Adapt: 2D to 3D

Expert guidance for migrating 2D games into the third dimension.

NEVER Do

NEVER directly replace Vector2 with Vector3(x, y, 0) — This creates a "flat 3D" game with no depth gameplay. Add Z-axis movement or camera rotation to justify 3D.
NEVER keep 2D collision layers — 2D and 3D physics use separate layer systems. You must reconfigure collision_layer/collision_mask for 3D nodes.
NEVER forget to add lighting — 3D without lights is pitch black (unless using unlit materials). Add at least one DirectionalLight3D.
NEVER use Camera2D follow logic in 3D — Camera3D needs spring arm or look-at logic. Direct position copying causes clipping and disorientation.
NEVER assume same performance — 3D is 5-10x more demanding. Budget for lower draw calls, smaller viewport resolution on mobile.
NEVER use the rotation property for complex 3D logic — 3D rotation uses Euler angles. Interpolating Euler angles causes unpredictable paths and Gimbal Lock. Always use Quaternion for 3D rotation interpolation or the Basis matrix for directional vectors.
NEVER ignore metric scaling — 3D physics and lighting assume 1 unit = 1 meter. Scaling models inside the engine introduces precision errors. Export assets from DCCs at the correct metric scale.
NEVER disable physics interpolation when using custom camera follow scripts — Updating camera position in _process to follow a body moving in _physics_process causes jitter. Use Node3D.get_global_transform_interpolated() for smooth transforms.