DIRECT SCREEN ACCESS

i found few or nothing on direct screen access , i suppose it is the best method to access the screen while writing a video codec , so , i try some code like :

   TPckgBuf<TScreenInfoV01> info;
   TUint16 *screenBase;
   UserSvr::ScreenInfo(info);   

   TInt sWidth = info().iScreenSize.iWidth;
   TInt sHeight = info().iScreenSize.iHeight;
    TBool isValid = info().iScreenAddressValid;

Starting from the CONSOLE HELLO WORLD epoc example (the one provided with series60 sdk) , but i always getting a FALSE when calling iScreenAddressValid and can't understand why.
Does anyone knows how to make a direct screen access correctly from an application?
Does the type of application affect the results of screen access?

tHAnKs