choosing the right engine is extremely important so i spent an entire day going through the options. modern engines tend to abstract things too much with concepts like actors and such. it’s a completely different paradigm from what the game did. directx 8 uses a very direct rendering approach. so we needed something barebones to replicate that. i ultimately chose raylib for my rewrite. so far im happy with the choice.

the engine handles things like creating a game window, drawing textures, playing sounds, streaming music, and handling input. so basically it takes care of some of the stuff grim.dll does. the only thing left to do is to fill in the entire game.

the rewrite

up to this point i was greedily documenting every behavior we could infer from the executable, but we had no code of our own besides the format loaders.

i think of it like getting a noisy picture in path tracing rendering. some details start to come through but the picture is not fully clear yet. we have random things documented but we can’t know for sure if this knowledge is sufficient to reimplement the entire game. there is only one way to know.

we need to change our render settings to scanline. just kidding, but that’s how i thought about it. when my version could boot into the menu, we would have uncovered all the missing pieces that lead up to that point. when we get to the gameplay, we’d have to implement the most systems to get there. our documentation helps, but this forward path leaves no system untouched and eventually we have our working game.

on day 6, when i got to the main menu, i found a funny path that would allow me to cover a lot of ground. i noticed the game still had demo teaser code intact, but game_is_full_version() was hardcoded to 1 in this gog version. naturally, i could write a frida script to uncrack the game and make it behave like a shareware, even though this version never intended such functionality.

the rendering in this game is pretty simple. there is a ground framebuffer (in opengl, i believe, the correct term is render target). first the game generates a terrain, i went great to lengths to get it right. we literally have test fixtures that assert we generate the exact same picture from the same seed. then this framebuffer is used to render all sorts of decals, like bodies, blood, bullet casings, scorch marks. this simple technique allows the game to visually transform the battlefield during gameplay.

the creatures come from sprite atlases you’ve seen above. the projectiles render with either beautiful traces that stay in the air, or at most with a simple texture and additive glow.

so rendering was not a huge obstacle, the hard part was that everything in this game is hardcoded in the exe. for example, all quest spawn scenarios are just one massive 3950 line switch statement. the indirection i mentioned has caused me a bunch of headaches, and i hit off-by-one errors for some things a few times, before they got pinned down with runtime analysis.

obviously, i needed our version more testable, so some of it was destined to be refactored into composable and testable bits. testing is important, because you can turn runtime captures into fixtures, and prove that your behavior is identical. for example, this is how quest builders look in my version (and spawn templates are another work of art).

@register_quest(
    level="2.5",
    title="Sweep Stakes",
    time_limit_ms=35000,
    start_weapon_id=6,
    unlock_perk_id=PerkId.BARREL_GREASER,
)
def build_2_5_sweep_stakes(ctx: QuestContext, rng: random.Random | None = None) -> list[SpawnEntry]:
    rng = rng or random.Random()
    entries: list[SpawnEntry] = []
    center_x, center_y = center_point(ctx.width, ctx.height)
    trigger = 2000
    step = 2000
    while step > 720:
        angle = random_angle(rng)
        for x, y in radial_points(center_x, center_y, angle, 0x54, 0xFC, 0x2A):
            heading = heading_from_center(x, y, center_x, center_y)
            entries.append(
                spawn(
                    x=x,
                    y=y,
                    heading=heading,
                    spawn_id=SpawnId.ALIEN_AI7_ORBITER_36,
                    trigger_ms=trigger,
                    count=1,
                )
            )
        trigger += max(step, 600)
        step -= 0x50
    return entries