How to get dimensions of image file in Delphi?

Question

I want to know width and height of an image file before opening that file.

So, how to do that?

EDIT: This refers to jpg, bmp, png and gif types of image fil

Answer 1

Since GetGIFSize in Rafael answer is broken and utterly complicated here is my personal version of it:

function GetGifSize(var Stream: TMemoryStream; var Width: Word; var Height: Word): Boolean;
var
    HeaderStr: AnsiString;

begin
    Result := False;
    Width := 0;
    Height := 0;

    //GIF header is 13 bytes in length
    if Stream.Size > 13 then
    begin
        SetString(HeaderStr, PAnsiChar(Stream.Memory), 6);
        if (HeaderStr = 'GIF89a') or (HeaderStr = 'GIF87a') then
        begin
            Stream.Seek(6, soFromBeginning);
            Stream.Read(Width, 2);  //Width is located at bytes 7-8
            Stream.Read(Height, 2); //Height is located at bytes 9-10

            Result := True;
        end;
    end;
end;

Found it by reading the RFC.