So how about some arena-based blasting on the C64 for a quiet but annoyingly warm Sunday evening then? Well, there’s always Death Weapon from… erm, yours truly on the C64CD label that popped up as a last minute entry into RGCD’s 16K cartridge competition. So why not grab a joystick and blast those nasty little aliens into oblivion!
The gameplay was originally based on Jeff J Gosden’s Death Dealer from 1989 but had elements from other titles grafted into it and one thing to look out for in particular is the control scheme; moving the joystick guides the Death Weapon around the screen but it fires backwards unless the fire button is held down. That’s a combination of what Death Dealer does and the Hallucin-O-Bomblets sub game of Jeff Minter’s Batalyx, which was also the inspiration for the enemies materialising into the playfield rather than emerging fully formed from the borders. Another title that had ideas unceremoniously robbed from it was Intensity by Andrew Braybrook, more specifically it influenced the background graphics and the pretty parallax starfield behind them.
There’s a couple of graphical tricks going on to make elements of the game appear to be using colours not in the regular C64 palette as well, parts of the background use PAL blending – when two colours of the same luminance are placed next to each other on odd and even scanlines – to get what appear to be new colours and some of the enemy sprites are utilising a similar trick, alternating between two colours every frame so they appear to blend together. This is best experienced with a good old CRT display, as are the player’s bullets which are generated from one sprite which flickers back and forth between the two jobs at 50 frames a second.
None of this is new of course, but there’s some reasonably well commented source code over at Github for anyone fancying a prod around.
The start of a new month sees a new C64 game from yours truly… well, “new” in the sense it’s a reworking of something I released last year under the C64CD label. Stercore XD is a horizontally scrolling shoot ’em up and I’m sure my regular fan has just passed out in shock at such an out of character move on my part. Sarcasm aside, I might as well wibble on a bit about technical details since the game isn’t exactly a complex beast with an engaging back story.
The screen scrolls at five pixels a frame – a little slower than the original Stercore on the Spectrum or the direct C64 port which are moving at eight pixels – and uses a wider map which leaves gaps for tidy background colour changes. That map is 2,340 tiles wide, making the background around 240 screens in total and it all barrels past in a little shy of five minutes during play. Since Stercore XD has been squarely aimed at the RGCD competition it needed to run from a 16K cartridge, so the bulk of the game is compressed with Exomizer but the unrolled chunk of background scroll code required to move two and a half times what the Spectrum is dealing with was generated on start up.
Stercore had player and enemy objects passing between two layers and this has been faithfully replicated with hardware sprites in Stercore XD, relying on the hardware sprite priority register and using a similar approach to games like Implosion, Dan Dare or Shadow Skimmer. One background layer is always the background colour the character mulitcolour which that doesn’t get priority over the sprites, the other layer uses the remaining multicolour and character colour, with the latter mostly being used to add dark and light detail. This technique is accurate to half pixels and requires no processor overheads for masking but does require a lot of extra juggling when drawing the graphics.
After that it’s got a slightly extended and tweaked version of the original theme tune which was just an extra half pattern from the original Autotracker-generated tune used to “create” the original Stercore music and, more importantly to the gameplay, there’s a simple attack wave driver which was repurposed from Super Hyperzap. As is usually the case there’s source code – and in this case, work files for those wanting to understand the sprite priority thing – over at GitHub for folks wanting to prod around, although that source isn’t my tidiest piece of work even after it was “sanitised”!
Last time I talked about the new, C64-specific version of C64CD blaster Stercore and this month continues that “trend”. It’s now officially called Stercore XD and I’ve actually got the majority of it written and working somehow!
Memory is getting very tight but all of the graphics are done now including some sprite animations – one of the aliens and the explosion have been “inherited” from Super Hyperzap – and everything just about fits into the lower half of the C64’s RAM after decrunching until the code grabs around 10K more whilst unrolling the background scroller. Spacial awareness is important here because I’m planning to put the final release version of Stercore XD out as a cartridge image for the RGCD competition. To that end the build process automagically makes a working CRT image for testing.
I still have a “to do” list including tweaks to the collision detection, a rewrite of the enemy movement code to optimise it, adding explosions and bolting in the simple attack wave driver from Super Hyperzap with a couple of small tweaks but, since there’s currently only about 280 bytes of ROM left in the 16K block the game has to occupy as a cartridge, some concessions might be necessary. To give a sense of scale, this paragraph is 457 bytes…