  We present first results on the quantitative spectroscopic analysis of the
photospheric-phase of Type II supernovae (SN). The analyses are based on the
non-LTE model atmosphere code, CMFGEN, of Hillier & Miller (1998). A key asset
of CMFGEN is its thorough treatment of line-blanketing due to metal species.
  From its applicability to hot star environments, the main modifications to
the source code were to allow a linear velocity law, a power-law density
distribution, an adaptive grid to handle the steep H recombination/ionization
front occurring in cooler SN models, and a routine to compute the gray
temperature structure in the presence of large velocities. In this first paper
we demonstrate the ability of CMFGEN to reproduce, with a high level of
accuracy, the UV and optical observations of a sample of well observed Type II
SN, i.e. SN1987A and SN1999em, at representative stages of their photospheric
evolution. Two principal stages of SN are modeled - that where hydrogen is
fully ionized, and that in which H is only partially ionized. We provide
detailed and accurate line identifications for our sample of epochs. We discuss
the impact on the SED of varying the density exponent, the spatial scale of the
SN, the value of Rmax and the metallicity; we illustrate the formation sites
for selected lines. Both Hydrogen Balmer and Helium lines (when present) are
very well reproduced, even for our adopted CNO-equilibrium abundance values
adequate for a massive supergiant progenitor star. NII lines are also predicted
in the blue-wing of both Hbeta and HeI 5875A, and thus not associated with
peculiar absorption/emission far above the photosphere. We finally present a
pedagogical discussion of P-Cygni profile formation and explain, e.g., the
noticeable blue-shift of Halpha in early time spectra (much abridged abstract).
