game: add shader to indicate low hp

scenes/managers/ui/main_ui.gd

@@ -5,9 +5,35 @@ extends Control
 @onready var player_ui: PlayerUI = $CanvasLayer/PlayerUI
 @onready var debug_ui: DebugUI = $CanvasLayer/DebugUI
 @onready var level_up_ui: LevelUpUI = $CanvasLayer/LevelUpUI
+@onready var low_hp_indicator: ColorRect = $CanvasLayer/LowHPIndicator
+
+var elapsed_time: float
 
 
 func _ready() -> void:
 	pause_ui.visible = false
 	player_ui.visible = true
 	level_up_ui.visible = false
+	low_hp_indicator.visible = false
+	GlobalConst.sig_on_player_hp_change.connect(_on_player_hp_change)
+	GlobalConst.sig_toggle_low_hp_shader.connect(_on_toggle_low_hp_shader)
+
+
+func _process(delta: float) -> void:
+	elapsed_time += delta
+	if low_hp_indicator.visible:
+		low_hp_indicator.material.set_shader_parameter("time", elapsed_time)
+
+
+func _on_player_hp_change(hp: float, max_hp: float):
+	print_debug("got sig")
+	var percent_hp = hp / max_hp * 100
+	if percent_hp < 40:
+		low_hp_indicator.visible = true
+		low_hp_indicator.material.set_shader_parameter("damage_amount", 1 - (percent_hp / 100))
+	else:
+		low_hp_indicator.visible = false
+
+
+func _on_toggle_low_hp_shader(toggled_on: bool):
+	low_hp_indicator.visible = !low_hp_indicator.visible