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Simple Dynamic Shadows in Java2D
import java.awt.BorderLayout;
import java.awt.Canvas;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.Polygon;
import java.awt.RadialGradientPaint;
import java.awt.RenderingHints;
import java.awt.Shape;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Point2D;
import java.awt.geom.Rectangle2D;
import java.awt.image.BufferStrategy;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
* @author Matt DesLauriers, mattdesl
public class ShadowTest {
public static void main(String[] args) {
new ShadowTest().run();
/** The gradient radius for our shadow. */
protected final static float GRADIENT_SIZE = 200f;
/** The fractions for our shadow gradient, going from 0.0 (black) to 1.0 (transparent). */
protected final static float[] GRADIENT_FRACTIONS = new float[]{0f, 1f};
/** The colors for our shadow, going from opaque black to transparent black. */
protected final static Color[] GRADIENT_COLORS = new Color[] {, new Color(0f,0f,0f,0f) };
/** A Polygon object which we will re-use for each shadow geometry. */
protected final static Polygon POLYGON = new Polygon();
/** The size of the game canvas, initially 640x480. */
private static int width = 640, height = 480;
/** Returns the width of the game canvas. */
public static int getWidth() {
return Math.max(0, width);
/** Returns the height of the game canvas. */
public static int getHeight() {
return Math.max(0, height);
/** The buffer strategy used for smooth active rendering. */
protected BufferStrategy strategy;
/** True if the game loop is running. */
protected boolean running;
/** The current frames per second, used for debugging performance. */
protected int fps = 60;
/** Timer used for adding entities every N seconds. */
private float tickTimer;
/** A list of entities to render. */
protected List<Shape> entities = new ArrayList<Shape>();
/** The mouse position */
protected int mouseX, mouseY;
/////// GUI VARIABLES ///////
/** The frame for our GUI. */
protected JFrame frame = new JFrame("Shooter Game");
/** Constructs a new Game object; run() should be called to start the rendering. */
public ShadowTest() {
//set up our Canvas which will be used for active rendering (game loop)
final Canvas canvas = new Canvas();
canvas.setPreferredSize(new Dimension(width, height));
canvas.addComponentListener(new ComponentAdapter() { //on resize...
public void componentResized(ComponentEvent arg0) {
width = canvas.getWidth();
height = canvas.getHeight();
//set background as per assignment requirement
//listen to mouse events
canvas.addMouseMotionListener(new MouseMoveListener());
frame.add(canvas, BorderLayout.CENTER);
//pack to proper size
//center frame on user's screen
//show the frame
//set up a buffer strategy used for smooth rendering
//needs to be called after JFrame is valid and visible
strategy = canvas.getBufferStrategy();
/** Stops the game loop. */
public void stop() {
running = false;
/** Called to initialize the game loop. */
public void run() {
int frames = 0;
long lastTime, lastSec;
lastTime = lastSec = System.nanoTime();
running = true;
while (running) {
long deltaTime = System.nanoTime() - lastTime;
lastTime += deltaTime;
//update the game by a little
update(deltaTime / 1e9);
//pretty standard buffer strategy game loop
do {
do {
Graphics2D g = (Graphics2D) strategy.getDrawGraphics();
//clear screen
g.clearRect(0, 0, width, height);
} while (strategy.contentsRestored());;
} while (strategy.contentsLost());
//count FPS
if (System.nanoTime() - lastSec >= 1e9) {
fps = frames;
frames = 0;
lastSec += 1e9;
//sync frame rate to 60 FPS
do {
} while (System.nanoTime() - lastTime < 1e9/60);
/** Called on first run to initialize the game and any resources. */
protected void init() {
//add the first entity
entities.add(new Ellipse2D.Float(125, 125, 50, 50));
entities.add(new Rectangle2D.Float(125, 225, 20, 20));
/** Updates the game's entities. */
protected void update(double deltaTime) {
/** Called to render the frame. */
protected void render(Graphics2D g) {
//we'll use nice quality interpolation
g.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g.drawString("FPS: "+fps, 10, 20);
//render the shadows first
//render each entity
for (int i=0; i<entities.size(); i++) {
Shape e = entities.get(i);
* Renders the shadows using simple vector math. The steps are as follows:
* <pre>
* for each entity
* if entity is not moving:
* ignore entity
* if entity is too far from mouse:
* ignore entity
* determine unit vector from mouse to entity center
* get perpendicular of unit vector
* Create Points A + B:
* extrude perpendicular in either direction, by the half-size of the entity
* Create Points C + D:
* extrude A + B away from mouse position
* construct polygon with points A, B, C, D
* render with RadialGradientPaint to give it a "fade-out" appearance
* </pre>
* @param g the graphics to use for rendering
protected void renderShadows(Graphics2D g) {
//old Paint object for resetting it later
Paint oldPaint = g.getPaint();
//minimum distance (squared) which will save us some checks
//amount to extrude our shadow polygon by
//use a large enough value to ensure that it is way off screen
//we'll use a radial gradient from the mouse center
final Paint GRADIENT_PAINT = new RadialGradientPaint(new Point2D.Float(mouseX, mouseY),
final Point2D.Float mouse = new Point2D.Float(mouseX, mouseY);
//for each entity
for (int i=0; i<entities.size(); i++) {
Shape e = entities.get(i);
Rectangle2D bounds = e.getBounds2D();
//radius of Entity's bounding circle
float r = (float)bounds.getWidth()/2f;
//get center of entity
float cx = (float)bounds.getX() + r;
float cy = (float)bounds.getY() + r;
//get direction from mouse to entity center
float dx = cx - mouse.x;
float dy = cy - mouse.y;
//get euclidean distance from mouse to center
float distSq = dx * dx + dy * dy; //avoid sqrt for performance
//if the entity is outside of the shadow radius, then ignore
if (distSq > minDistSq)
//normalize the direction to a unit vector
float len = (float)Math.sqrt(distSq);
float nx = dx;
float ny = dy;
if (len != 0) { //avoid division by 0
nx /= len;
ny /= len;
//get perpendicular of unit vector
float px = -ny;
float py = nx;
//our perpendicular points in either direction from radius
Point2D.Float A = new Point2D.Float(cx - px * r, cy - py * r);
Point2D.Float B = new Point2D.Float(cx + px * r, cy + py * r);
//project the points by our SHADOW_EXTRUDE amount
Point2D.Float C = project(mouse, A, SHADOW_EXTRUDE);
Point2D.Float D = project(mouse, B, SHADOW_EXTRUDE);
//construct a polygon from our points
POLYGON.addPoint((int)A.x, (int)A.y);
POLYGON.addPoint((int)B.x, (int)B.y);
POLYGON.addPoint((int)D.x, (int)D.y);
POLYGON.addPoint((int)C.x, (int)C.y);
//fill the polygon with the gradient paint
//reset to old Paint object
/** Projects a point from end along the vector (end - start) by the given scalar amount. */
private Point2D.Float project(Point2D.Float start, Point2D.Float end, float scalar) {
float dx = end.x - start.x;
float dy = end.y - start.y;
//euclidean length
float len = (float)Math.sqrt(dx * dx + dy * dy);
//normalize to unit vector
if (len != 0) { //avoid division by 0
dx /= len;
dy /= len;
//multiply by scalar amount
dx *= scalar;
dy *= scalar;
return new Point2D.Float(end.x + dx, end.y + dy);
/** Mouse motion listener for dynamic 2D shadows. */
private class MouseMoveListener extends MouseMotionAdapter {
public void mouseMoved(MouseEvent e) {
mouseX = e.getX();
mouseY = e.getY();
public void mouseDragged(MouseEvent e) {
mouseX = e.getX();
mouseY = e.getY();
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