We present a photometric study of the H \alpha 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 H \alpha emission that arises from the diffuse ionized gas is about 15–30 % of the total H \alpha 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 H \alpha 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 H \alpha 
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 H \alpha luminosity density and the star formation rate density of the local Universe are 10 ^ { 39.3 \pm 0.2 } erg s ^ { -1 } Mpc ^ { -3 } , and \rho _ { SFR } = 0.016 ^ { +0.007 } _ { -0.004 } \mathcal { M } _ { \sun } 
yr ^ { -1 } Mpc ^ { -3 } ( H _ { 0 } = 50 km s ^ { -1 } Mpc ^ { -1 } , \Omega _ { \mathrm { M } } = 1.0 , \Lambda = 0 ) .
The corresponding values for the ‘ concordance cosmology ’ ( H _ { 0 } = 70 km s ^ { -1 } Mpc ^ { -1 } , \Omega _ { \mathrm { M } } = 0.3 , \Lambda = 0.7 ) are 10 ^ { 39.5 \pm 0.2 } erg s ^ { -1 } Mpc ^ { -3 } \rho _ { SFR } = 0.029 ^ { +0.008 } _ { -0.005 } \mathcal { M } _ { \sun } yr ^ { -1 } Mpc ^ { -3 } .