How to 'Get' a Specific Point From A Shape Object Derived from AffineTransform
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As a self-project, I\'m trying to make the game \'Asteroids\'.
Currently, I\'m stuck on trying to figure out how to make it so the lasers fired from my ship appear from
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You can use AffineTransform.transform(Point2D, Point2D) to transform a single point on your polygon.
Things would be a lot simpler for you if instead of trying to move the ship by using a rotation transform you kept a single
(x,y)location of where the ship is. You'd move the ship's location inmove()instead of trying to translate the polygon. Then when you want to paint the ship you e.g. do:// Optionally copying the Graphics so the // transform doesn't affect later painting. Graphics2D temp = (Graphics2D) g2d.create(); temp.translate(ship.locX, ship.locY); temp.rotate(ship.angle); temp.draw(ship);To move a point based on speed you can do this to find the movement vector:
double velX = speed * Math.cos(angle); double velY = speed * Math.sin(angle); locX += timeElapsed * velX; locY += timeElapsed * velY;That is essentially a conversion from polar to Cartesian coordinates. The x and y velocities are the legs of a triangle whose hypotenuse is
speedand whose known angle isangle:/| / | / | / | speed / | / | / |velY / angle | /)_______| velXThere's an example of doing movement this way in an answer of mine here: https://stackoverflow.com/a/43692434/2891664.
For your comments:
Are you saying that, unlike my initial move function, just to make
shiphold a single point, and thus I would only translate that instead?More or less, yes. You'd still have a polygon to hold the ship's shape, but the points on the polygon would be relative to
(0,0).Suppose the following definitions:
- Each
(x,y)point on the polygon ispi. (In other words, one ofp0,p1,p2andp3.) - The
(x,y)coordinates of the translation areT
Then, after translating the
Graphics2D, eachpicoordinate becomespi+Ton the panel. So if your polygon points are defined relative to(0,0)then translating to the ship's(locX,locY)will move the polygon to a location relative to(locX,locY).It could be simplest then to define the point which is the tip of the polygon as being
(0,0)so that after the translation the tip of the ship is the ship's location:// Your original points: int xPoints[] = {800, 780, 800, 820}; int yPoints[] = {400, 460, 440, 460}; // Become these points relative to (0,0): int xPoints[] = {0, -20, 0, 20}; int yPoints[] = {0, 60, 40, 60};And to e.g. start the ship in the same place, you would initialize its location to
(800,400).
I was thinking about this again and realized the rotation is a little more complicated, because you probably don't want to rotate the ship around the tip. You probably want to rotate the ship around its center.
So, here's an MCVE demonstrating how to do all of this.
package mcve.game; import javax.swing.*; import java.awt.event.*; import java.awt.geom.*; import java.awt.Polygon; import java.awt.RenderingHints; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.Color; import java.awt.Dimension; import java.awt.Rectangle; import java.awt.Insets; import java.awt.Toolkit; import java.awt.GraphicsConfiguration; import java.util.Set; import java.util.HashSet; import java.util.List; import java.util.ArrayList; public class MovementExample implements ActionListener { public static void main(String[] args) { SwingUtilities.invokeLater(MovementExample::new); } final int fps = 60; final int period = 1000 / fps; final JFrame frame; final GamePanel panel; final Controls controls; final Ship ship; final List<Bullet> bullets = new ArrayList<>(); MovementExample() { frame = new JFrame("Movement Example"); Dimension size = getMaximumWindowSize(frame); size.width /= 2; size.height /= 2; frame.setPreferredSize(size); panel = new GamePanel(); frame.setContentPane(panel); frame.pack(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setLocationRelativeTo(null); frame.setVisible(true); controls = new Controls(); ship = new Ship(panel.getWidth() / 2, panel.getHeight() / 2); new Timer(period, this).start(); } @Override public void actionPerformed(ActionEvent e) { double secondsElapsed = 1.0 / fps; ship.update(secondsElapsed); bullets.forEach(b -> b.update(secondsElapsed)); Rectangle bounds = panel.getBounds(); bullets.removeIf(b -> !bounds.contains(b.locX, b.locY)); panel.repaint(); } class GamePanel extends JPanel { GamePanel() { setBackground(Color.WHITE); } @Override protected void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2 = (Graphics2D) g.create(); g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON); if (ship != null) { ship.draw(g2); } bullets.forEach(b -> b.draw(g2)); g2.dispose(); } } abstract class AbstractGameObject { double maxSpeed; double rotationAngle; double locX; double locY; double velX; double velY; AbstractGameObject(double initialX, double initialY) { locX = initialX; locY = initialY; } abstract void update(double secondsElapsed); abstract void draw(Graphics2D g2); } class Ship extends AbstractGameObject { Polygon shape; double rotationRate; Ship(double initialX, double initialY) { super(initialX, initialY); maxSpeed = 128; // pixels/second rotationAngle = Math.PI * 3 / 2; rotationRate = (2 * Math.PI) / 2; // radians/second int xPoints[] = {0, -20, 0, 20}; int yPoints[] = {0, 60, 40, 60}; shape = new Polygon(xPoints, yPoints, 4); } Point2D.Double getTip() { Point2D.Double center = getCenter(); // The tip is at (0,0) and it's already centered // on the x-axis origin, so the distance from the // tip to the center is just center.y. double distance = center.y; // Then find the location of the tip, relative // to the center. double tipX = distance * Math.cos(rotationAngle); double tipY = distance * Math.sin(rotationAngle); // Now find the actual location of the center. center.x += locX; center.y += locY; // And return the actual location of the tip, relative // to the actual location of the center. return new Point2D.Double(tipX + center.x, tipY + center.y); } Point2D.Double getCenter() { // Returns the center point of the ship, // relative to (0,0). Point2D.Double center = new Point2D.Double(); for (int i = 0; i < shape.npoints; ++i) { center.x += shape.xpoints[i]; center.y += shape.ypoints[i]; } center.x /= shape.npoints; center.y /= shape.npoints; return center; } @Override void update(double secondsElapsed) { // See my answer here: https://stackoverflow.com/a/43692434/2891664 // for a discussion of why this logic is the way it is. double speed = 0; if (controls.isUpHeld()) { speed += maxSpeed; } if (controls.isDownHeld()) { speed -= maxSpeed; } velX = speed * Math.cos(rotationAngle); velY = speed * Math.sin(rotationAngle); locX += secondsElapsed * velX; locY += secondsElapsed * velY; double rotation = 0; if (controls.isLeftHeld()) { rotation -= rotationRate; } if (controls.isRightHeld()) { rotation += rotationRate; } rotationAngle += secondsElapsed * rotation; // Cap the angle so it can never e.g. get so // large that it loses precision. if (rotationAngle > 2 * Math.PI) { rotationAngle -= 2 * Math.PI; } if (controls.isFireHeld()) { Point2D.Double tipLoc = getTip(); Bullet bullet = new Bullet(tipLoc.x, tipLoc.y, rotationAngle); bullets.add(bullet); } } @Override void draw(Graphics2D g2) { Graphics2D copy = (Graphics2D) g2.create(); copy.setColor(Color.RED); // Translate to the ship's location. copy.translate(locX, locY); // Rotate the ship around its center. Point2D.Double center = getCenter(); // The PI/2 offset is necessary because the // polygon points are defined with the ship // already vertical, i.e. at an angle of -PI/2. copy.rotate(rotationAngle + (Math.PI / 2), center.x, center.y); copy.fill(shape); } } class Bullet extends AbstractGameObject { Ellipse2D.Double shape = new Ellipse2D.Double(); Bullet(double initialX, double initialY, double initialRotation) { super(initialX, initialY); maxSpeed = 512; rotationAngle = initialRotation; velX = maxSpeed * Math.cos(rotationAngle); velY = maxSpeed * Math.sin(rotationAngle); double radius = 3; shape.setFrame(-radius, -radius, 2 * radius, 2 * radius); } @Override void update(double secondsElapsed) { locX += secondsElapsed * velX; locY += secondsElapsed * velY; } @Override void draw(Graphics2D g2) { Graphics2D copy = (Graphics2D) g2.create(); copy.setColor(Color.BLACK); copy.translate(locX, locY); copy.fill(shape); } } // See https://docs.oracle.com/javase/tutorial/uiswing/misc/keybinding.html class Controls { final Set<Integer> keysHeld = new HashSet<>(); Controls() { bind(KeyEvent.VK_A, "left"); bind(KeyEvent.VK_D, "right"); bind(KeyEvent.VK_W, "up"); bind(KeyEvent.VK_S, "down"); bind(KeyEvent.VK_SPACE, "fire"); } boolean isLeftHeld() { return keysHeld.contains(KeyEvent.VK_A); } boolean isRightHeld() { return keysHeld.contains(KeyEvent.VK_D); } boolean isUpHeld() { return keysHeld.contains(KeyEvent.VK_W); } boolean isDownHeld() { return keysHeld.contains(KeyEvent.VK_S); } boolean isFireHeld() { return keysHeld.contains(KeyEvent.VK_SPACE); } void bind(int keyCode, String name) { bind(keyCode, name, true); bind(keyCode, name, false); } void bind(int keyCode, String name, boolean isOnRelease) { KeyStroke stroke = KeyStroke.getKeyStroke(keyCode, 0, isOnRelease); name += isOnRelease ? ".released" : ".pressed"; panel.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW) .put(stroke, name); panel.getActionMap() .put(name, new AbstractAction() { @Override public void actionPerformed(ActionEvent e) { if (isOnRelease) { keysHeld.remove(keyCode); } else { keysHeld.add(keyCode); } } }); } } // This returns the usable size of the display which // the JFrame resides in, as described here: // http://docs.oracle.com/javase/8/docs/api/java/awt/GraphicsEnvironment.html#getMaximumWindowBounds-- static Dimension getMaximumWindowSize(JFrame frame) { GraphicsConfiguration config = frame.getGraphicsConfiguration(); Dimension size = config.getBounds().getSize(); Insets insets = Toolkit.getDefaultToolkit().getScreenInsets(config); size.width -= insets.left + insets.right; size.height -= insets.top + insets.bottom; return size; } }There are other ways the tip of the ship could be calculated, but the way I did it in the MCVE is this:
- Get the center point of the ship, relative to
(0,0). - Get the distance from the center point to the tip. The tip is at
(0,0)so this is just the y-coordinate of the center. - Then calculate the
(x,y)location of the tip, relative to the center. This is done in a very similar way to the figure above for speed and velocity, but the hypotenuse is the distance between the center and the tip of the ship. - Translate the center to be relative to the ship's location.
- Translate the location of the tip (which is relative to the center) to be relative to the ship's location.
It could also all be done with an
AffineTransform, similar to what you are doing in the code in the question, but you'd set it on every update. Something like this:AffineTransform transform = new AffineTransform(); @Override void update(double secondsElapsed) { ... // Clear the previous translation and rotation. transform.setToIdentity(); // Set to current. transform.translate(locX, locY); Point2D.Double center = getCenter(); transform.rotate(rotationAngle + (Math.PI / 2), center.x, center.y); if (controls.isFireHeld()) { Point2D.Double tip = new Point2D.Double(0, 0); transform.transform(tip, tip); Bullet bullet = new Bullet(tip.x, tip.y, rotationAngle); bullets.add(bullet); } }You could still use a transform to do calculations that way, but you don't end up with any strangeness from depending on the transform for movement. (In the code in the question, the ship is e.g. only ever moved along the y-axis. The apparent sideways movement is due to the series of rotation concatenations.)
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