Initial implementation of LowHealthLayer

osu.Game/Configuration/OsuConfigManager.cs

@@ -88,6 +88,7 @@ namespace osu.Game.Configuration
             Set(OsuSetting.ShowInterface, true);
             Set(OsuSetting.ShowProgressGraph, true);
             Set(OsuSetting.ShowHealthDisplayWhenCantFail, true);
+            Set(OsuSetting.FadePlayfieldWhenLowHealth, true);
             Set(OsuSetting.KeyOverlay, false);
             Set(OsuSetting.ScoreMeter, ScoreMeterType.HitErrorBoth);
 
@@ -183,6 +184,7 @@ namespace osu.Game.Configuration
         ShowInterface,
         ShowProgressGraph,
         ShowHealthDisplayWhenCantFail,
+        FadePlayfieldWhenLowHealth,
         MouseDisableButtons,
         MouseDisableWheel,
         AudioOffset,

osu.Game/Overlays/Settings/Sections/Gameplay/GeneralSettings.cs

@@ -53,6 +53,12 @@ namespace osu.Game.Overlays.Settings.Sections.Gameplay
                     Keywords = new[] { "hp", "bar" }
                 },
                 new SettingsCheckbox
+                {
+                    LabelText = "Fade playfield to red when health is low",
+                    Bindable = config.GetBindable<bool>(OsuSetting.FadePlayfieldWhenLowHealth),
+                    Keywords = new[] { "hp", "playfield", "health" }
+                },
+                new SettingsCheckbox
                 {
                     LabelText = "Always show key overlay",
                     Bindable = config.GetBindable<bool>(OsuSetting.KeyOverlay)

osu.Game/Screens/Play/HUD/LowHealthLayer.cs

@@ -0,0 +1,47 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Allocation;
+using osu.Framework.Bindables;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Shapes;
+using osu.Game.Configuration;
+using osu.Game.Graphics;
+
+namespace osu.Game.Screens.Play.HUD
+{
+    public class LowHealthLayer : HealthDisplay
+    {
+        private const float max_alpha = 0.4f;
+
+        private const double fade_time = 300;
+
+        private readonly Box box;
+
+        private Bindable<bool> configFadeRedWhenLowHealth;
+
+        public LowHealthLayer()
+        {
+            Child = box = new Box
+            {
+                RelativeSizeAxes = Axes.Both,
+                Alpha = 0
+            };
+        }
+
+        [BackgroundDependencyLoader]
+        private void load(OsuConfigManager config, OsuColour color)
+        {
+            configFadeRedWhenLowHealth = config.GetBindable<bool>(OsuSetting.FadePlayfieldWhenLowHealth);
+            box.Colour = color.Red;
+
+            configFadeRedWhenLowHealth.BindValueChanged(value =>
+            {
+                if (value.NewValue)
+                    this.FadeIn(fade_time, Easing.OutQuint);
+                else
+                    this.FadeOut(fade_time, Easing.OutQuint);
+            }, true);
+        }
+    }
+}