Comment by II2II
1 day ago
It's a good description, but I'm going to add a couple of details since details that are obvious to someone who lived through that era may not be obvious to those who came after.
> Obviously it could have not been a mask of pixels
To be more specific about your explanation of too much memory: many early GUIs were 1 bit-per-pixel, so the bitmask would use the same amount of memory as the window contents.
There was another advantage to the complexity of only drawing regions: the OS could tell the application when a region was exposed, so you only had to redraw a region if it was exposed and needed an update or it was just exposed. Unless you were doing something complex and could justify buffering the results, you were probably re-rendering it. (At least that is my recollections from making a Mandelbrot fractal program for a compact Mac, several decades back.)
And even ignoring memory requirements, an uncompressed bitmap mask would have taken a lot of time to process (especially considering when combining regions where one was not a multiple of 8 pixels shifted with respect to the other. With just the horizontal coordinates of inversions, it takes the same amount of time for a region 8 pixels wide and 800 pixels wide, given the same shape complexity.