TL;DW: Memory constrains prevented the inclusion of enemies in the game (due to the space taken by rotoscoped images), so he got the idea of xor-ing frame pixels together to get the outline of the main character, kind of a shadow, and made that the enemy. He also squeezed a bit more memory from an "auxiliary" memory unit to animate guards.
Hi, saw this thread and wanted to jump in. It's a good question not answered in the 20-minute ArsTechnica video: since Shadowman was born out of the necessity of being squeezed for memory on the Apple II, how was I then able to add guards, skeletons, and a Vizier?
To refresh my own memory (which is more than 48K, but often foggy), I checked my journal. Part of the answer is in this entry from 11/20/1988:
"...Put in footsteps, and improved the chomping jaws. But the real breakthrough this week was invisible: I moved a bunch of stuff around so the main game code can use the auxiliary language card. Basically, I've just freed up an extra 12K. That gives me some breathing room I'll sorely need if I'm going to put in all this sword fighting."
And a few months later, on 2/16/89:
"A lot of heavy work, most of it invisible, but which will make it much easier for me to implement the enemy guards in their flowing robes and turbans."
The journal doesn't give more technical detail than that (if I'd imagined anyone would be interested in this stuff 30 years later, I'd have written more!) but the code is on github, so anyone who's motivated enough should be able to reconstruct how the final code uses the auxiliary memory. In my recollection (foggy, as warned -- it's been a while), the "auxiliary language card" was an extra 16K of RAM that Apple had squeezed into the latest generation of Apple II computers, but that was difficult to use because it wasn't contiguous with the rest of RAM. I seem to recall it shared addresses already used by the main memory, so you had to flip between which part you were using. In November '88, seeing how much Shadowman and swordfighting and Shadowman improved the game motivated me to invest the time to rewrite/strategically relocate a bunch of existing code to free up 12K that I could use to implement enemies.
As for the multiple enemies, I'm pretty sure that each level contained exactly one enemy, whose shape table was packaged with the level load. The floppy disk access (very quick, thanks to Roland Gustafsson's magical 18-sector routines) when the level was loaded would swap out the previous level's enemy with the new one. That's why you'll notice that the levels containing the "special enemies" (skeleton, Jaffar, and the Fat Guard) had no normal guards in the rest of the level -- they couldn't.
Such great memories of playing PoP in my teens! I wish I could get as excited about some new game/software as I did back then. In those days there really were some titles that would come along and just knock your socks off. Doesn't happen much anymore...a sign of the times I guess.
I can't reply to the parent anymore, but I just went back to HN after a week of busy work: thanks a lot for the details, jmechner, that's quite interesting :)
I don't have enough time on my hands to dig into the old source code (nor I am particularly interested in the Apple II), but I'm glad to know it's available, should I or some others be suddenly curious to dig in. I much prefer such code to be opened than to be lost to history!
So, it's a bit like register banks [1] or an MMU, where you have to page in and out part of the address space, I guess?
I've seen some people [1] argue that we needed less powerful machines and to break backwards compatibility, for people to get creative again. Maybe there is some truth to that?
Kind of light on details, though :)