[racket] fruit flies

Robby --

I don't know enough about how the internal drawing routine of DrRacket
works, so you could help clarify that.

In 2htdp/universe, the act of "movement" is represented by
reconstructing the entire scene.  Of course, there may be sharing:

(define bg ... something complex ...)

;; at time 1
(place-image I 50 50 bg)

;; at time 2
(place-image I 70 70 bg)

When this is finally rendered as a single image in the window, how
much work is done?  Is it proportional to the size of the scene, or is
it only proportional to the pixels that changed between the scene at
time 1 and the scene at time 2?  Does the library perform a diff
between the two scene graphs?

In Whalesong's web-world, the act of "movement" is represented by a
cursor finding the object and functionally updating its CSS.  After
web-world does its tree-diff, it uses a *mutative update* to change
the CSS attributes.  My understanding is that the browser DOM
implementation then uses this information to update the screen as
efficiently as possible (using Z depth to avoid unnecessary redraws,
clipping, bitblit, etc).

Thus, in web-world, the amount of work should be proportional to the
number of affected pixels and *independent* of the number of
unaffected ones.  Is this true of 2htdp/image/universe?

Shriram