Added easing functions for all-around better animations.

These are Robert Penner's easing functions (http://robertpenner.com/easing/), refactored by CharlotteGore to only take a t parameter (https://github.com/CharlotteGore/functional-easing). Licensed under BSD (the former) and MIT (the latter).

Related changes:
- Added "AnimatedValue" utility class for updating values used in animations.
- MenuButton now uses AnimatedValue to handle its animations (still linear by default).
- Added in-out-back easings on logo, mods, and various other elements; added out-bounce easings on button menu.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>

src/itdelatrisu/opsu/GameMod.java

@@ -19,6 +19,7 @@
 package itdelatrisu.opsu;
 
 import itdelatrisu.opsu.ui.MenuButton;
+import itdelatrisu.opsu.ui.animations.AnimationEquation;
 
 import java.util.Arrays;
 import java.util.Collections;
@@ -199,6 +200,8 @@ public enum GameMod {
 			mod.button = new MenuButton(img,
 					baseX + (offsetX * mod.categoryIndex) + img.getWidth() / 2f,
 					mod.category.getY());
+			mod.button.setHoverAnimationDuration(300);
+			mod.button.setHoverAnimationEquation(AnimationEquation.IN_OUT_BACK);
 			mod.button.setHoverExpand(1.2f);
 			mod.button.setHoverRotate(10f);
 

src/itdelatrisu/opsu/Utils.java

@@ -239,6 +239,22 @@ public class Utils {
 		return val;
 	}
 
+	/**
+	 * Clamps a value between a lower and upper bound.
+	 * @param val the value to clamp
+	 * @param low the lower bound
+	 * @param high the upper bound
+	 * @return the clamped value
+	 * @author fluddokt
+	 */
+	public static int clamp(int val, int low, int high) {
+		if (val < low)
+			return low;
+		if (val > high)
+			return high;
+		return val;
+	}
+
 	/**
 	 * Clamps a value between a lower and upper bound.
 	 * @param val the value to clamp

src/itdelatrisu/opsu/states/ButtonMenu.java

@@ -31,6 +31,8 @@ import itdelatrisu.opsu.beatmap.BeatmapSetList;
 import itdelatrisu.opsu.beatmap.BeatmapSetNode;
 import itdelatrisu.opsu.ui.MenuButton;
 import itdelatrisu.opsu.ui.UI;
+import itdelatrisu.opsu.ui.animations.AnimatedValue;
+import itdelatrisu.opsu.ui.animations.AnimationEquation;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -261,8 +263,11 @@ public class ButtonMenu extends BasicGameState {
 		/** The actual title string list, generated upon entering the state. */
 		private List<String> actualTitle;
 
+		/** The horizontal center offset, used for the initial button animation. */
+		private AnimatedValue centerOffset;
+
 		/** Initial x coordinate offsets left/right of center (for shifting animation), times width. (TODO) */
-		private static final float OFFSET_WIDTH_RATIO = 1 / 18f;
+		private static final float OFFSET_WIDTH_RATIO = 1 / 25f;
 
 		/**
 		 * Constructor.
@@ -336,18 +341,14 @@ public class ButtonMenu extends BasicGameState {
 		 */
 		public void update(GameContainer container, int delta, int mouseX, int mouseY) {
 			float center = container.getWidth() / 2f;
+			boolean centerOffsetUpdated = centerOffset.update(delta);
+			float centerOffsetX = centerOffset.getValue();
 			for (int i = 0; i < buttons.length; i++) {
 				menuButtons[i].hoverUpdate(delta, mouseX, mouseY);
 
 				// move button to center
-				float x = menuButtons[i].getX();
+				if (centerOffsetUpdated)
-				if (i % 2 == 0) {
+					menuButtons[i].setX((i % 2 == 0) ? center + centerOffsetX : center - centerOffsetX);
-					if (x < center)
-						menuButtons[i].setX(Math.min(x + (delta / 5f), center));
-				} else {
-					if (x > center)
-						menuButtons[i].setX(Math.max(x - (delta / 5f), center));
-				}
 			}
 		}
 
@@ -404,9 +405,10 @@ public class ButtonMenu extends BasicGameState {
 		 */
 		public void enter(GameContainer container, StateBasedGame game) {
 			float center = container.getWidth() / 2f;
-			float centerOffset = container.getWidth() * OFFSET_WIDTH_RATIO;
+			float centerOffsetX = container.getWidth() * OFFSET_WIDTH_RATIO;
+			centerOffset = new AnimatedValue(700, centerOffsetX, 0, AnimationEquation.OUT_BOUNCE);
 			for (int i = 0; i < buttons.length; i++) {
-				menuButtons[i].setX(center + ((i % 2 == 0) ? centerOffset * -1 : centerOffset));
+				menuButtons[i].setX(center + ((i % 2 == 0) ? centerOffsetX : centerOffsetX * -1));
 				menuButtons[i].resetHover();
 			}
 

src/itdelatrisu/opsu/states/Game.java

@@ -48,6 +48,7 @@ import itdelatrisu.opsu.replay.Replay;
 import itdelatrisu.opsu.replay.ReplayFrame;
 import itdelatrisu.opsu.ui.MenuButton;
 import itdelatrisu.opsu.ui.UI;
+import itdelatrisu.opsu.ui.animations.AnimationEquation;
 
 import java.io.File;
 import java.util.LinkedList;
@@ -1425,6 +1426,8 @@ public class Game extends BasicGameState {
 			Image skip = GameImage.SKIP.getImage();
 			skipButton = new MenuButton(skip, width - skip.getWidth() / 2f, height - (skip.getHeight() / 2f));
 		}
+		skipButton.setHoverAnimationDuration(350);
+		skipButton.setHoverAnimationEquation(AnimationEquation.IN_OUT_BACK);
 		skipButton.setHoverExpand(1.1f, MenuButton.Expand.UP_LEFT);
 
 		// load other images...

src/itdelatrisu/opsu/states/GamePauseMenu.java

@@ -27,6 +27,7 @@ import itdelatrisu.opsu.audio.SoundController;
 import itdelatrisu.opsu.audio.SoundEffect;
 import itdelatrisu.opsu.ui.MenuButton;
 import itdelatrisu.opsu.ui.UI;
+import itdelatrisu.opsu.ui.animations.AnimationEquation;
 
 import org.lwjgl.input.Keyboard;
 import org.newdawn.slick.Color;
@@ -227,6 +228,14 @@ public class GamePauseMenu extends BasicGameState {
 		continueButton = new MenuButton(GameImage.PAUSE_CONTINUE.getImage(), width / 2f, height * 0.25f);
 		retryButton = new MenuButton(GameImage.PAUSE_RETRY.getImage(), width / 2f, height * 0.5f);
 		backButton = new MenuButton(GameImage.PAUSE_BACK.getImage(), width / 2f, height * 0.75f);
+		final int buttonAnimationDuration = 300;
+		continueButton.setHoverAnimationDuration(buttonAnimationDuration);
+		retryButton.setHoverAnimationDuration(buttonAnimationDuration);
+		backButton.setHoverAnimationDuration(buttonAnimationDuration);
+		final AnimationEquation buttonAnimationEquation = AnimationEquation.IN_OUT_BACK;
+		continueButton.setHoverAnimationEquation(buttonAnimationEquation);
+		retryButton.setHoverAnimationEquation(buttonAnimationEquation);
+		backButton.setHoverAnimationEquation(buttonAnimationEquation);
 		continueButton.setHoverExpand();
 		retryButton.setHoverExpand();
 		backButton.setHoverExpand();

src/itdelatrisu/opsu/states/MainMenu.java

@@ -33,6 +33,7 @@ import itdelatrisu.opsu.downloads.Updater;
 import itdelatrisu.opsu.states.ButtonMenu.MenuState;
 import itdelatrisu.opsu.ui.MenuButton;
 import itdelatrisu.opsu.ui.MenuButton.Expand;
+import itdelatrisu.opsu.ui.animations.AnimationEquation;
 import itdelatrisu.opsu.ui.UI;
 
 import java.awt.Desktop;
@@ -145,9 +146,18 @@ public class MainMenu extends BasicGameState {
 		exitButton = new MenuButton(exitImg,
 				width * 0.75f - exitOffset, (height / 2) + (exitImg.getHeight() / 2f)
 		);
-		logo.setHoverExpand(1.05f);
+		final int logoAnimationDuration = 350;
-		playButton.setHoverExpand(1.05f);
+		logo.setHoverAnimationDuration(logoAnimationDuration);
-		exitButton.setHoverExpand(1.05f);
+		playButton.setHoverAnimationDuration(logoAnimationDuration);
+		exitButton.setHoverAnimationDuration(logoAnimationDuration);
+		final AnimationEquation logoAnimationEquation = AnimationEquation.IN_OUT_BACK;
+		logo.setHoverAnimationEquation(logoAnimationEquation);
+		playButton.setHoverAnimationEquation(logoAnimationEquation);
+		exitButton.setHoverAnimationEquation(logoAnimationEquation);
+		final float logoHoverScale = 1.1f;
+		logo.setHoverExpand(logoHoverScale);
+		playButton.setHoverExpand(logoHoverScale);
+		exitButton.setHoverExpand(logoHoverScale);
 
 		// initialize music buttons
 		int musicWidth  = GameImage.MUSIC_PLAY.getImage().getWidth();
@@ -170,6 +180,8 @@ public class MainMenu extends BasicGameState {
 		// initialize downloads button
 		Image dlImg = GameImage.DOWNLOADS.getImage();
 		downloadsButton = new MenuButton(dlImg, width - dlImg.getWidth() / 2f, height / 2f);
+		downloadsButton.setHoverAnimationDuration(350);
+		downloadsButton.setHoverAnimationEquation(AnimationEquation.IN_OUT_BACK);
 		downloadsButton.setHoverExpand(1.03f, Expand.LEFT);
 
 		// initialize repository button
@@ -179,6 +191,8 @@ public class MainMenu extends BasicGameState {
 			repoButton = new MenuButton(repoImg,
 					startX - repoImg.getWidth(), startY - repoImg.getHeight()
 			);
+			repoButton.setHoverAnimationDuration(350);
+			repoButton.setHoverAnimationEquation(AnimationEquation.IN_OUT_BACK);
 			repoButton.setHoverExpand();
 			startX -= repoImg.getWidth() * 1.75f;
 		} else

src/itdelatrisu/opsu/ui/MenuButton.java

@@ -18,7 +18,8 @@
 
 package itdelatrisu.opsu.ui;
 
-import itdelatrisu.opsu.Utils;
+import itdelatrisu.opsu.ui.animations.AnimatedValue;
+import itdelatrisu.opsu.ui.animations.AnimationEquation;
 
 import org.newdawn.slick.Animation;
 import org.newdawn.slick.Color;
@@ -63,11 +64,23 @@ public class MenuButton {
 	/** The hover actions for this button. */
 	private int hoverEffect = 0;
 
-	/** The current and max scale of the button. */
+	/** The hover animation duration, in milliseconds. */
-	private float scale = 1f, hoverScale = 1.25f;
+	private int animationDuration = 100;
 
-	/** The current and base alpha level of the button. */
+	/** The hover animation equation. */
-	private float alpha = 1f, baseAlpha = 0.75f;
+	private AnimationEquation animationEqn = AnimationEquation.LINEAR;
+
+	/** The scale of the button. */
+	private AnimatedValue scale;
+
+	/** The default max scale of the button. */
+	private static final float DEFAULT_SCALE_MAX = 1.25f;
+
+	/** The alpha level of the button. */
+	private AnimatedValue alpha;
+
+	/** The default base alpha level of the button. */
+	private static final float DEFAULT_ALPHA_BASE = 0.75f;
 
 	/** The scaled expansion direction for the button. */
 	private Expand dir = Expand.CENTER;
@@ -75,8 +88,11 @@ public class MenuButton {
 	/** Scaled expansion directions. */
 	public enum Expand { CENTER, UP, RIGHT, LEFT, DOWN, UP_RIGHT, UP_LEFT, DOWN_RIGHT, DOWN_LEFT; }
 
-	/** The current and max rotation angles  of the button. */
+	/** The rotation angle of the button. */
-	private float angle = 0f, maxAngle = 30f;
+	private AnimatedValue angle;
+
+	/** The default max rotation angle of the button. */
+	private static final float DEFAULT_ANGLE_MAX = 30f;
 
 	/**
 	 * Creates a new button from an Image.
@@ -192,15 +208,15 @@ public class MenuButton {
 				float oldAlpha = image.getAlpha();
 				float oldAngle = image.getRotation();
 				if ((hoverEffect & EFFECT_EXPAND) > 0) {
-					if (scale != 1f) {
+					if (scale.getValue() != 1f) {
-						image = image.getScaledCopy(scale);
+						image = image.getScaledCopy(scale.getValue());
 						image.setAlpha(oldAlpha);
 					}
 				}
 				if ((hoverEffect & EFFECT_FADE) > 0)
-					image.setAlpha(alpha);
+					image.setAlpha(alpha.getValue());
 				if ((hoverEffect & EFFECT_ROTATE) > 0)
-					image.setRotation(angle);
+					image.setRotation(angle.getValue());
 				image.draw(x - xRadius, y - yRadius, filter);
 				if (image == this.img) {
 					image.setAlpha(oldAlpha);
@@ -217,9 +233,10 @@ public class MenuButton {
 				imgR.draw(x + xRadius - imgR.getWidth(), y - yRadius, filter);
 			} else if ((hoverEffect & EFFECT_FADE) > 0) {
 				float a = image.getAlpha(), aL = imgL.getAlpha(), aR = imgR.getAlpha();
-				image.setAlpha(alpha);
+				float currentAlpha = alpha.getValue();
-				imgL.setAlpha(alpha);
+				image.setAlpha(currentAlpha);
-				imgR.setAlpha(alpha);
+				imgL.setAlpha(currentAlpha);
+				imgR.setAlpha(currentAlpha);
 				image.draw(x - xRadius + imgL.getWidth(), y - yRadius, filter);
 				imgL.draw(x - xRadius, y - yRadius, filter);
 				imgR.draw(x + xRadius - imgR.getWidth(), y - yRadius, filter);
@@ -267,28 +284,61 @@ public class MenuButton {
 	 */
 	public void resetHover() {
 		if ((hoverEffect & EFFECT_EXPAND) > 0) {
-			this.scale = 1f;
+			scale.setTime(0);
 			setHoverRadius();
 		}
 		if ((hoverEffect & EFFECT_FADE) > 0)
-			this.alpha = baseAlpha;
+			alpha.setTime(0);
 		if ((hoverEffect & EFFECT_ROTATE) > 0)
-			this.angle = 0f;
+			angle.setTime(0);
 	}
 
 	/**
 	 * Removes all hover effects that have been set for the button.
 	 */
-	public void removeHoverEffects() { hoverEffect = 0; }
+	public void removeHoverEffects() {
+		this.hoverEffect = 0;
+		this.scale = null;
+		this.alpha = null;
+		this.angle = null;
+	}
+
+	/**
+	 * Sets the hover animation duration.
+	 * @param duration the duration, in milliseconds
+	 */
+	public void setHoverAnimationDuration(int duration) {
+		this.animationDuration = duration;
+		if (scale != null)
+			scale.setDuration(duration);
+		if (alpha != null)
+			alpha.setDuration(duration);
+		if (angle != null)
+			angle.setDuration(duration);
+	}
+
+	/**
+	 * Sets the hover animation equation.
+	 * @param eqn the equation to use
+	 */
+	public void setHoverAnimationEquation(AnimationEquation eqn) {
+		this.animationEqn = eqn;
+		if (scale != null)
+			scale.setEquation(eqn);
+		if (alpha != null)
+			alpha.setEquation(eqn);
+		if (angle != null)
+			angle.setEquation(eqn);
+	}
 
 	/**
 	 * Sets the "expand" hover effect.
 	 */
-	public void setHoverExpand() { hoverEffect |= EFFECT_EXPAND; }
+	public void setHoverExpand() { setHoverExpand(DEFAULT_SCALE_MAX, this.dir); }
 
 	/**
 	 * Sets the "expand" hover effect.
-	 * @param scale the maximum scale factor (default 1.25f)
+	 * @param scale the maximum scale factor
 	 */
 	public void setHoverExpand(float scale) { setHoverExpand(scale, this.dir); }
 
@@ -296,45 +346,45 @@ public class MenuButton {
 	 * Sets the "expand" hover effect.
 	 * @param dir the expansion direction
 	 */
-	public void setHoverExpand(Expand dir) { setHoverExpand(this.hoverScale, dir); }
+	public void setHoverExpand(Expand dir) { setHoverExpand(DEFAULT_SCALE_MAX, dir); }
 
 	/**
 	 * Sets the "expand" hover effect.
-	 * @param scale the maximum scale factor (default 1.25f)
+	 * @param scale the maximum scale factor
 	 * @param dir the expansion direction
 	 */
 	public void setHoverExpand(float scale, Expand dir) {
 		hoverEffect |= EFFECT_EXPAND;
-		this.hoverScale = scale;
+		this.scale = new AnimatedValue(animationDuration, 1f, scale, animationEqn);
 		this.dir = dir;
 	}
 
 	/**
 	 * Sets the "fade" hover effect.
 	 */
-	public void setHoverFade() { hoverEffect |= EFFECT_FADE; }
+	public void setHoverFade() { setHoverFade(DEFAULT_ALPHA_BASE); }
 
 	/**
 	 * Sets the "fade" hover effect.
-	 * @param baseAlpha the base alpha level to fade in from (default 0.7f)
+	 * @param baseAlpha the base alpha level to fade in from
 	 */
 	public void setHoverFade(float baseAlpha) {
 		hoverEffect |= EFFECT_FADE;
-		this.baseAlpha = baseAlpha;
+		this.alpha = new AnimatedValue(animationDuration, baseAlpha, 1f, animationEqn);
 	}
 
 	/**
 	 * Sets the "rotate" hover effect.
 	 */
-	public void setHoverRotate() { hoverEffect |= EFFECT_ROTATE; }
+	public void setHoverRotate() { setHoverRotate(DEFAULT_ANGLE_MAX); }
 
 	/**
 	 * Sets the "rotate" hover effect.
-	 * @param maxAngle the maximum rotation angle, in degrees (default 30f)
+	 * @param maxAngle the maximum rotation angle, in degrees
 	 */
 	public void setHoverRotate(float maxAngle) {
 		hoverEffect |= EFFECT_ROTATE;
-		this.maxAngle = maxAngle;
+		this.angle = new AnimatedValue(animationDuration, 0f, maxAngle, animationEqn);
 	}
 
 	/**
@@ -371,45 +421,21 @@ public class MenuButton {
 		if (hoverEffect == 0)
 			return;
 
+		int d = delta * (isHover ? 1 : -1);
+
 		// scale the button
 		if ((hoverEffect & EFFECT_EXPAND) > 0) {
-			int sign = 0;
+			if (scale.update(d))
-			if (isHover && scale < hoverScale)
-				sign = 1;
-			else if (!isHover && scale > 1f)
-				sign = -1;
-			if (sign != 0) {
-				scale = Utils.getBoundedValue(scale, sign * (hoverScale - 1f) * delta / 100f, 1, hoverScale);
 				setHoverRadius();
 		}
-		}
 
 		// fade the button
-		if ((hoverEffect & EFFECT_FADE) > 0) {
+		if ((hoverEffect & EFFECT_FADE) > 0)
-			int sign = 0;
+			alpha.update(d);
-			if (isHover && alpha < 1f)
-				sign = 1;
-			else if (!isHover && alpha > baseAlpha)
-				sign = -1;
-			if (sign != 0)
-				alpha = Utils.getBoundedValue(alpha, sign * (1f - baseAlpha) * delta / 200f, baseAlpha, 1f);
-		}
 
 		// rotate the button
-		if ((hoverEffect & EFFECT_ROTATE) > 0) {
+		if ((hoverEffect & EFFECT_ROTATE) > 0)
-			int sign = 0;
+			angle.update(d);
-			boolean right = (maxAngle > 0);
-			if (isHover && angle != maxAngle)
-				sign = (right) ? 1 : -1;
-			else if (!isHover && angle != 0)
-				sign = (right) ? -1 : 1;
-			if (sign != 0) {
-				float diff = sign * Math.abs(maxAngle) * delta / 125f;
-				angle = (right) ?
-					Utils.getBoundedValue(angle, diff, 0, maxAngle) :
-					Utils.getBoundedValue(angle, diff, maxAngle, 0);
-			}
-		}
 	}
 
 	/**
@@ -422,10 +448,11 @@ public class MenuButton {
 			image = anim.getCurrentFrame();
 
 		int xOffset = 0, yOffset = 0;
+		float currentScale = scale.getValue();
 		if (dir != Expand.CENTER) {
 			// offset by difference between normal/scaled image dimensions
-			xOffset = (int) ((scale - 1f) * image.getWidth());
+			xOffset = (int) ((currentScale - 1f) * image.getWidth());
-			yOffset = (int) ((scale - 1f) * image.getHeight());
+			yOffset = (int) ((currentScale - 1f) * image.getHeight());
 			if (dir == Expand.UP || dir == Expand.DOWN)
 				xOffset = 0;    // no horizontal offset
 			if (dir == Expand.RIGHT || dir == Expand.LEFT)
@@ -435,7 +462,7 @@ public class MenuButton {
 			if (dir == Expand.DOWN ||  dir == Expand.DOWN_LEFT || dir == Expand.DOWN_RIGHT)
 				yOffset *= -1;  // flip y for down
 		}
-		this.xRadius = ((image.getWidth() * scale) + xOffset) / 2f;
+		this.xRadius = ((image.getWidth() * currentScale) + xOffset) / 2f;
-		this.yRadius = ((image.getHeight() * scale) + yOffset) / 2f;
+		this.yRadius = ((image.getHeight() * currentScale) + yOffset) / 2f;
 	}
 }

src/itdelatrisu/opsu/ui/UI.java

@@ -26,6 +26,7 @@ import itdelatrisu.opsu.Utils;
 import itdelatrisu.opsu.audio.SoundController;
 import itdelatrisu.opsu.beatmap.BeatmapParser;
 import itdelatrisu.opsu.replay.ReplayImporter;
+import itdelatrisu.opsu.ui.animations.AnimationEquation;
 
 import javax.swing.JOptionPane;
 import javax.swing.UIManager;
@@ -106,6 +107,8 @@ public class UI {
 			Image back = GameImage.MENU_BACK.getImage();
 			backButton = new MenuButton(back, back.getWidth() / 2f, container.getHeight() - (back.getHeight() / 2f));
 		}
+		backButton.setHoverAnimationDuration(350);
+		backButton.setHoverAnimationEquation(AnimationEquation.IN_OUT_BACK);
 		backButton.setHoverExpand(MenuButton.Expand.UP_RIGHT);
 	}
 

src/itdelatrisu/opsu/ui/animations/AnimatedValue.java

@@ -0,0 +1,124 @@
+/*
+ * opsu! - an open-source osu! client
+ * Copyright (C) 2014, 2015 Jeffrey Han
+ *
+ * opsu! is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * opsu! is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with opsu!.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package itdelatrisu.opsu.ui.animations;
+
+import itdelatrisu.opsu.Utils;
+
+/**
+ * Utility class for updating a value using an animation equation.
+ */
+public class AnimatedValue {
+	/** The animation duration, in milliseconds. */
+	private int duration;
+
+	/** The current time, in milliseconds. */
+	private int time;
+
+	/** The base value. */
+	private float base;
+
+	/** The maximum difference from the base value. */
+	private float diff;
+
+	/** The current value. */
+	private float value;
+
+	/** The animation equation to use. */
+	private AnimationEquation eqn;
+
+	/**
+	 * Constructor.
+	 * @param duration the total animation duration, in milliseconds
+	 * @param min the minimum value
+	 * @param max the maximum value
+	 * @param eqn the animation equation to use
+	 */
+	public AnimatedValue(int duration, float min, float max, AnimationEquation eqn) {
+		this.time = 0;
+		this.duration = duration;
+		this.value = min;
+		this.base = min;
+		this.diff = max - min;
+		this.eqn = eqn;
+	}
+
+	/**
+	 * Returns the current value.
+	 */
+	public float getValue() { return value; }
+
+	/**
+	 * Returns the current animation time, in milliseconds.
+	 */
+	public int getTime() { return time; }
+
+	/**
+	 * Sets the animation time manually.
+	 * @param time the new time, in milliseconds
+	 */
+	public void setTime(int time) {
+		this.time = Utils.clamp(time, 0, duration);
+		updateValue();
+	}
+
+	/**
+	 * Sets the animation duration.
+	 * @param duration the new duration, in milliseconds
+	 */
+	public void setDuration(int duration) {
+		this.duration = duration;
+		int newTime = Utils.clamp(time, 0, duration);
+		if (time != newTime) {
+			this.time = newTime;
+			updateValue();
+		}
+	}
+
+	/**
+	 * Sets the animation equation to use.
+	 * @param eqn the new equation
+	 */
+	public void setEquation(AnimationEquation eqn) {
+		this.eqn = eqn;
+		updateValue();
+	}
+
+	/**
+	 * Updates the animation by a delta interval.
+	 * @param delta the delta interval since the last call.
+	 * @return true if an update was applied, false if the animation was not updated
+	 */
+	public boolean update(int delta) {
+		int newTime = Utils.getBoundedValue(time, delta, 0, duration);
+		if (time != newTime) {
+			this.time = newTime;
+			updateValue();
+			return true;
+		}
+		return false;
+	}
+
+	/**
+	 * Recalculates the value by applying the animation equation with the current time.
+	 */
+	private void updateValue() {
+		float t = eqn.calc((float) time / duration);
+		this.value = base + (t * diff);
+	}
+}

src/itdelatrisu/opsu/ui/animations/AnimationEquation.java

@@ -0,0 +1,309 @@
+/*
+ * opsu! - an open-source osu! client
+ * Copyright (C) 2014, 2015 Jeffrey Han
+ *
+ * opsu! is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * opsu! is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with opsu!.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package itdelatrisu.opsu.ui.animations;
+
+
+/*
+ * These equations are copyright (c) 2001 Robert Penner, all rights reserved,
+ * and are open source under the BSD License.
+ * http://www.opensource.org/licenses/bsd-license.php
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ * - Redistributions of source code must retain the above copyright notice,
+ *   this list of conditions and the following disclaimer.
+ * - Redistributions in binary form must reproduce the above copyright notice,
+ *   this list of conditions and the following disclaimer in the documentation
+ *   and/or other materials provided with the distribution.
+ * - Neither the name of the author nor the names of contributors may be used
+ *   to endorse or promote products derived from this software without specific
+ *   prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/**
+ * Easing functions for animations.
+ *
+ * @author Robert Penner (<a href="http://robertpenner.com/easing/">http://robertpenner.com/easing/</a>)
+ * @author CharlotteGore (<a href="https://github.com/CharlotteGore/functional-easing">https://github.com/CharlotteGore/functional-easing</a>)
+ */
+public enum AnimationEquation {
+	/* Linear */
+	LINEAR {
+		@Override
+		public float calc(float t) { return t; }
+	},
+
+	/* Quadratic */
+	IN_QUAD {
+		@Override
+		public float calc(float t) { return t * t; }
+	},
+	OUT_QUAD {
+		@Override
+		public float calc(float t) { return -1 * t * (t - 2); }
+	},
+	IN_OUT_QUAD {
+		@Override
+		public float calc(float t) {
+			t = t * 2;
+			if (t < 1)
+				return 0.5f * t * t;
+			t = t - 1;
+			return -0.5f * (t * (t - 2) - 1);
+		}
+	},
+
+	/* Cubic */
+	IN_CUBIC {
+		@Override
+		public float calc(float t) { return t * t * t; }
+	},
+	OUT_CUBIC {
+		@Override
+		public float calc(float t) {
+			t = t - 1;
+			return t * t * t + 1;
+		}
+	},
+	IN_OUT_CUBIC {
+		@Override
+		public float calc(float t) {
+			t = t * 2;
+			if (t < 1)
+				return 0.5f * t * t * t;
+			t = t - 2;
+			return 0.5f * (t * t * t + 2);
+		}
+	},
+
+	/* Quartic */
+	IN_QUART {
+		@Override
+		public float calc(float t) { return t * t * t * t; }
+	},
+	OUT_QUART {
+		@Override
+		public float calc(float t) {
+			t = t - 1;
+			return -1 * (t * t * t * t - 1);
+		}
+	},
+	IN_OUT_QUART {
+		@Override
+		public float calc(float t) {
+			t = t * 2;
+			if (t < 1)
+				return 0.5f * t * t * t * t;
+			t = t - 2;
+			return -0.5f * (t * t * t * t - 2);
+		}
+	},
+
+	/* Quintic */
+	IN_QUINT {
+		@Override
+		public float calc(float t) { return t * t * t * t * t; }
+	},
+	OUT_QUINT {
+		@Override
+		public float calc(float t) {
+			t = t - 1;
+			return (t * t * t * t * t + 1);
+		}
+	},
+	IN_OUT_QUINT {
+		@Override
+		public float calc(float t) {
+			t = t * 2;
+			if (t < 1)
+				return 0.5f * t * t * t * t * t;
+			t = t - 2;
+			return 0.5f * (t * t * t * t * t + 2);
+		}
+	},
+
+	/* Sine */
+	IN_SINE {
+		@Override
+		public float calc(float t) { return -1 * (float) Math.cos(t * (Math.PI / 2)) + 1; }
+	},
+	OUT_SINE {
+		@Override
+		public float calc(float t) { return (float) Math.sin(t * (Math.PI / 2)); }
+	},
+	IN_OUT_SINE {
+		@Override
+		public float calc(float t) { return (float) (Math.cos(Math.PI * t) - 1) / -2; }
+	},
+
+	/* Exponential */
+	IN_EXPO {
+		@Override
+		public float calc(float t) { return (t == 0) ? 0 : (float) Math.pow(2, 10 * (t - 1)); }
+	},
+	OUT_EXPO {
+		@Override
+		public float calc(float t) { return (t == 1) ? 1 : (float) -Math.pow(2, -10 * t) + 1; }
+	},
+	IN_OUT_EXPO {
+		@Override
+		public float calc(float t) {
+			if (t == 0 || t == 1)
+				return t;
+			t = t * 2;
+			if (t < 1)
+				return 0.5f * (float) Math.pow(2, 10 * (t - 1));
+			t = t - 1;
+			return 0.5f * ((float) -Math.pow(2, -10 * t) + 2);
+		}
+	},
+
+	/* Circular */
+	IN_CIRC {
+		@Override
+		public float calc(float t) { return -1 * ((float) Math.sqrt(1 - t * t) - 1); }
+	},
+	OUT_CIRC {
+		@Override
+		public float calc(float t) {
+			t = t - 1;
+			return (float) Math.sqrt(1 - t * t);
+		}
+	},
+	IN_OUT_CIRC {
+		@Override
+		public float calc(float t) {
+			t = t * 2;
+			if (t < 1)
+				return -0.5f * ((float) Math.sqrt(1 - t * t) - 1);
+			t = t - 2;
+			return 0.5f * ((float) Math.sqrt(1 - t * t) + 1);
+		}
+	},
+
+	/* Back */
+	IN_BACK {
+		@Override
+		public float calc(float t) { return t * t * ((OVERSHOOT + 1) * t - OVERSHOOT); }
+	},
+	OUT_BACK {
+		@Override
+		public float calc(float t) {
+			t = t - 1;
+			return t * t * ((OVERSHOOT + 1) * t + OVERSHOOT) + 1;
+		}
+	},
+	IN_OUT_BACK {
+		@Override
+		public float calc(float t) {
+			float overshoot = OVERSHOOT * 1.525f;
+			t = t * 2;
+			if (t < 1)
+				return 0.5f * (t * t * ((overshoot + 1) * t - overshoot));
+			t = t - 2;
+			return 0.5f * (t * t * ((overshoot + 1) * t + overshoot) + 2);
+		}
+	},
+
+	/* Bounce */
+	IN_BOUNCE {
+		@Override
+		public float calc(float t) { return 1 - OUT_BOUNCE.calc(1 - t); }
+	},
+	OUT_BOUNCE {
+		@Override
+		public float calc(float t) {
+			if (t < 0.36363636f)
+				return 7.5625f * t * t;
+			else if (t < 0.72727273f) {
+				t = t - 0.54545454f;
+				return 7.5625f * t * t + 0.75f;
+			} else if (t < 0.90909091f) {
+				t = t - 0.81818182f;
+				return 7.5625f * t * t + 0.9375f;
+			} else {
+				t = t - 0.95454546f;
+				return 7.5625f * t * t + 0.984375f;
+			}
+		}
+	},
+	IN_OUT_BOUNCE {
+		@Override
+		public float calc(float t) {
+			if (t < 0.5f)
+				return IN_BOUNCE.calc(t * 2) * 0.5f;
+			return OUT_BOUNCE.calc(t * 2 - 1) * 0.5f + 0.5f;
+		}
+	},
+
+	/* Elastic */
+	IN_ELASTIC {
+		@Override
+		public float calc(float t) {
+			if (t == 0 || t == 1)
+				return t;
+			float period = 0.3f;
+			t = t - 1;
+			return -((float) Math.pow(2, 10 * t) * (float) Math.sin(((t - period / 4) * (Math.PI * 2)) / period));
+		}
+	},
+	OUT_ELASTIC {
+		@Override
+		public float calc(float t) {
+			if (t == 0 || t == 1)
+				return t;
+			float period = 0.3f;
+			return (float) Math.pow(2, -10 * t) * (float) Math.sin((t - period / 4) * (Math.PI * 2) / period) + 1;
+		}
+	},
+	IN_OUT_ELASTIC {
+		@Override
+		public float calc(float t) {
+			if (t == 0 || t == 1)
+				return t;
+			float period = 0.44999996f;
+			t = t * 2 - 1;
+			if (t < 0)
+				return -0.5f * ((float) Math.pow(2, 10 * t) * (float) Math.sin((t - period / 4) * (Math.PI * 2) / period));
+			return (float) Math.pow(2, -10 * t) * (float) Math.sin((t - period / 4) * (Math.PI * 2) / period) * 0.5f + 1;
+		}
+	};
+
+	/** Overshoot constant for "back" easings. */
+	private static final float OVERSHOOT = 1.70158f;
+
+	/**
+	 * Calculates a new {@code t} value using the animation equation.
+	 * @param t the raw {@code t} value [0,1]
+	 * @return the new {@code t} value [0,1]
+	 */
+	public abstract float calc(float t);
+}