Martydom of St Processo and St Martiniano was executed for an altar of the right transept of St Peter's Basilica. The references to the Martyrdom of St Erasmus, painted by Poussin for a nearby altar, are clear, so much so that the two works were considered at first as being in competition with one another.
Contemporaries acknowledged that the purely Caravaggio style painting (due to its realism and the particular use of light) of Valentin exceeded that of Poussin in naturalism, force, richness and harmony of colour.
