Janis Joplin’s raspy wail and the tunes she wrote somehow reached into reservoirs of sadness we never knew we had. Comparisons have been made to Amy Winehouse, some accurate, some not. They each had their own demons. But the shared theme of almost all their songs was a desperate loneliness. This sympathetic, conventionally structured cultural excavation offers a new element: the reading (in a spot-on voiceover by Cat Power, capturing the singer’s spirit) of several heartbreakingly optimistic letters that Janis wrote to her family back in Texas. An engrossing tribute to a brilliant talent. (103 min)