Загружать большие битовые изображения с небольшим использованием памяти

function MyGetMem(Size: DWORD): Pointer;
begin
  Result := Pointer(GlobalAlloc(GPTR, Size));
end;

procedure MyFreeMem(p: Pointer);
begin
  if p = nil then Exit;
  GlobalFree(THandle(p));
end;

{ This code will fill a bitmap by stretching an image coming from a big bitmap on disk. 

 FileName - Name of the uncompressed bitmap to read 
 DestBitmap - Target bitmap  where the bitmap on disk will be resampled. 
 BufferSize - The size of a memory buffer used for reading scanlines from the physical bitmap on disk. 
   This value will decide how many scanlines can be read from disk at the same time, with always a 
   minimum value of 2 scanlines. 

 Will return false on error. 
}
function GetDIBInBands(const FileName: string;
  DestBitmap: TBitmap; BufferSize: Integer;
  out TotalBitmapWidth, TotalBitmapHeight: Integer): Boolean;
var
  FileSize: integer;    // calculated file size 
  ImageSize: integer;    // calculated image size 
  dest_MaxScans: integer;  // number of scanline from source bitmap 
  dsty_top: Integer;    // used to calculate number of passes 
  NumPasses: integer;    // number of passed needed 
  dest_Residual: integer;  // number of scanlines on last band 
  Stream: TStream;    // stream used for opening the bitmap 
  bmf: TBITMAPFILEHEADER;  // the bitmap header 
  lpBitmapInfo: PBITMAPINFO;  // bitmap info record 
  BitmapHeaderSize: integer;  // size of header of bitmap 
  SourceIsTopDown: Boolean;  // is reversed bitmap ? 
  SourceBytesPerScanLine: integer;  // number of bytes per scanline 
  SourceLastScanLine: Extended;     // last scanline processes 
  SourceBandHeight: Extended;       // 
  BitmapInfo: PBITMAPINFO;
  img_start: integer;
  img_end: integer;
  img_numscans: integer;
  OffsetInFile: integer;
  OldHeight: Integer;
  bits: Pointer;
  CurrentTop: Integer;
  CurrentBottom: Integer;
begin
  Result := False;

  // open the big bitmap 
  Stream := TFileStream.Create(FileName, fmOpenRead or fmShareDenyWrite);

  // total size of bitmap 
  FileSize := Stream.Size;
  // read the header 
  Stream.ReadBuffer(bmf, SizeOf(TBITMAPFILEHEADER));
  // calculate header size 
  BitmapHeaderSize := bmf.bfOffBits - SizeOf(TBITMAPFILEHEADER);
  // calculate size of bitmap bits 
  ImageSize := FileSize - Integer(bmf.bfOffBits);
  // check for valid bitmap and exit if not 
  if ((bmf.bfType <> $4D42) or
     (Integer(bmf.bfOffBits) < 1) or
     (FileSize < 1) or (BitmapHeaderSize < 1) or (ImageSize < 1) or
     (FileSize < (SizeOf(TBITMAPFILEHEADER) + BitmapHeaderSize + ImageSize))) then
  begin
    Stream.Free;
    Exit;
  end;
  lpBitmapInfo := MyGetMem(BitmapHeaderSize);
  try
    Stream.ReadBuffer(lpBitmapInfo^, BitmapHeaderSize);
    // check for uncompressed bitmap 
    if ((lpBitmapInfo^.bmiHeader.biCompression = BI_RLE4) or
       (lpBitmapInfo^.bmiHeader.biCompression = BI_RLE8)) then
    begin
      Exit;
    end;

    // bitmap dimensions 
    TotalBitmapWidth  := lpBitmapInfo^.bmiHeader.biWidth;
    TotalBitmapHeight := abs(lpBitmapInfo^.bmiHeader.biHeight);

    // is reversed order ? 
    SourceIsTopDown := (lpBitmapInfo^.bmiHeader.biHeight < 0);

    // calculate number of bytes used per scanline 
    SourceBytesPerScanLine := ((((lpBitmapInfo^.bmiHeader.biWidth *
       lpBitmapInfo^.bmiHeader.biBitCount) + 31) and not 31) div 8);

    // adjust buffer size 
    if BufferSize < Abs(SourceBytesPerScanLine) then
       BufferSize := Abs(SourceBytesPerScanLine);

    // calculate number of scanlines for every pass on the destination bitmap 
    dest_MaxScans := round(BufferSize / abs(SourceBytesPerScanLine));
    dest_MaxScans := round(dest_MaxScans * (DestBitmap.Height / TotalBitmapHeight));

    if dest_MaxScans < 2 then
      dest_MaxScans := 2;         // at least two scan lines 

    // is not big enough ? 
    if dest_MaxScans > TotalBitmapHeight then
       dest_MaxScans := TotalBitmapHeight;

    { count the number of passes needed to fill the destination bitmap }
    dsty_top  := 0;
    NumPasses := 0;
    while (dsty_Top + dest_MaxScans) <= DestBitmap.Height do
    begin
      Inc(NumPasses);
      Inc(dsty_top, dest_MaxScans);
    end;
    if NumPasses = 0 then Exit;

    // calculate scanlines on last pass 
    dest_Residual := DestBitmap.Height mod dest_MaxScans;

    // now calculate how many scanlines in source bitmap needed for every band on the destination bitmap 
    SourceBandHeight := (TotalBitmapHeight * (1 - (dest_Residual / DestBitmap.Height))) /
       NumPasses;

    // initialize first band 
    CurrentTop    := 0;
    CurrentBottom := dest_MaxScans;

    // a floating point used in order to not loose last scanline precision on source bitmap 
    // because every band on target could be a fraction (not integral) on the source bitmap 
    SourceLastScanLine := 0.0;

    while CurrentTop < DestBitmap.Height do
    begin
      // scanline start of band in source bitmap 
      img_start          := Round(SourceLastScanLine);
      SourceLastScanLine := SourceLastScanLine + SourceBandHeight;
      // scanline finish of band in source bitmap 
      img_end := Round(SourceLastScanLine);
      if img_end > TotalBitmapHeight - 1 then
         img_end := TotalBitmapHeight - 1;
      img_numscans := img_end - img_start;
      if img_numscans < 1 then Break;
      OldHeight := lpBitmapInfo^.bmiHeader.biHeight;
      if SourceIsTopDown then
        lpBitmapInfo^.bmiHeader.biHeight := -img_numscans
      else
        lpBitmapInfo^.bmiHeader.biHeight := img_numscans;

      // memory used to read only the current band 
      bits := MyGetMem(Abs(SourceBytesPerScanLine) * img_numscans);

      try
        // calculate offset of band on disk 
        OffsetInFile := TotalBitmapHeight - (img_start + img_numscans);
        Stream.Seek(Integer(bmf.bfOffBits) + (OffsetInFile * abs(SourceBytesPerScanLine)),
          soFromBeginning);
        Stream.ReadBuffer(bits^, abs(SourceBytesPerScanLine) * img_numscans);

        SetStretchBltMode(DestBitmap.Canvas.Handle, COLORONCOLOR);
        // now stretch the band readed to the destination bitmap 
        StretchDIBits(DestBitmap.Canvas.Handle,
           0,
           CurrentTop,
           DestBitmap.Width,
           Abs(CurrentBottom - CurrentTop),
           0,
           0,
           TotalBitmapWidth,
           img_numscans,
           Bits,
           lpBitmapInfo^,
           DIB_RGB_COLORS, SRCCOPY);
      finally
        MyFreeMem(bits);
        lpBitmapInfo^.bmiHeader.biHeight := OldHeight;
      end;

      CurrentTop    := CurrentBottom;
      CurrentBottom := CurrentTop + dest_MaxScans;
      if CurrentBottom > DestBitmap.Height then
        CurrentBottom := DestBitmap.Height;
    end;
  finally
    Stream.Free;
    MyFreeMem(lpBitmapInfo);
  end;
  Result := True;
end;

// example of usage 
procedure TForm1.Button1Click(Sender: TObject);
var
  bmw, bmh: Integer;
  Bitmap: TBitmap;
begin
  Bitmap := TBitmap.Create;
  with TOpenDialog.Create(nil) do
    try
      DefaultExt := 'BMP';
      Filter := 'Bitmaps (*.bmp)|*.bmp';
      Title := 'Define bitmap to display';
      if not Execute then Exit;
      { define the size of the required bitmap }
      Bitmap.Width       := Self.ClientWidth;
      Bitmap.Height      := Self.ClientHeight;
      Bitmap.PixelFormat := pf24Bit;
      Screen.Cursor      := crHourglass;
      // use 100 KB of buffer 
      if not GetDIBInBands(FileName, Bitmap, 100 * 1024, bmw, bmh) then Exit;
      // original bitmap width = bmw 
      // original bitmap height = bmh 
      Self.Canvas.Draw(0,0,Bitmap);
    finally
      Free;
      Bitmap.Free;
      Screen.Cursor := crDefault;
    end;
 end;