{the following code constructs a relative URL
pass the directory path of the file recieving the link
and the path and filename of the file to be linked
and the return value is a relative URL
note that if you construct a URL from disk both URLs
must be on the same drive (as though on the same server)
if you connect to a file on another server you do not use relative URLs}
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, FileCtrl;

type
  TForm1 = class(TForm)
    Button1: TButton;
    OpenDialog1: TOpenDialog;
    DirectoryListBox1: TDirectoryListBox;
    DriveComboBox1: TDriveComboBox;
    procedure Button1Click(Sender: TObject);
    function DoLink(RootsPath, PagesPath : string):string;
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);
var s, t : string;
begin

    s := DirectoryListBox1.Directory;   {the path of the file accepting link}
    OpenDialog1.Execute;
    t := OpenDialog1.filename;    {the path and filename of file to be linked}

    ShowMessage(s + chr(13) + t + chr(13) + DoLink(s, t));
end;

function TForm1.DoLink(RootsPath, PagesPath : string):string;
var i, j, k : integer;  found : boolean;
begin

       if pos(RootsPath, PagesPath) <> 0 then begin
          Result := copy(PagesPath, length(RootsPath) + 2,
                length(PagesPath) - Length(RootsPath) + 2);
          exit;
       end
       else begin
            i := 0;  found := false;
            repeat
               repeat
                 i := i + 1;
               until (RootsPath[i] = '\') or (i = length(PagesPath))
                     or (i = length(RootsPath));

               if i >= length(PagesPath) then found := true
               else
                  if  copy(RootsPath, 1, i) <> copy(PagesPath, 1, i)  then
                   found := true;
               if not found then k := i;
            until found or (i = length(PagesPath)) or (i = length(RootsPath));

            j := 1;
            If i <> length(RootsPath) then
              repeat
                  i := i + 1;
                  if (RootsPath[i] = '\') or (i = length(Rootspath)) then
                     j := j + 1;
              until (i = length(RootsPath));

            for i := 1 to j do
               Result := Result + '../';

            Result := Result + copy(PagesPath, k + 1, length(PagesPath) - k + 1);
            for i := 1 to length(Result) do
               if Result[i] = '\' then Result[i] := '/';

       end;

end;

end.