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