FPS Demo — James Liu

01 · Goal

Simple rules become a recognizable weapon feel.

A Unity systems study connecting shot dispersion, movement momentum, and accuracy to weapon feel.

Implementation details

The core loop of an FPS is familiar, so identity comes from the implementation details: how shots disperse, how quickly movement settles, and how accuracy responds to player intent.

02 · Bullet spread

A normal distribution creates natural shot dispersion.

Box–Muller sampling creates a tunable spread pattern with shots concentrated near the center.

Bullet directionAim stays intentional. Spread adds variation.

AimCamera ray → targetDirection from the bullet spawn

RandomnessBox–Muller offsetsNormally distributed X / Y values

TuningSpread intensityScale the random deviation

COMBINE

ShotFinal directionAim direction + scaled offsets

Technical details & source excerpt

The weapon uses the Box–Muller transform to generate horizontal and vertical offsets from a bivariate normal distribution. Most shots cluster near the center while a smaller number fall farther away, producing a more believable pattern than uniform randomness.

Raycasts establish the camera's intended target point
Spread intensity scales the generated deviation
The distribution remains centered and tunable

Centered bullet spreadWeaponController.cs · C#

Ray ray = Camera.main.ViewportPointToRay(
    new Vector3(0.5f, 0.5f, 0f)
);
Vector3 targetPoint = Physics.Raycast(ray, out RaycastHit hit)
    ? hit.point
    : ray.GetPoint(100f);

Vector3 direction =
    (targetPoint - bulletSpawn.position).normalized;

float u1 = 1f - UnityEngine.Random.value;
float u2 = 1f - UnityEngine.Random.value;
float normalX = Mathf.Sqrt(-2f * Mathf.Log(u1))
    * Mathf.Sin(2f * Mathf.PI * u2);
float normalY = Mathf.Sqrt(-2f * Mathf.Log(u1))
    * Mathf.Cos(2f * Mathf.PI * u2);

return direction + new Vector3(
    normalX * spreadIntensity,
    normalY * spreadIntensity,
    0f
);

The Box–Muller transform concentrates most shots near the reticle while still allowing occasional wider deviation—a more natural pattern than uniform offsets.

03 · Movement

Precision requires deliberate counter-strafing.

Counter-strafing makes precise shots depend on deliberate movement control.

Movement & accuracyControl momentum to recover precision

MoveBuild momentumAccuracy responds to movement

Counter-strafeApply opposing inputBring movement to a stop

FireShoot while stationaryHighest precision

Implementation details

Movement preserves enough momentum that stopping accurately requires opposing input. Shooting is most precise when stationary, encouraging players to actively balance mobility, timing, and aim instead of treating movement and accuracy as separate systems.