I wasn’t planning to post an update so soon, but this morning when I came into work I was told that my classes were cancelled for the day. This happens occasionally at the end of the school year and generally means that I have a full day at my desk to do whatever I want. I was up late last night writing my last post, so NES programming was on my mind when I woke up in the morning.
Sequences
Today’s mission was to get Sequences up and running. A Sequence is a series of monster images at the top of the screen that indicates to the player what they should whack next. I want the Sequence engine done before I start work on monsters. It doesn’t make sense to do the monsters first. The Sequence engine is like the backbone of the game.
Here’s what I needed for Sequences:
- An init routine that selects a starting Sequence based on Level/Sublevel.
- A routine to draw a sequence to the screen
- A routine to close an individual Slot and advance the engine to the next monster in the Sequence. This includes some helper routines to update graphics.
- A routine to select a (random) new Sequence after all Slots in the current Sequence are closed.
Graphics
I decided to start by drawing a 16×16 metatile to represent a closed slot. I had time, so I took my time and ended up redrawing a lot of the dummy graphics for the Sequence Indicator:
new graphics for the Sequence Indicator.
I went for a metal look to start because it is the easiest for me to draw. For the final game I want to have several tilesets for different Level themes. I’ve always sucked at art, so I was quite surprised and happy that my tiles ended up looking so good (to my eyes anyway). I’m especially proud of my keyhole.
Loading Sequences
Loading a Sequence was pretty straight forward to code. I have a table of pointers to tables of pointers to tables of pointers: Levels->Sublevels->Sequences. Once I get down to the Sequences, I choose one randomly. My random number generator is extremely basic: I merely increment a variable once per frame. So far it does the job.
Each sequence is a series of bytes representing a monster ID. Since Sequences can be of variable size (4, 5 or 6 monsters), I have a terminator ($FF) to signal to the engine when the Sequence ends. Loading a sequence copies this series of bytes (including the terminator) to an array in RAM and sets an index variable to 0.
Drawing the sequence was pretty straight-forward too. I cleverly designed the game board so that each Slot location corresponds to a 16×16 square in the bottom-right quadrant of an attribute block. This means that the only thing that differs between the 6 Slots as far as drawing is concerned is the low byte of the target PPU address. Code is so much cleaner when everything lines up like this.
Closing Slots
Since there are no monsters yet, I have the game close a Slot every time a hole is whacked. On the game logic level, all I have to do is increment the sequence index variable and check for the terminator.
Graphically it is no more complicated than drawing a monster image into a Slot. At least it shouldn’t have been, but I got greedy and added a 3-frame animation. It took me all day to finish as I had never done background animation before today, but it was good to get my hands dirty and figure it out. I learned a lot. As a final touch, I added a door-slam sound effect at the end of the animation, which incidentally forced me to code pitch envelopes for the noise channel into my sound engine. Here’s the latest video:
Conclusion
There are still some things I want to add the the Sequence engine. I want to animate the Slots opening when a new Sequence is displayed. I also need to add in some mechanism to advance Levels/Sublevels after a specified number of Sequences are cleared. What I mean is if the player is on Level 1-1, and they knock off, say 8 Sequences, it sends them to level 1-2, which would have a different set of Sequences for the engine to choose from. These features can wait for another day though, as I’m all Nintendo’d out for today.
(BTW, Google is the ultimate word-usage authority: it’s “Nintendo’d out”, not “Nintendoed out”)
