Piero della Francesca was not just looking at painters while he was in Florence. In the two years he worked there, 1439-1441, he absorbed visual stimuli from sculpture, architecture, and painting. He is never documented in Florence again, at least to produce works of art, but he carries with him through the whole of his artistic life, in Borgo San Sepolcro, Arezzo, Urbino, Rimini, Monterchi, the seeds of what art was as it was created in the great excitement nest of 1440 Florence. Even if he never documents in his treatises or elsewhere the art he was exposed to in that period, we know he saw things just by their inclusion in his own paintings.
What did he see, then?
Donatello's tomb of Pope John the XXIII (the Antipope) provided the open curtained tent for three of Piero's works:
Madonna della Misericordia, 1445-1462, Borgo San Sepolcro
Dream of Constantine, 1450's, San Francesco, Arezzo
Madonna del Parto, after 1457, orig. Santa Maria di Momentana,
The literal protection over the dead body of the pope in Donatello's design is transformed by Piero into painted cloth-covered protection in death provided by Christian belief. Piero even includes in all three works Donatello's symmetrical bas-relief angels on the sarcophagus below the pope's body, and his virtue figures below them.
In the Misericordia the virtues are changed to the kneeling members of the Confraternity of the Misericordia who commissioned the painting, members who include the artist himself looking up next to the figure wearing the black hood (worn by Misericordia members even today).
In the Constantine scene, the angels and virtues are changed to soldiers guarding Constantine while he is visited by the angel with cross who converts him to Christianity.
In the Madonna del Parto, the symmetrical angels stand up and hold the curtain on either side of the Madonna (they are identical twins but reversed, probably from a pounced drawing that was just turned over for reuse.)
The Donatello tomb sculpture is also one of the sources for the shell niche in Piero's Brera Madonna of 1472-74.
The head of the Madonna is moved below the shell niche and the stretched-out body of the pope becomes the body of the Christ child lying on his mother's lap. Piero's shell is deeper than that of Donatello's, though, and more clearly a scallop shell. Other sources for the Brera shell are Brunelleschi's sculptural green and white marble shells in the tribune niches of the Florentine Duomo
and the shells in the niches of the intarsia of the North Sacristy:
Piero turns Brunelleschi's shell upside down to make it his own creation. His ideas, though, are guided by what is au courant in Florence around 1440. Donatello and Brunelleschi are only two of the shapers of his consciousness. Blog C for What Piero Saw will give other examples of Florentine influence in Piero's work.