  We develop a method to estimate photometric metallicities by simultaneously
fitting the dereddened colors u-g, g-r, r-i and i-z from the SDSS with those
predicted by the metallicity-dependent stellar loci. The method is tested with
a spectroscopic sample of main-sequence stars in Stripe 82 selected from the
SDSS DR9 and three open clusters. With 1 per cent photometry, the method is
capable of delivering photometric metallicities precise to about 0.05, 0.12,
and 0.18 dex at metallicities of 0.0, -1.0, and -2.0, respectively, comparable
to the precision achievable with low-resolution spectroscopy at a
signal-to-noise ratio of 10. We apply this method to the re-calibrated Stripe
82 catalog and derive metallicities for about 0.5 million stars of colors 0.3 <
g-i < 1.6 mag and distances between 0.3 -- 18 kpc. Potential systematics in the
metallicities thus derived, due to the contamination of giants and binaries,
are investigated. Photometric distances are also calculated. About 91, 72, and
53 per cent of the sample stars are brighter than r = 20.5, 19.5, and 18.5 mag,
respectively. The median metallicity errors are around 0.19, 0.16, 0.11, and
0.085 dex for the whole sample, and for stars brighter than r = 20.5, 19.5, and
18.5 mag, respectively. The median distance errors are 8.8, 8.4, 7.7, and 7.3
per cent for the aforementioned four groups of stars, respectively. The data
are publicly available. Potential applications of the data in studies of the
distribution, (sub)structure, and chemistry of the Galactic stellar
populations, are briefly discussed. The results will be presented in future
papers.
