Auto invert preview colors based on the screen color

Basically, when the background color of the screen at certain points is dark, the preview at that spot is white, and when it is bright, the preview is white at that spot. This helps to keep the previews always distinguishable, regardless of the background color.

src/Shaders/AutoInvertColors.shader

@@ -0,0 +1,25 @@
+shader_type canvas_item;
+
+
+vec3 rgb2hsb(vec3 c){
+	vec4 K = vec4(0.0, -1.0 / 3.0, 2.0 / 3.0, -1.0);
+	vec4 p = mix(vec4(c.bg, K.wz),
+				vec4(c.gb, K.xy),
+				step(c.b, c.g));
+	vec4 q = mix(vec4(p.xyw, c.r),
+				vec4(c.r, p.yzx),
+				step(p.x, c.r));
+	float d = q.x - min(q.w, q.y);
+	float e = 1.0e-10;
+	return vec3(abs(q.z + (q.w - q.y) / (6.0 * d + e)),
+				d / (q.x + e),
+				q.x);
+}
+
+
+void fragment() {
+	vec3 screen_color = rgb2hsb(textureLod(SCREEN_TEXTURE, SCREEN_UV, 0.0).rgb);
+	if(screen_color.z < 0.5){
+		COLOR.rgb = vec3(1.0) - COLOR.rgb;
+	}
+}

src/Tools/LineTool.gd

@@ -112,7 +112,7 @@ func draw_preview() -> void:
 		canvas.draw_set_transform(start - t_offsetv, canvas.rotation, canvas.scale)
 
 		for line in _create_polylines(indicator):
-			canvas.draw_polyline(PoolVector2Array(line), tool_slot.color)
+			canvas.draw_polyline(PoolVector2Array(line), Color.black)
 
 		canvas.draw_set_transform(canvas.position, canvas.rotation, canvas.scale)
 

src/Tools/ShapeDrawer.gd

@@ -105,7 +105,7 @@ func draw_preview() -> void:
 		canvas.draw_set_transform(rect.position - t_offsetv, canvas.rotation, canvas.scale)
 
 		for line in _create_polylines(indicator):
-			canvas.draw_polyline(PoolVector2Array(line), tool_slot.color)
+			canvas.draw_polyline(PoolVector2Array(line), Color.black)
 
 		canvas.draw_set_transform(canvas.position, canvas.rotation, canvas.scale)
 

src/UI/Canvas/Canvas.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=12 format=2]
+[gd_scene load_steps=14 format=2]
 
 [ext_resource path="res://src/UI/Canvas/Canvas.gd" type="Script" id=1]
 [ext_resource path="res://src/UI/Canvas/Grid.gd" type="Script" id=2]
@@ -9,6 +9,7 @@
 [ext_resource path="res://src/UI/Canvas/Previews.gd" type="Script" id=7]
 [ext_resource path="res://src/UI/Canvas/Selection.gd" type="Script" id=8]
 [ext_resource path="res://src/Shaders/MarchingAntsOutline.shader" type="Shader" id=9]
+[ext_resource path="res://src/Shaders/AutoInvertColors.shader" type="Shader" id=10]
 
 [sub_resource type="CanvasItemMaterial" id=1]
 blend_mode = 4
@@ -22,6 +23,9 @@ shader_param/width = 0.05
 shader_param/frequency = 200.0
 shader_param/stripe_direction = 0.5
 
+[sub_resource type="ShaderMaterial" id=4]
+shader = ExtResource( 10 )
+
 [node name="Canvas" type="Node2D"]
 script = ExtResource( 1 )
 
@@ -58,4 +62,5 @@ centered = false
 script = ExtResource( 3 )
 
 [node name="Previews" type="Node2D" parent="."]
+material = SubResource( 4 )
 script = ExtResource( 7 )