Well, in fact I didnt understand very well :/
I'm not sure if I will be able to help (as Im not very good in programming), but I love to see which kind of problems do ppl have, so if (er, when) I have the same problems I know how to fix, and I love games of isometric view, and I try to make them, so I have a good chance of having same problems as other iso games programmers :P

Cya,
-RoTTer

Make sure you use the lPitch member of the data returned from a DDSURFACE->Lock() call, not the assumed surface width (640, 800, etc) or else it'll look wonky.

Most likely, you're going to need to store information seperate to the actual graphics of objects. In the case of a building, you could have perhaps a bounding polygon of some kind(a quadrilateral would do fine), or maybe some kind of mask for it, another graphic with only two colors, white or black, one of which tells where the building is, and one of which doesn't. That mask could be used for collision detection, or just to see whether or not you should draw over it.

There are a lot of ways to do this, and how you do it will pretty much depend on your own preferences, and what you need it for.

Good luck,
Jonathan

I am in the middle of programming a game using standard square tiles.

I need to know how can detect a non square object as part of the graphics, for example an isometric looking building, I want the building to be only detected around the edges not as part of the whole tile.

if anyone needs to know more please let me know my description is pretty vague.