  We present a photometric study of the Halpha emission in the Universidad
Complutense de Madrid (UCM) Survey galaxies. This work complements our
previously-published spectroscopic data. We study the location of the
star-forming knots, their intensity, concentration, and the relationship of
these properties with those of the host galaxy. We also estimate that the
amount of Halpha emission that arises from the diffuse ionized gas is about
15-30% of the total Halpha flux for a typical UCM galaxy. This percentage seems
to be independent of the Hubble type. Conversely, we found that an `average'
UCM galaxy harbours a star formation event with 30% of its Halpha luminosity
arising from a nuclear component. The implications of these results for
higher-redshift studies are discussed, including the effects of galaxy size and
the depth of the observations. A correlation between the SFR and the Balmer
decrement is observed, but such correlation breaks down for large values of the
extinction. Finally, we recalculate the Halpha luminosity function and star
formation rate density of the local Universe using the new imaging data. Our
results point out that, on average, spectroscopic observations detected about
one third of the total emission-line flux of a typical UCM galaxy. The new
values obtained for the Halpha luminosity density and the star formation rate
density of the local Universe are 10^(39.3+/-0.2) erg s-1 Mpc-3, and
rho_SFR=0.016^(+0.007)_(-0.004) Mass_sun yr-1 Mpc-3 (H_0=50 km s-1 Mpc-1,
Omega_M=1.0, Lambda=0). The corresponding values for the `concordance
cosmology' (H_0=70 km s-1 Mpc-1, Omega_M=0.3, Lambda=0.7) are 10^(39.5+/-0.2)
erg s-1 Mpc-3 rho_SFR=0.029^(+0.008)_(-0.005) Mass_sun yr-1 Mpc-3.
