very starty setup

root.gd

@@ -0,0 +1,30 @@
+extends Node2D
+
+@onready var sim_viewport := $SubViewport
+@onready var display_sprite := $Sprite2D
+
+var tex_a: Texture2D
+var tex_b: Texture2D
+var swap = false
+
+func _ready():
+	# Create textures
+	tex_a = ImageTexture.create_from_image(
+		Image.create(sim_viewport.size.x, sim_viewport.size.y, false, Image.FORMAT_RGBAF)
+	)
+	tex_b = tex_a.duplicate()
+	
+	# Assign one texture to the simulation shader as input
+	$SubViewport/ColorRect.material.set("shader_parameter/trail_tex", tex_a)
+	display_sprite.texture = tex_a
+
+func _process(_delta):
+	# Swap textures each frame
+	swap = !swap
+	var src = tex_a if swap else tex_b
+	var dst = tex_b if swap else tex_a
+	
+	# Assign src as input and dst as render target
+	$SubViewport/ColorRect.material.set("shader_parameter/trail_tex", src)
+	sim_viewport.render_target_update_mode = SubViewport.UPDATE_ALWAYS
+	display_sprite.texture = dst

root.gd.uid

@@ -0,0 +1 @@
+uid://d1bbllvcal2uw

root.gdshader

@@ -1,5 +1,7 @@
 shader_type canvas_item;
 
+uniform sampler2D trail_tex;
+
 uniform float sensor_distance = 2.0;
 uniform float sensor_angle = 15.0;
 uniform float rotation_angle = 23.0;
@@ -7,10 +9,11 @@ uniform float move_distance = 2.67;
 uniform float decay_factor = 0.75;
 
 void fragment() {
-	COLOR = vec4(0.05, 0.05, 0.05, 1.0);
+    vec2 uv = UV;
-}
+    // Read current trail value
-
+    vec4 trail = texture(trail_tex, uv);
-//void light() {
+    // Apply decay
-//	// Called for every pixel for every light affecting the CanvasItem.
+    trail *= decay_factor;
-//	// Uncomment to replace the default light processing function with this one.
+    
-//}
+    COLOR = trail;
+}

root.tscn

@@ -1,6 +1,15 @@
-[gd_scene load_steps=4 format=3 uid="uid://bf4ohdg0sfl8l"]
+[gd_scene load_steps=5 format=3 uid="uid://bf4ohdg0sfl8l"]
 
 [ext_resource type="Shader" uid="uid://b3wu436nky225" path="res://root.gdshader" id="1_pq8q7"]
+[ext_resource type="Script" uid="uid://d1bbllvcal2uw" path="res://root.gd" id="1_pyidc"]
+
+[sub_resource type="ShaderMaterial" id="ShaderMaterial_pq8q7"]
+shader = ExtResource("1_pq8q7")
+shader_parameter/sensor_distance = 2.0
+shader_parameter/sensor_angle = 15.0
+shader_parameter/rotation_angle = 23.0
+shader_parameter/move_distance = 2.67
+shader_parameter/decay_factor = 0.75
 
 [sub_resource type="ShaderMaterial" id="ShaderMaterial_pyidc"]
 shader = ExtResource("1_pq8q7")
@@ -10,12 +19,15 @@ shader_parameter/rotation_angle = 23.0
 shader_parameter/move_distance = 2.67
 shader_parameter/decay_factor = 0.75
 
-[sub_resource type="QuadMesh" id="QuadMesh_vvh5c"]
-material = SubResource("ShaderMaterial_pyidc")
-size = Vector2(1152, 648)
-
 [node name="Node2D" type="Node2D"]
+script = ExtResource("1_pyidc")
 
-[node name="MeshInstance2D" type="MeshInstance2D" parent="."]
+[node name="Sprite2D" type="Sprite2D" parent="."]
-position = Vector2(576, 324)
+material = SubResource("ShaderMaterial_pq8q7")
-mesh = SubResource("QuadMesh_vvh5c")
+
+[node name="SubViewport" type="SubViewport" parent="."]
+
+[node name="ColorRect" type="ColorRect" parent="SubViewport"]
+material = SubResource("ShaderMaterial_pyidc")
+offset_right = 40.0
+offset_bottom = 40.0