For the moment, I have sacrificed speed with a routine that can handle three different fills - black, white, or 50% dithered. The routine will always blank the wall, mask out the pattern you wish to fill then OR it - which works nicely, but wastes time if you are filling plain black or white walls.
The result is quite pleasing:
As you can also see, I added scrolling skies.
The dithered walls look a bit nicer on hardware. The option to switch the initial dither pattern on alternate frames (resulting in a pseudo-greyscale) can be switched on or off, as it looks rather bad in emulators.
There is one horrible problem with the physical LCD, though - it really cannot cope with columns of alternating pixels. Compare the two shots, one being much closer to the wall (and so having more columns of alternating pixels):
Not so good. Hopefully the world in-game will have enough content to break up the columns, but I'll have to wait and see on that count.
One major concern I've had is keeping the speed of the engine constant. Just moving the player X units per frame is not an option, as the framerate can vary wildly. I need some sort of timing!
Luckily, the calculator has fairly limited timing hardware built-in. I can use this timer via interrupts. There are various pieces of hardware that can trigger interrupts - including the On key, activity on the link IO port (any line held low will trigger an interrupt) or this timer. The TIOS seems to use this timer to scan the keypad a few times a second, so it can read a key and store it for the key reading routines.
Fortunately, it is possible to take advantage of interrupt mode 2 on these calculators to install your own handler. When the engine is initialised, it adds the new handler - which is very simple. All it does is increment a 16-bit counter!
I added a piece of code that read this value and set it to zero each frame, displaying the result (ticks). I then added a piece of code that toggled the data lines of the link IO port, and used a PC program to monitor this rate (I used this technique before to work out interrupt-based timing for the 60Hz CHIP-8 clock). By comparing two extremes - a large viewpoint where I could see all of the walls, close up and a viewpoint where I could see nothing, I could see that one view ran at 16FPS and accounted for 20 ticks, and another ran at 44FPS and accounted for 8 ticks. This shows that one tick is approximately 3ms long, and that a FPS counter could be implemented by dividing 333 by the tick count.
A quick check with my PC program shows that the counter is fairly accurate (close enough for my liking). I can now use these tick counts to scale the movement of the player, so the speed is consistent. Another advantage is that I can set the SE edition calculators to 15MHz, and they can benefit from more fluid framerates (nb: any program I run will still need to run merrily on the 6MHz calcs for me to be happy. I'm not going SE-only!)
That's not to say I can now go and build an exciting level and release a nifty demo. There are still big holes in the engine - simple graphics bugs, like the wall filling code still not supporting top edge slopes with a y difference of over 127: (overflow-related bug)
...to there still not being any proper occlusion (sorting)...
I'd like to get occlusion up and running, so I can throw a nifty level together (for example, showing off non-90? walls). Thing is, it's tricky trying to show off cool stuff and not walk into an area where you can see the world isn't quite as solid as it should be. [sad]