Set up framework for more advance pickup behaviours.

resources/pickups/pickup_behavior.gd

@@ -0,0 +1,43 @@
+class_name PickupBehavior extends Resource
+
+## Base resource that defines how a pickup moves and looks on screen.
+## Assign to [member pickup_behavior] on a PickupBehaviorController.
+
+@export_group("Animation")
+## Whether the pickup bobs up and down in place.
+@export var bob_enabled: bool = true
+## Bob oscillation frequency in cycles per second.
+@export var bob_frequency: float = 2.0
+## Bob vertical amplitude in pixels (peak-to-center).
+@export var bob_amplitude: float = 4.0
+
+## Whether the pickup subtly shakes (jitter) in place.
+@export var shake_enabled: bool = false
+## Shake frequency in Hz (how fast the jitter oscillates).
+@export var shake_frequency: float = 8.0
+## Shake amplitude in pixels (peak-to-center).
+@export var shake_amplitude: float = 1.5
+
+## Optional fade-in duration (0 = no fade).
+@export var fade_in_duration: float = 0.0
+## Optional fade-out duration when the pickup is about to be collected
+## (used by controller; 0 = no fade).
+@export var fade_out_duration: float = 0.0
+
+var _elapsed: float = 0.0
+
+
+func start() -> void:
+    _elapsed = 0.0
+
+
+func process(delta: float) -> float:
+    # Returns the current opacity factor (0..1) based on fade timers.
+    _elapsed += delta
+
+    var target_alpha: float = 1.0
+
+    if fade_in_duration > 0 and _elapsed < fade_in_duration:
+        target_alpha = clampf(_elapsed / fade_in_duration, 0.0, 1.0)
+
+    return target_alpha