[JKCalhoun]

With overlapping rectangular windows (slightly simpler case than ones with rounded corners) you can expect visible regions of windows that are not foremost to be, for example, perhaps "L" shaped, perhaps "T" shaped (if there are many windows and they overlap left and right edges). Bill's region structure was, as I understand it, more or less a RLE (run-length encoded) representation of the visible rows of a window's bounds. The region for the topmost window (not occluded in any way) would indicate the top row as running from 0 to width-of-window (or right edge of the display if clipped by the display). I believe too there was a shortcut to indicate "oh, and the following rows are identical" so that an un-occluded rectangular window would have a pretty compact region representation.

Windows partly obscured would have rows that may not begin at 0, may not continue to width-of-window. Window regions could even have holes if a skinnier window was on top and within the width of the larger background window.

The cleverness, I think, was then to write fast routines to add, subtract, intersect, and union regions, and rectangles of this structure. Never mind quickly traversing them, clipping to them, etc.

[duskwuff]

The QuickDraw source code refers to the contents of the Region structure as an "unpacked array of sorted inversion points". It's a little short on details, but you can sort of get a sense of how it works by looking at the implementation of PtInRgn(Point, RegionHandle):

https://github.com/historicalsource/supermario/blob/9dd3c4be...

As far as I can tell, it's a bounding box (in typical L/T/R/B format), followed by a sequence of the X/Y coordinates of every "corner" inside the region. It's fairly compact for most region shapes which arise from overlapping rectangular windows, and very fast to perform hit tests on.

[JKCalhoun]

Thanks for digging deeper.

[gblargg]

The key seems to have been recognizing the utility of the region concept and making it fundamental to the QuickDraw API (and the clever representation that made finding the main rectangular portions easy). This insulated QuickDraw from the complexity of windowing system operations. Once you go implementing region operations you probably find that it's fairly efficient to work out the major rectangular regions so you can use normal graphics operations on them, leaving small areas that can just be done inefficiently as a bunch of tiny rectangles. All this work for clipped graphics was applicable to far more than just redrawing obscured window content, so it could justify more engineering time to polishing it. Given how easy they were to use, more things could leverage the optimization (e.g. using them to redraw only the dirty region when a window was uncovered).