Function can_fog

src/monmove.c:2364–2371

monster can change form into a fog if necessary */

/* monster can change form into a fog if necessary */
boolean
can_fog(struct monst *mtmp)
{
    if (!(svm.mvitals[PM_FOG_CLOUD].mvflags & G_GENOD) && is_vampshifter(mtmp)
        && !Protection_from_shape_changers && !stuff_prevents_passage(mtmp))
        return TRUE;
    return FALSE;
}

/* this is called when a vampire turns into a fog cloud in order to move
   under a closed door; if it was sensed via telepathy or seen via