“Ruin porn” is the trend — fed in part by the recession — in which decaying, abandoned buildings have become a popular photographic subject, not just for architectural studies, but for fashion shoots. The latest installation of the Hirshhorn’s Directions series focuses on the work of two artists whose work “explores the ruins of the 20th century.” But the work of Mario Garcia Torres and Cyprien Gaillard is not just ruin porn: their art traffics not only in decaying structures but in obsolescent art forms as well.