Augmentinel — Internals

Introduction

Augmentinel is re-skinned version of the Geoff Crammond classic: The Sentinel.

It provides the original gameplay experience, enhancing the best features from the different game ports.

Code Execution

The original game code runs as it would in a Spectrum emulator. The lower 16K of the memory map contains the Spectrum ROM image, and the upper 48K contains a snapshot image taken from the game at the main menu. The Z80 code is interpreted by a CPU core in 69888-cycle blocks every 1/50th of a second, to maintain the original game speed. However, unlike a traditional Spectrum emulator, the display map isn’t converted to a screen image and all I/O requests are ignored (reads return 0xff).

To maintain control over the running program we use the same technique as TileMap. After the snapshot is loaded a few code patches are made to advance from the menu into the game, and skip any key press prompts. The bulk of the real work is performed inside code hooks, which are breakpoints set at carefully selected locations in the original Sentinel game code. I spent a couple of months reverse-engineering the original game code to learn how it worked, to determine where should be hooked. 14 hooks are used to track and modify the entire game state – more on those below.

I used the Spectrum version of the game as it’s the one I’m most familiar with, but it could just as easily been the original BBC Micro version instead. The core of all game versions were converted from the original BBC Micro 6502 code to ensure each plays the same. Crammond wrote a 6502 to Z80 converter for the CPC version, which was also used as a starting point for the Spectrum version. There are still small sections of unused CPC code and data to be found in the Spectrum version! The x86 and 68000 code appears to have been given similar treatment, making it easy to find equivalent routines in any version.

To give you some idea of how similar the converted code is, here is the map look-up routine in 6502 (BBC), Z80 (Spectrum), and x86 (PC):

LDA     obj_x
ASL     A
ASL     A
ASL     A
AND     #$E0
ORA     obj_z
TAY
LDA     obj_x
AND     #3
CLC
ADC     #4 ; map_base MSB
STA     map_ptr_msb
LDA     (map_ptr_lsb),Y
CMP     #$C0
RTS
ld      a, (obj_x)
sla     a
sla     a
sla     a
and     0E0h
ld      hl, obj_z
or      (hl)
ld      e, a
ld      a, (obj_x)
and     3
add     a, 61h ; map_base MSB
ld      (map_ptr_msb), a
ld      hl, (map_ptr_lsb)
add     hl, de
ld      a, (hl)
cp      0C0h
ccf
ret
mov     al, obj_x
shl     ax, 1
shl     ax, 1
shl     ax, 1
and     ax, 0E0h
or      al, obj_z
mov     di, ax
mov     ah, obj_x
and     ax, 300h
add     ax, offset map_base
mov     bp, ax
mov     al, [bp+di]
cmp     al, 0C0h
cmc
retn

The code conversion is mostly 1:1 from 6502 to Z80, however the lack of indexed addressing flexibility on the Z80 requires some extra 16-bit arithmetic. The 6502 X and Y registers are held in the Z80 C and E registers, with the B and D registers holding zero for easy indexing use.

There are many other identifiable patterns in the converted code. In the Z80 and x86 example here there is a CCF/cmc instruction to invert the carry flag. This is needed because the 6502 carry flag has the opposite status after compare instructions.

It’d be a fun challenge to rewrite the conversion tool in Python, but I’ve resisted so far.

Resource Extraction

Augmentinel extracts almost all resources it needs directly from the original game data. This includes the vertex/index/face information needed for the game models, which can be loaded almost directly into buffers used by modern 3D hardware. Perhaps the only oddity is that the vertices are stored as polar coordinates rather than cartesian coordinates, but that was probably to simplify model rotation.

All game versions include the BBC Micro bitmap font (upper-case and digits), used for all in-game text. It’s usually drawn offset in different colours to give a 3D appearance. To match that I create extruded character models from the bitmap data, to give real 3D text. The same is done for the symbols used for the energy display, though they are displayed flat using an orthogonal view projection.

Perhaps the only resource embedded directly in Augmentinel is the 16-colour palette, taken from the EGA PC version. Three of the colour entries are dynamic, and change depending on the number of sentries present on the current landscape. The model face colours index into the palette, in some cases referencing the dynamic colours. Augmentinel maintains an equivalent colour look-up table in a shader constant buffer, to retain the same model colour flexibility.

Another area of extraction is the landscape itself, which is stored in a 32x32 block of memory. The landscape is also stored as vertices, to gives a 31x31 landscape area. The original game draws the landscape tile by tile in strips, in back-to-front order so closer sections correctly obscure those further back. Augmentinel creates a landscape model from the data, which can be drawn with a single D3D DrawIndexed call, with all the z-order issues handled by the GPU. Landscape 0000 uses just 4250 vertices and 5766 indices (31 * 31 * 6), which is trivial by modern standards! I’ll leave details of landscape generation and storage for a future article.

The large text used for the title screen (“The Sentinel”) and the 8-digit secret code is actually stored as models in the landscape map. Code converts the bitmap text data into special block models used to create the text, then positions the camera far enough away to view it (with slight pitch and rotation). Augmentinel extracts these placed models for display in the same way, though the modern planar 3D projection means it is missing the slight curve found on the original display.

Scene Lighting

To give an impression of lighting in the original game, sloped edges of landscapes are coloured black or white, depending on whether they’re facing front/back or left/right (respectively). That simple trick is surprisingly effective at giving a convincing 3D landscape, especially once combined with the two-colour checkered pattern of flat tiles.

The limited colour palette (just 4 active colours on the BBC Micro) meant it wasn’t possible to light game models properly. Neighbouring faces with the same colour are drawn with an outline to prevent the faces blending together. You can see this effect on the chest of the robot and face of the sentinel and sentries. The outlining is encoded in the face colours, so it’s not something the drawing code needs to determine itself.

Augmentinel has the luxury of applying simple lighting to the whole scene. It uses a background ambient lighting level, with two diffuse lights positioned at the front-left and back-right of the landscape. The positions and intensities were chosen to give a similar landscape lighting appearance to the original game.

My sloped landscape faces are coloured by mixing grey with the prominent landscape colour, as determined by sentry count, and then applying normal lighting. This gives an overall colour that matches the original landscapes. The back faces of the landscape should appear darker, so they’re given only a fraction of the ambient light level. This makes it clear that you are viewing under the edge of the landscape. The model faces no longer need outlines in Augmentinel, as they are lit depending on their angle to the lights. This gives a cleaner and more natural appearance than the original game.

The VGA PC version also includes some simulated fog to help convey object distance. This is achieved using 16 gradiations of the 16-colour EGA palette, with each step fading slightly more towards the sky colour. It’s used to darken the back of the preview landscape (black sky), and fade distant objects during the main game (blue sky).

Augmentinel applies exponential fog in the vertex shader, using the distance from the camera to each scene vertex. This results in smoother gradients than the VGA version, but can lead to subtle banding artefacts when moving the camera around, due to the change in distance to the view projection plane. The fog is manually reduced on the landscape preview and in sky view mode, as the greater viewing distance would otherwise make the whole scene too foggy.

Game State

Augmentinel follows the underlying game state to determine what it should do next. There are currently 9 internal states: Reset, TitleScreen, LandscapePreview, WrongCode, Game, SkyView, PlayerDead, ShowKiller, Complete.

Transitions between these states occur in the following hook handlers:

struct ISentinelEvents
{
    virtual void OnTitleScreen() = 0;
    virtual void OnLandscapeInput(int &landscape_bcd, uint32_t &secret_code_bcd) = 0;
    virtual void OnSentinelPlaced() = 0;
    virtual void OnLandscapeGenerated() = 0;
    virtual void OnWrongSecretCode() = 0;
    virtual void OnNewPlayerView() = 0;
    virtual void OnPlayerDead() = 0;
    virtual void OnInputAction(uint8_t &action) = 0;
    virtual void OnGameModelChanged(int id, bool player_initiated) = 0;
    virtual bool OnTargetActionTile(InputAction action, int &tile_x, int &tile_z) = 0;
    virtual void OnHideEnergyPanel() = 0;
    virtual void OnAddEnergySymbol(int symbol_idx, int x_offset) = 0;
    virtual void OnPlayTune(int n) = 0;
    virtual void OnSoundEffect(int n, int idx) = 0;
};

Here’s how they’re used in a typical game scenario.

This event-driven logic gives us control where we need it, but lets the game run as normal for most of the time.

Game Difficulty

The game timers control the overall game difficulty level, and they vary between different versions of the game. The sentinel rotates every 10 seconds on the PC, 12 seconds on the BBC/C64, and 15 seconds on the CPC/Spectrum. All player actions have a time cost, putting you closer to the next sentinel rotation, and the risk of being seen. Object creation and absorbtion costs 2 seconds, just like the original 8-bit versions.

The game timers are driven from an interrupt handler, so they run at a constant rate. They also run at all times interrupts are enabled, including when the screen is blue generating a new view after the player has moved! However, any due timers are only acted upon from non-interrupt processing, so you’ll often hear the sentinel turn just as a new view becomes visible and normal game loop resumes.

Augmentinel attempts to maintain the same pacing by keeping the time costs the same. It also exposes manual control of the timer frequency by the way of a difficulty setting. The default rotation rate is 12 seconds like the BBC version, but if you find it too easy you can reduce that to 10 (Hard) or 8 (Very Hard). New players can increase it to 20 (Very Easy) as they get to grips with how to play.

Virtual Reality

The Sentinel always seemed like a VR game ahead of its time, and it’s great to finally feel like you’re in the world. Looking around is so much more satisfying than panning the view with the mouse.

The implementation was relatively straight-forward using the OpenVR API, which requires rendering the scene separately for each eye to give the final stereo view for the headset. It did introduce some complications that I hadn’t considered in the flat version:

Questions? Please get in touch using the link below.