Update the background according to the currently selected beatmap and blur it within song select.

osu.Game/Screens/Backgrounds/BackgroundModeBeatmap.cs

@@ -0,0 +1,76 @@
+//Copyright (c) 2007-2016 ppy Pty Ltd <contact@ppy.sh>.
+//Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using OpenTK;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics.Transformations;
+using osu.Game.Beatmaps;
+using osu.Game.Graphics.Background;
+
+namespace osu.Game.Screens.Backgrounds
+{
+    public class BackgroundModeBeatmap : BackgroundMode
+    {
+        private Background background;
+
+        private WorkingBeatmap beatmap;
+
+        public WorkingBeatmap Beatmap
+        {
+            get
+            {
+                return beatmap;
+            }
+
+            set
+            {
+                if (beatmap == value)
+                    return;
+
+                beatmap = value;
+
+                Background oldBackground = background;
+
+                addBackground(background = new Background());
+                background.Sprite.Texture = beatmap.Background;
+
+                if (oldBackground != null)
+                {
+                    oldBackground.Depth = 1;
+                    oldBackground.Flush();
+                    oldBackground.FadeOut(500);
+                    oldBackground.Expire();
+
+                    background.BlurSigma = oldBackground.BlurSigma;
+                }
+            }
+        }
+
+        public BackgroundModeBeatmap(WorkingBeatmap beatmap)
+        {
+            Beatmap = beatmap;
+        }
+
+        private void addBackground(Background background)
+        {
+            background.CacheDrawnFrameBuffer = true;
+            Add(background);
+        }
+
+        public void BlurTo(Vector2 sigma, double duration)
+        {
+            background?.BlurTo(sigma, duration, EasingTypes.OutExpo);
+        }
+
+        protected override void Update()
+        {
+            base.Update();
+        }
+
+        public override bool Equals(BackgroundMode other)
+        {
+            return base.Equals(other) && beatmap == ((BackgroundModeBeatmap)other).Beatmap;
+        }
+    }
+}