The Arcade Blogger on the Development of Xevious

December 30th of last year, The Arcade Blogger did a piece on the creation of Namco’s classic vertical shooter Xevious.

Xevious was modestly successful in the US, where it was produced by Atari, but it Japan it did amazing numbers. Jeremy Parish (in his NES Works and related series) has mentioned several times that it was a vastly influential game in Japan, inspiring a whole generation of designers, and a whole bunch of clones and similar games. Its US release was around the time of the arcade crash, which was mostly an American thing. If it hadn’t had happened, maybe now we’d think about Xevious the way we consider Pac-Man.

The team behind Xevious

Xevious basically invented the vertical scrolling shooter where your ship has free movement of the screen. It also included a Bomb button to attack objects on the ground, displayed on the game’s background layer. It was a concept that would later be iterated upon in Konami’s Twinbee games.

Revealed in the article is an interesting fact. The scrolling background is stored in ROM as a huge 1024×2048 bitmapped image. That’s much wider than the screen is though. What the game does is send the player into a vertical portion of it 224 pixels wide.

When the player reaches the top, they wrap around to the bottom of another vertical stripe of the game world. In a complete loop, the player will travel from the bottom to the top 16 times. You can tell when you’re about to start another loop because the background will reach a place with trees all the way across!

You always start off a life in a tree-filled area because it begins you at the bottom of a stripe; each vertical pass over the map functions as a checkpoint. The stripes overlap somewhat, so you sometimes pass over an area you’ve seen before but offset by a bit.

For more facts on Xevious and its development, be sure to click through to the article!

The Arcade Blogger: The Development of Xevious

Classic Game Dev Andrew Braybook On Computer Conversions

Andred Braybrook is a legendary computer game dev from the Commodore 64 age, and before and after. In addition to the classic C64 games Uridium and Paradroid, which perform feats of scrolling that machine are really not designed for, he went on to many several other games, including the excellent computer ports of Taito’s sequel to Bubble Bobble, Rainbow Islands.

(Although Braybrook mentions that Taito hadn’t told him about the three secret islands that can appear at the end, so they got left out. They didn’t know about them either!)

Image from Braybrook’s blog

He has a blog post that details many aspects of he and his co-workers’ process back then that is fascinating to anyone with knowledge of these platforms, and even someone who doesn’t. Thanks to @acb@mastodon.social for the link!

Andrew Braybrook: How We Made Computer Game Conversions, from his blog

Breath of the Wild Cel Shading Break Glitch

The Legend of Zelda: Breath of the Wild is a gigantic game, and where content proliferates, so too do bugs. Many of these bugs are highly entertaining (my favorite is the bullet time bounce), but there are some that are just head-scratching, leaving one to wonder why does this happen? That the occur pulls back the curtain on the many technically complex things a big game like BotW does behind the scenes to realize its world, for, every step of a process that a system must go through is one more opportunity for something to go wrong.

Image from Nintendo Everything

Youtuber Jasper has made a 35 minute video about why, if Link stands in a specific spot in BotW, inside the broken corner of a stone wall, the cel shading usually applied to his model goes away, and he appears with normal light shading. In the way of Youtubers, the explanation is contained within a 35-minute discursive video that goes into the history of game lighting, why some older 3D games have graphics that have aged well while others don’t, the basics of cel shading, and still other topics. Here is that video, embedded:

The whole video is pretty interesting, and if you have the time and interest you should watch the whole thing. However, in the event that this is all tl;dw, allow me to summarize.

  1. Because Breath of the Wild is both a huge game and has a dynamic world, baking lighting in into textures would consume way too much storage and memory, so lighting has to be done dynamically.
  2. As an optimization measure, the more complex steps of cel shading are deferred to later in each frame’s rendering. The main rendering is done, then the cel shading is applied afterward, when the visibility of the area has been determined, so this effort-expensive process is only done for visible pixels.
  3. One of the deferred steps of rendering marks which of nine different kinds of material will be applied to each pixel. Terrain in BotW is not cel shaded, while characters link Link are, so they have different types of material that determine whether that shading is applied to them.
  4. In the location where Link’s cel shading disappears, there is a decal applied to the crumbling bridge that erroneously extends over the corner, and overwrites Link’s character material type with the terrain material, causing the cel shading not to be applied to him.

How Gravity Works in Super Mario Galaxy

Another Youtube video? Yeah I know. This one explains how gravity works in Super Mario Galaxy. It’s 29 minutes long. The basic gist is, there are eight kinds of invisible gravity field objects, based off of simple shapes, in the game, which are used in concert to create the various orientations that Mario switches to as he moves around: Parallel, Sphere, Cube, Disk, Torus, Cylinder, Wedge, Wire (basically an arbitrary path in space), and Cone, which is only used in two places.

An interesting fact from near the end of the video: gravity affects Mario’s shadow! Shadows point towards where Mario will fall, not according to how light strikes him, to give players a sense of where he is spatially in 3D space.

How Spherical Planets Bent the Rules in Super Mario Galaxy (Youtube, 30 minutes)

Super Mario Bros Level Glitches

The Youtube channel Retro Game Mechanics has done a series of three videos on glitches in Super Mario Bros. One involves using the NES game Tennis, which has a certain property of its code that allows you to load all kinds of funky levels in SMB.

They’re all interesting, but the one that floats my particular boat is the third, which turns into a deep dive in the compressed manner that Super Mario Bros. stores its levels in ROM, and uses to draw them during play in real time.

1. Access Glitch Worlds in Super Mario Bros. via NES Tennis (13 minutes)

2. Exploring Glitch Level Pointers In Super Mario Bros. (15 minutes)

3. Super Mario Bros. Glitch Levels Explained (30 minutes — this is the one embedded above)

Roguelike Celebration: Adam Newgas on Constraint-Based Generation

Honestly, I could devote a post to every Roguelike Celebration talk. I’ve been limiting myself to just one such post a week, on Saturdays. This one, a short sixteen minutes talk about terrain generation, is for the developers out there.

Constraint-based generation, also known as “wave function collapse,” is a system where, as objects are placed randomly during generation, the generator “solves” the world around them, placing later terrain as is necessitated by prior terrain. If the generator reaches a contradiction, a situation where there is no viable terrain that can be placed, it undoes the contradictory placement and continues from there.

It’s a technique that’s fairly popular in procedural generation circles, and among other games is used in Caves of Qud. It’s also fun to watch it work!

Metroid Prime’s Save System

Zoid Kirsh on Twitter (while Elon Musk hasn’t completely wrecked it yet) tweeted about how Metroid Prime’s save system works. Metroid Prime save files are less than 60 bytes long! A single Gamecube memory card block is eight kilobytes, so it’s a bit overkill, but it’s still nice when a developer is frugal!

Image, of Metroid Prime’s inventory screen, from Mobygames.

The way they explain it is that the game has a number of “world layers” which determine what is spawned in each area when it’s loaded. Which layer is active when a room is loaded is determined by a single bit in the save. That, plus some basic stats like health and ammo, and the record of object scans, all take up very little memory when bit-packed.

If Metroid Prime’s save file were 59 bytes long, that would mean it had 472 bits to work with. The passwords used by the original Metroid only stored 144 bits of data!

The Issues With NES Teenage Mutant Ninja Turtles

The title refers to the original NES TMNT, not the arcade version or the NES game based on it. This is the version that Konami released under their Ultra label. It sold well (real well!) but is widely considered an inferior game for a number of reasons. Those reasons are the subject of these three videos, from Youtube channel Displaced Gamers. I recommend them, even if I think every place they say gamer it would be more proper to say player.

The first video:

In a long and difficult game, one of the hardest sections comes relatively early. The only swimming section in the entire game, players must maneuver their supposedly-aquatic surrogates through a difficult course that has imprecise movement, water currents, high damage, instant kill hazards, a strict time limit, and, as the video shows, buggy implementation. Many players in the NES era gave up at this point, which is rather a shame considering it’s only at the end of level two. This video examines the code and demonstrates why it’s so challenging, and how it could be made fairer.

The second video:

TMNT has notoriously floaty jumps, a low frame rate, and a fairly weird implementation of gravity. Any platform game that allows players to adjust their jump height according to how long the hold down the jump button is fudging its physics behind the scenes, but TMNT does it rather poorly.

The third video:

Displaced Gamers examines additional problems with the game’s timing, particular with that of its input reading and attack animation. Like the other two videos, they suggest code changes (sometimes in the form of Game Genie codes) that fix the problem, if you happen to have a fondness for 6502 assembly. (I do!)

If you’d like to try NES Teenage Mutant Turtles, it’s included in the “Cowabunga Collection” that was released for Switch, Xbox X/S and Playstations 4 and 5. Fortunately, it also includes twelve much more playable titles.

Roguelike Celebration: Joel Ryan on Creating the Sil-Q Tileset

Sil-Q is an Angband variant. Joel Ryan, aka MicroChasm, made its tileset which shows a lot of care in its creation. Sil-Q’s tiles are modular, so humanoid monsters can hold weapons, and also have strong silhouettes to aid recognition. It’s full of the kinds of concerns pixel artists have to worry about!

Silhouettes of various monsters in Sil-Q
As a bonus, the talk provides this lineage of Sil-Q!

Roguelike Celebration: Common Pitfalls With Roguelike Traps

From Roguelike Celebration 2022, Reed Lockwood’s talk on trap design in roguelikes. Traps are an essential part of a D&D-style dungeon exploration sim, but are very easy to get wrong, either by making them too strong or, conversely, too weak. Some interesting ideas here!

Fabian Sanglard on Sprite Creation on the Capcom CPS-1

Street Fighter II has some really complex spritework! Its characters don’t actually use traditional sprites, but more what amounts to custom tile layers for each one. This helped unlock characters from being mostly rectangular, and allow them to have poses with radically different shapes.

Fabian Sanglard has a great article about how character art was created for that system that’s well worth your time to have a look at. Well, maybe it’s worth it? I don’t know you. For all I know you’re looking for sewing advice. This is probably the wrong site for that. No offense to all you clothes-makers out there.

ROMs were expensive, and Street Fighter II required a lot of them, so it was important to make the most out of each one. A big rectangular shape around Edmond Honda would contain a lot of empty, wasted space. Imagine how much space they would have wasted with Dhalsim’s long stretchy legs! With this system, they only had to include the graphics data that would actually contain pixels.

This was in 1991, mind you. More recent development practices would probably have the data be compressed in storage, which would take care of all those empty pixels, or at least they could make a tool to handle figuring out which tiles should contain data. What the Street Fighter II artists had to do was create physical representations of each character on a physical board, chop that up into squares, and figure out what each tile had to contain, a laborious process.

Fabian reckons this system was used for other CPS-1 games, going back to at least Forgotten Worlds. Looking at the tile layouts of CPS-2 titles, it seems a lot more evident that they used a packing tool to handle fitting their characters into the memory space. For more info, please go check out the article!

How David Crane Got Good Music Out Of The Atari VCS For Pitfall II

Back in 2013, David Crane chimed in on a thread about Pitfall II. The Atari VCS (a.k.a. 2600) was not known for the quality of its music. For sound effects, especially noise effects like blasts and booms, it was fine, but its TIA chip didn’t have the frequency resolution to produce every musical note precisely, meaning some of it notes would sound a bit off.

Pitfall II’s music, some of the best on the system in the classic era

There was technically a way to produce almost arbitrary waveforms, though like many techniques on the system it was processor-intensive. It involved changing the volume on one of its sound channels in real time to simulate the waveform of the sound you wanted to make. That was fine so long as you didn’t need the processor to do anything else, and sadly, on the VCS, just displaying graphics relied heavily on the processor.

Pitfall II, VCS/2600 version. Image from Mobygames.

David Crane managed to get decent polyphonic music out of the VCS by using Pitfall II’s DPC chip, which Crane created himself, as a co-processor that figured out the right values to set the volume to produce the mixed waveform for the music at a specific time, which the machine’s overworked 6507 CPU could then read and send to the right volume register in the TIA every scanline. The process is explained (to the understanding of a sufficiently technical frame of mind) here. I think I understand it myself!

The fact that David Crane is still around, and so willing to discuss the many tricks he came up with to make his games, is a great blessing, as is the existence of the AtariAge forums themselves, which are a trove of classic gaming information.

Nowadays this technique has been refined and utilized in homebrew cartridge productions. A particular standout is the music from Champ Games’ version of Mappy, which is frankly amazing. Check it out: