[XeTeX] How does xetex handle non-native image files such as eps?

[Yuan Qi]

Peter Dyballa <Peter_Dyballa at ...> writes:
 
> You should not need to care for this. XeTeX does not need an external  
> file that contains the picture's dimensions, it either extracts these  
> values itself (pdfTeX is able to retrieve the dimensions of an  
> included graphics file and assign them to dimen variables) or leaves  
> this to the output driver, since the XDV file (as a DVI file too)  
> only contains a reference. Since \includegraphics not always sets the  
> dimensions XeTeX obviously has the ability to extract the values from  
> the graphics file and reserve space for it in the page's layout.

Are you sure about this? Without bb information, how would XeTeX be
able to determine where to place (float) the figure? Isn't the layout 
set in stone once the dvi/xdv file is compiled?