }
{                          [url]www.luckie-online.de[/url]                                }
{                                                                              }
{******************************************************************************}

program OffScreenBmp;

uses
  Windows, Messages;

const
  CLASSNAME = 'WndClass';
  APPNAME = 'OffScreen-Bitmap';
  WINDOWWIDTH = 500;
  WINDOWHEIGHT = 350;

var
  hdcMem: HDC;
  hBmp: THandle;
  hBmpOld: THandle;

function WndProc(hWnd: HWND; uMsg: UINT; wParam: wParam; lParam: LParam):
  lresult; stdcall;
var
  rect: TRect;
  RedBrush: HBRUSH;
  ps: TPaintStruct;
  dc: HDC;
  ptStart, ptEnd: TPoint;
begin
  Result := 0;
  case uMsg of
    WM_CREATE:
      begin
        // Create a screen compatible DC in memory
        hdcMem := CreateCompatibleDC(0);
        // Create a Bitmap compatible to the window
        hBmp := CreateCompatibleBitmap(GetDC(hWnd), WINDOWWIDTH, WINDOWHEIGHT);
        // Select Bitmap into the memory DC and saving the old one
        hBmpOld := SelectObject(hdcMem, hBmp);
        // Release the Bitmap
        DeleteObject(hBmp);

        rect.Left := 0;
        rect.Top := 0;
        rect.Right := WINDOWWIDTH;
        rect.Bottom := WINDOWHEIGHT;
        RedBrush := CreateSolidBrush(RGB(255, 0, 0));
        // paint red rectangle on the Bitmap in memory
        FillRect(hdcMem, rect, RedBrush);
        DeleteObject(RedBrush);
      end;
    WM_PAINT:
      begin
        dc := BeginPaint(hWnd, ps);
        // everytime window needs to be painted, copy the Bitmap in memory
        // onto the window
        BitBlt(dc, 0, 0, WINDOWWIDTH, WINDOWHEIGHT, hdcMem, 0, 0, SRCCOPY);
        EndPaint(hWnd, ps);
      end;
    WM_LBUTTONDOWN:
      begin
        // starting coordinates
        ptStart.X := LoWord(lParam);
        ptStart.Y := HiWord(lParam);
      end;
    WM_LBUTTONUP:
      begin
        // ending coordinates
        ptEnd.X := LoWord(lParam);
        ptEnd.Y := HiWord(lParam);
        // paint rectangle on Bitmap in memory
        Rectangle(hdcMem, ptStart.X, ptStart.Y, ptEnd.X, ptEnd.Y);
        // declare window invalide, force WM_PAINT
        InvalidateRect(hWnd, nil, False);
      end;
    WM_DESTROY:
      begin
        SelectObject(hdcMem, hBmpOld);
        DeleteObject(hdcMem);
        PostQuitMessage(0);
      end;
  else
    Result := DefWindowProc(hWnd, uMsg, wParam, lParam);
  end;
end;

var
  wc: TWndClassEx = (
    cbSize: SizeOf(TWndClassEx);
    Style: CS_HREDRAW or CS_VREDRAW;
    lpfnWndProc: @WndProc;
    cbClsExtra: 0;
    cbWndExtra: 0;
    lpszMenuName: nil;
    lpszClassName: ClassName;
    hIconSm: 0;
    );
  msg: TMsg;
begin
  wc.hInstance := hInstance;
  wc.hbrBackground := GetStockObject(WHITE_BRUSH);
  wc.hIcon := LoadIcon(0, IDI_APPLICATION);
  wc.hCursor := LoadCursor(0, IDC_ARROW);

  RegisterClassEx(wc);
  CreateWindowEx(0, ClassName, AppName, WS_OVERLAPPEDWINDOW or
    WS_VISIBLE or WS_SYSMENU or WS_MINIMIZEBOX or WS_MAXIMIZEBOX or WS_SIZEBOX,
    Integer(CW_USEDEFAULT), Integer(CW_USEDEFAULT), WINDOWWIDTH, WINDOWHEIGHT,
    0, 0, hInstance, nil);

  while true do
  begin
    if not GetMessage(msg, 0, 0, 0) then
      break;
    begin
      TranslateMessage(msg);
      DispatchMessage(msg);
    end;
  end;
  ExitCode := msg.wParam;
end.