A Vehicle Look-Back Camera for Halo PC

Halo PC has no dedicated control for checking directly behind a vehicle. You can turn the camera manually, but vehicle steering is tied to the direction the player is looking. During a race, moving the mouse or controller to look behind you also makes the vehicle steer that way, potentially pulling you off your line or sending you off the track.

I wanted a held control that would reverse only the vehicle camera, without changing the player's look direction, steering input, or vehicle physics. Melee was the first candidate because it has little use in the driver's seat, but the state I found behaved like a toggle rather than a reliable held input.

Reload became the practical alternative. Holding it now gives the driver a rear-facing view, and releasing it restores whichever vehicle camera was active before.

Finding a control that stayed held

The first implementation treated a melee-related value on the player as the physical button state. Runtime testing showed that it latched instead: pressing melee changed the value, but holding and releasing it did not produce the continuous state the camera needed.

Changing the edge handling did not alter that underlying behaviour. The biped and vehicle unit-control fields exposed action state, not the raw hold state I was trying to read.

I returned to Halo PC's processed controls structure. Chimera already contains a byte signature for locating it in the retail client, so the Lua script uses the same pattern instead of relying on a fixed absolute address.

Reload is available at offset +0x0D:

local CONTROLS_RELOAD = 0x0D

local reload_held =
    read_u8(controls_address + CONTROLS_RELOAD) ~= 0

The script samples that byte during preframe. It can distinguish press, hold, and release without modifying the player's look direction or any steering data.

Checking that the player is driving

Reload should not reverse the camera while the player is on foot or sitting in another vehicle seat.

The script reads the biped's parent object identifier at +0x11C, resolves that object, and requires object type 1 at +0xB4. It then reads the vehicle's driver identifier at +0x324 and checks that it resolves to the local biped.

biped + 0x11C   parent object id
parent + 0xB4   object type
vehicle + 0x324 driver object id

Only the local driver reaches the look-back transform. Passenger and gunner cameras remain unchanged.

Rotating the complete camera frame

Chimera's precamera callback supplies the camera position, forward vector, and up vector. I rotate all three by 180 degrees around the vehicle's up axis at +0x80.

The position is rotated relative to the vehicle origin at +0x5C. The forward and up vectors are rotated directly.

Reversing only the forward vector was not enough. Keeping the camera position and both orientation vectors together avoids a view that feels inverted or detached from the vehicle.

Using the vehicle's up axis also keeps the camera aligned while the vehicle is climbing, rolling, or travelling through the air.

A Chimera callback mismatch

The first callback registration included an explicit priority:

set_callback("precamera", "on_precamera", "after")

Chimera rejected it with a wrong-number-of-arguments error. The implementation contains code for parsing a third priority argument, but the preceding argument-count check accepts only one or two arguments in the tested version.

The supported registration worked:

set_callback("precamera", "on_precamera")

The callback failure was separate from the camera calculations.

Making it work with the first-person camera

The original third-person transform rotated the camera position around the vehicle. Once the separate first-person script moved that camera to the driver's head, the same orbit carried it across the cabin towards the passenger side.

The compatibility branch compares the incoming camera with the stock cyborg head position at +0x7E8. A distance below 0.35 world units identifies the head-positioned view.

For that view, look-back keeps the camera near the driver, moves it another 0.22 units forward to place it ahead of the visor, and rotates the forward and up vectors by 180 degrees. The ordinary third-person camera retains its original position orbit.

head distance threshold  0.35
first-person offset    0.22 forward

The tested script order applies first-person before look-back, so reload produces the same deliberate rear view in either mode.

The remaining collision problem

The reversed camera can clip through level geometry and reveal the other side of nearby walls.

Halo PC performs its normal vehicle-camera obstruction check before the Lua precamera callback. The script then moves the already-resolved camera after collision has been calculated, so the transformed position never passes through the native trace.

A collision-safe version would need to invoke or reproduce Halo PC's camera obstruction test at the new position. That remains outside this prototype.

Small delivery checks

Entering or leaving a vehicle while reload is held returns safely to the applicable camera state. Passenger and gunner behaviour stays unchanged, while death was only tested informally because the transition completes quickly.

Retail Halo PC vehicles cannot be destroyed through normal gameplay, so vehicle destruction was not part of the test matrix.

The finished rear view

Holding the configured reload input now gives the driver a rear-facing view from either Halo PC's stock third-person vehicle camera or the experimental first-person camera. Because the script transforms only the final camera frame, the player's actual look direction remains unchanged, so the vehicle continues steering exactly as it did before the rear view was activated. Releasing reload restores the normal view immediately.

The remaining compromise is camera collision. Solving it requires more than rotating the final camera frame in Lua, but the look-back behaviour itself is complete and usable.

Known issues

One known issue is interaction with the custom nametag script. Nametag positions are calculated using a world-to-screen projection before the rear-view camera transform is applied. Because the camera is reversed later in the frame, the scene renders facing backwards while the nametags retain coordinates calculated for the original forward-facing view.

As a result, nametags may appear in incorrect screen positions while the rear-view camera is active.

The current workaround is to rename the nametag script to `zz_nametags.lua`, forcing it to load later so its projection runs after the look-back camera script. This is only a temporary ordering fix until Chimera's callback priority argument is corrected and scripts can explicitly control callback order.