well ,thank u very much.but i m still confused what should i do with the projection matrix in 3dgs to figur that out? the only way to change the image i know is by vertex shader......or there is some other ways to do that ? plz lead me a liite more or give me some links ?i have no idea what to search...

btw: if ur dll works u could do multi-touch with 3dgs,that will be wonderful.u can also take a look at nui.org(i not sure).there is something about how to get a better result(with some image filter)