Kyrie, Johannes Ockeghem

Kyrie from Missa prolationum, Johannes Ockeghem.
There was something that caught my eye in the score of this piece while I was listening to it and trying to follow the score. The Alto voice in the very beginning was singing just the same melody as the Soprano, but proportionally slower. I've done some research and found that such thing is called a prolation canon (or a mensuration canon/proportional canon). And Missa prolationum is called so because this technique is used in all parts. As I understand it's extremely hard to write such a canon, and to make it sound this smooth and beautiful is even harder, and there are really a few examples of these.
The harmony feels full of light and the rhythm keeps my attention through the whole piece which is quite unusual for complex music when you sometimes lose focus. Here, because of canon structure and accompanying melodies in lower voices, and because the piece is partly using all 4 voices and partly only 2 it's really entertaining to listen to it.
I think the most appealing thing here is the structure, rhythm and compositional technique, while the harmony seems almost perfect too.
Talking about times when this music was created, I think of church choirs, and some castle-like places where the composer maybe lived, but the historical point of view is quite hard for me to have.
Interesting detail that I found is that Ockeghem for some time had a position of "left-hand choir singer" which meant that he sang composed music while "right-handers" sang chant. As I understood singing in a choir was a way to become a professional musician or composer at that time. What is also interesting is how many places he had opportunity to live in during his long career.
The dating for this piece is not precise. The best you can say - mid 15th century or maybe it's second half. Since Ockenghem lived 1410(?) - 1497, Kyrie from Missa Prolationum might be his middle or late work.