switch to less stuttery timing method

Scenes/Play.tscn

@@ -61,7 +61,7 @@ uv1_scale = Vector3(-1, 1, 1)
 [sub_resource type="ViewportTexture" id="ViewportTexture_w20vk"]
 viewport_path = NodePath("Mask")
 
-[sub_resource type="ShaderMaterial" id="ShaderMaterial_bs53h"]
+[sub_resource type="ShaderMaterial" id="ShaderMaterial_osg3y"]
 resource_local_to_scene = true
 shader = ExtResource("3_rjbyl")
 shader_parameter/mask = SubResource("ViewportTexture_w20vk")
@@ -140,7 +140,7 @@ anchor_mode = 0
 editor_draw_screen = false
 
 [node name="Background" parent="2D Viewport/Viewport Control" instance=ExtResource("2_8g6gv")]
-material = SubResource("ShaderMaterial_bs53h")
+material = SubResource("ShaderMaterial_osg3y")
 layout_mode = 1
 offset_top = -1920.0
 offset_right = 1920.0
@@ -187,8 +187,8 @@ layout_mode = 1
 anchors_preset = 2
 anchor_top = 1.0
 anchor_bottom = 1.0
-offset_top = -1820.0
+offset_top = -1920.0
-offset_bottom = -1820.0
+offset_bottom = -1920.0
 grow_vertical = 0
 
 [node name="ViewportView Left" type="SubViewportContainer" parent="2D Viewport/Viewport Control"]
@@ -232,21 +232,14 @@ position = Vector2(0, -1920)
 offset = Vector2(1921, 1920)
 anchor_mode = 0
 
-[node name="ViewportView" type="SubViewportContainer" parent="."]
-modulate = Color(1, 1, 1, 0.329412)
-anchors_preset = 7
-anchor_left = 0.5
-anchor_top = 1.0
-anchor_right = 0.5
-anchor_bottom = 1.0
-offset_left = -960.0
-offset_top = -1920.0
-offset_right = 960.0
-grow_horizontal = 2
-grow_vertical = 0
-
 [node name="AudioStreamPlayer BGM" type="AudioStreamPlayer" parent="."]
 script = ExtResource("4_c2dke")
 
 [node name="AudioStreamPlayer SFX" type="AudioStreamPlayer" parent="."]
 script = ExtResource("5_owrd5")
+
+[node name="Dev Area" type="Control" parent="."]
+layout_mode = 3
+anchors_preset = 0
+offset_right = 1920.0
+offset_bottom = 1920.0

Scripts/Scenes/Play/Audio/BGM.cs

@@ -4,6 +4,19 @@ using System;
 
 public partial class BGM : AudioStreamPlayer
 {
+	// latency compensation
+	private ulong timeStartUsec;
+	private float timeDelay;
+	
+	public float CurTime
+	{
+		get
+		{
+			float time = (Time.GetTicksUsec() - timeStartUsec) / 1000000f;
+			return time - timeDelay;
+		}
+	}
+
 	public void LoadFromUser(string path)
 	{
 		if (!path.StartsWith("user://"))
@@ -43,4 +56,11 @@ public partial class BGM : AudioStreamPlayer
 				break;
 		}
 	}
+
+	public void Play()
+	{
+		timeStartUsec = Time.GetTicksUsec();
+		timeDelay = (float) (AudioServer.GetTimeToNextMix() + AudioServer.GetOutputLatency());
+		base.Play();
+	}
 }

Scripts/Scenes/Play/Play.cs

@@ -124,7 +124,7 @@ namespace WacK.Scenes
 			double time = bgmController.GetPlaybackPosition() + AudioServer.GetTimeSinceLastMix() - AudioServer.GetOutputLatency();
 			
 			var nPos = noteDisplay.Position;
-			nPos.Y = ((float)time * scrollPxPerSec) + 1920;
+			nPos.Y = (bgmController.CurTime * scrollPxPerSec) + 1910;
 			noteDisplay.Position = nPos;
 			scrollDisplay.Position = nPos;
         }