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Sega's Alien Storm is undoubtedly one of the most interesting of the latest wave of coin-ops. Not only does it have the standard blow-your-socks-off graphic routines, it also has an unusual range of gameplay which has pushed Tiertex to what looks to be their finest hour yet on the C64. Robin Hogg interviews programmer Mike Ager about US Gold's latest coin-op extravaganza.

The mega-blastin' coin-op is set on Earth, some undefined moment in the future when games programmers' favourite nightmare has come true - aliens have landed. Apparently this bunch of aliens have been forced to leave their home planet "through lack of environmental awareness" - hmmm, ah well US Gold always did like to be trendy.

Anyway, a huge mothership has arrived in orbit and sent forth all manner of weird creatures. The first aliens are capable of disguising themselves as plant pots, dustbins and other household objects which really makes our alien-busting heroes' shoot-em-'up activities seem a tiny little bit odd! Fortunately the aliens are quick to get confident enough to appear in all their natural hideousness and mass panic results!

The original coin-op was a three-player game with Gordon, Karla and Scooter - AKA the Alien Busters - taking on the evil and nasty baddies simultaneously, each with their own unique experimental weapon. The C64 version allows just two players on screen. However Tiertex "are trying to allow selection from all three characters, but with memory so tight each player will probably be the same character". But biggest challenge is that Alien Storm includes three totally different games, this has meant it's "taking longer than expected to write".

To refresh your memory, the coin-op has six missions each split into three sections using the different game styles. Section one is a slow-scrolling fight scene where you battle aliens at close range, desperately trying to avoid being poisoned, bitten, punched, whipped, thrown or eaten! Survive this and there's a 3D shoot-out section, you have a first-person perspective of a ship which parallax-scrolls by with various aliens suddenly leaping out at you. Then finally it's a fast-scrolling running section were you chase after the aliens, trying to stop them escaping.

The specific team responsible for converting this monster coin-op are graphic artist Wayne Billingham and ex-Software Creations programmer Mike Ager (Peter Pack Rat, Denarius and Sly Spy: Secret Agent).

"It will, of course, have to be multi-load as the sections have very varied graphics and code. We are trying to lessen the loads partly by changing the order of stages within a level and putting different stages into one load."

Programming "started with the slow walking section then the running stage and finally the harder 3D section. The majority of the game code is written for all three sections but the aliens which appear in later stages still need to be put in as well as linking the levels."

Unsurprisingly the 3D sequence provided the biggest programming challenge. "It is by far the trickiest on all formats, being a four-overlay parallax scroller with sprites needing to walk in between all levels. On the Commodore, the levels are built from characters and shifted in a frame like the arcade.

"The aliens run between the shelves but their size is limited. One alien runs behind the shelves then jumps at the player. This alien peeps over the shelf to check there aren't too many sprites already on-screen before expanding to its full size."

Tiertex are famed for their quality game presentation and Storm has a nice intro where the team scramble from hamburger stall. Unfortunately "due to development time and graphics room the introductions will most likely be cut."

It's a shame but no-one likes being slowed down by lengthy intros loading, the important thing is the gameplay which the team are confident of capturing. They certainly like the original coin-op. "The 3D section has to be the most fun, destroying the backdrops and causing mayhem all round. The alien graphics are interesting as they are quite frankly repulsive!"

The actual programming itself of Storm is being done on an Atari ST, a common system that's also used by Ocean among others. It's the ST's memory that gives more room for the team to mess around with, and the finished code can then be downloaded to the C64 for testing. "Graphics are being grabbed directly from the arcade board using Amiga HAM digitizers - these are used as a guide for C64 graphics. Actual sprites have had to be drawn by hand for the C64 because of sprite constraints. Backdrop graphics were converted using a utility on an Amiga."

Packing all this into a C64 is quite a challenge, in fact "there are sixteen different alien types in total. The number of animation frames ranges from three, for the blobs, to around twelve for the tentacle beast which is built from seven sprites. The largest graphic so far is the 3D section gremlin which, when jumping at you, expands to ten sprites being 84 x 96 pixels. The carrot-shaped mutating alien should be four sprites wide by six sprites high when it's put in."

All this leaves little room for music which is "composed on a synth then uploaded via Midi to an ST, then given C64 voices."

Although Storm still has a way to go before being completed, it's looking very hot. As you can see from the screenshots, the graphics are big, bold and very colourful. The imaginative coin-op variety has been perfectly preseved with some effective animation - the main character's big, somersaulting leaps are particularly nice - and scrolling on all the sections is silky smooth. As long as multi-loading isn't too bad - and the levels are big enough to minimize irritation - Tiertex's latest seems surely destined to be a big summer hit.

Alien Storm is due out from US Gold in September (prices to be announced). Check out next month's Zaap! for an exclusive review.

The Manchester-based programming company was founded three-and-a-half years ago by physicist Dr John Prince and electronics engineer Donald Campbell.

From the start, Tiertex was dedicated to coin-op conversions and US Gold with Spectrum 720 Deg being their debut product.

Subsequently they've done only three original titles: Indiana Jones & The Last Crusade: The Action Game (in association with Lucasfilm), Italy 1990, Italy 1990 Winners Edition and Strider II. The latter was US Gold's own sequel to their conversion of the original Capcom coin-op, but as yet talk of turning it into a coin-op temains just that.

For the moment, the company has its hands full not only with Alien Storm, but also Mercs and a couple of other top secret projects.

Softology: Last Duel, Human Killing Machine, Streetfighter, Thunderblade, Rolling Thunder, 1943, Black Tiger, Dynasty Wars, Indiana Jones: The Action Game, Italy 1990, Italy 1990: Winners Edition, Strider, Strider II, Days Of Thunder and UN Squadron.

Company motto: Filling the screen with magic!