In The History of Constantine, how is the The Baptism of Constantine of the Rubens' designs elucidated?

sketch: private collection, 17.75" x 22.813" (45.085 x 57.945 cm.)tapestry: 15' 8" x 17' 10.75" (477.52 x 545.465 cm.) In order to maintain a safe distance from the Arian Heresy condemned by the Council of Nicaea, Rubens ignored Eusebius' account for once and instead used the much later Vita Silvestri, a medieval collection of legends. Thus, Constantine is here safely baptized by Pope Sylvester in Rome, rather than Nicomedia as actually occurred. The main difference between sketch and tapestry is the fact that the later includes Constantine's crown on a nearby cushion, a reference to a legend that a spontaneous flame appeared over the baptismal font and would not be extinguished until Constantine removed his crown.
Rome Arian Heresy Nicomedia Council of Nicaea tapestry Pope Sylvester Eusebius