Hazy　images　have　limited　visibility　and　low　contrast.　The　degradation　is　expressed　by　transmission　map,　which　is　one　of　the　most　important　estimates　of　single　image　dehazing.　Transmission　map　estimation　is　an　underconstraint　problem,　and　lots　of　priors　have　been　proposed.　Among　them,　the　dark　channel　prior　is　widely　recognized.　However,　traditional　methods　have　not　fully　exploited　its　power　due　to　improper　assumptions　or　operations,　which　cause　unwanted　artifacts.　The　postrefinement　algorithms　employed　to　remove　these　artifacts　in　turn　undermine　the　merits　of　the　prior.　In　this　letter,　a　novel　method　for　estimating　transmission　map　by　energy　minimization　is　proposed　to　solve　this　problem.　The　energy　function　combines　the　dark　channel　prior　with　piecewise　smoothness.　The　method　is　compared　to　the　state-of-theart　methods　and　shows　outstanding　performance.