Re: Requesting tips, comm

"Knute Johnson" <knute.johnson@THRWHITE.remove-dii-this>
Wed, 27 Apr 2011 15:32:23 GMT
Oliver Wong wrote:

"Knute Johnson" <> wrote in message

For active rendering I would take your rendering out of the EDT
altogether. The problem that you are going to run into is that
eventually too much will be happening on the EDT and you won't be able
to do any user input processing or that it will delay your rendering.

    This key piece of information is pretty central to my
(mis?)understanding. I thought it's written that whenever you touch Swing
components, you should do so from within the EDT. If that's the case,
shouldn't you perform painting of JFrames and other widgets within the

Unless you are trying to do high performance active rendering. Then you
  do your rendering in another thread. The normal Swing component
drawing is disabled and you won't interfere with it at all.

You only need to create the BufferStrategy once not every time in the
rendering loop.

    Right: in the code I posted, I actually create the BufferStrategy
once, but call getBufferStrategy() repeatedly from within the loop (as
opposed to, in your code, calling getBufferStrategy once and storing its
return value somewhere). From peering into the source code, it looks like
getBufferStrategy() is just a plain getter anyway.

Yes you are correct. In your case it is just one less method call in
the render loop to do it the way I did.

 I like to use java.util.Timer to drive the loop but you can use

    The Timer class seems to have its own internal "task queue". From the
Timer tasks should complete quickly. If a timer task takes excessive time
to complete, it "hogs" the timer's task execution thread. This can, in
turn, delay the execution of subsequent tasks, which may "bunch up" and
execute in rapid succession when (and if) the offending task finally

Each Timer object creates a thread. If you only schedule one task it
won't run into problems with queue delays. I did test this with
something else I was working on and it is better to create a new Timer
for each task.

    I haven't spent too much time analyzing the impact of this (presumably
there will only be one if I use Timer.schedule() at multiple locations in
my code), but it sounds like I can avoid worrying about it entirely with a
tight loop and Thread.sleep(1).

    I realize that the javadocs themselves claim that your design is
appropriate for animations (It says "Fixed-delay execution is appropriate
for recurring activities that require 'smoothness.' In other words, it is
appropriate for activities where it is more important to keep the
frequency accurate in the short run than in the long run. This includes
most animation tasks"), but this fixed-delay behaviour sounds like it's
incompatible with the way *I* want rendering to work in my engine.

I know but I can tell you after a lot of experimentation with adjusting
timers and fixed rate Timers that it looks better with fixed delay.

    It's not shown in my code, but in a "real" engine, I usually calculate
the amount of time that has passed between each frame of animation (I've
got this library which tries to use native C code on platforms it
recognizes, and falls back on System.currentTimeMillis() on those it
doesn't), and then I use that elapsed time to calculate whether to skip
ahead a couple of frames (on computers that are a bit slow for this game)
or to interpolate between two frames of animations (on computers that are
a bit fast for this game).

I don't know how real game developers handle timing. I do know that
with a fair amount of testing I have gone back to Timer.schedule(). It
does not produce that accurate a result on Windows XP computers that I
have tested it on. It is just better than the alternatives I have tried.

I don't have any real experience with XP timing but I would be very
curious to know if it is possible to make a JNI method that could sleep
very accurately. I would like to find something that could be accurate
to 250 microseconds +- to try in my timing loop. The accuracy of
Timer.schedule() is no where near that close. If you ran the test code
I posted you can see the jitter caused by the timing errors.

    For example, I might have a scripting language which says "It should
take the character 2 seconds to walk from point A to point B", and the
engine will smoothly animate a sprite moving from point A to point B at
the highest frame rate that the computer can handle.

I think what you will find with that method is that your frame rate is
constantly changing as thread timing and system load changes. But I
will be interested in your results.

    Long story short, the "Fixed-delay execution" system seems like it'll
work counter to my goal there.

I'm not trying to talk you out of it just trying to give you what little
I know and have tried.

 As Daniel said I would do my user input event driven.

    Really? I find a polling-based input system much more natural to
program for, in the context of making a game. For example, if the player
presses the "Right" cursor key, I want Sonic the Hedgehog to start running
towards the right. And if in the next frame, he's still holding down the
"right" cursor I want Sonic to keep running (otherwise, he should start to
brake, and slow down). And the longer the button is held down, the more
Sonic accelerates (up to a certain terminal velocity).

    With a polling system, the logic would look something like:

  if (rightButtonIsDown()) {
    if (sonic.xVel < MAX_XVEL) {
      sonic.xVel += 1.0;
  } else if (leftButtonIsDown()) {
    if (sonic.xVel > -MAX_XVEL) {
      sonic.xVel -= 1.0;
  } else {
    sonic.xVel /= 2; /*Quickly approaches zero.*/

    The only way I could think of implementing this with Swing's event
model would be to set a flag when a keypress occurs, and then unset a flag
when the key is released... which is pretty much emulating a polling
system ontop of Swing's event system... which is pretty much what the code
I posted does.

I used the old event model for the Pong game I wrote. That works well
and for that sort of input will probably be better. I was thinking
JTextFields and Buttons before. Pong game and source is at It does not use active rendering.

 You can use a Frame or JFrame but you have to deal with the insets and
title bar your self. If you use an undecorated frame you might as well
use a window.

    Right, the insets are a bit of a pain, and I sometimes wish JFrame had
a setSizeWithoutInsets() method in addition to its standard setSize()
method. But I *do* want a decorate frame (at least when the game is not
running in full screen mode), so that the player can move the window
around and resize it. Of course, if the player switches the game to
fullscreen mode, then at that point, the window should be undecorated.

I haven't used full screen exclusive mode much because most of what I
have been working on uses multiple monitors and still requires user
input. That won't work with FSEM. I don't know if it is possible to
remove the decoration on a window once it has been created or not.

Below is how I would do it. See if you like any of it.

[informative code snipped]

    Thanks. You introduced me to the WindowAdapter class, and it looks
like I'll have to re-read the javadocs for BufferStrategy (since it's not
clear to me what you're trying to do with your calls to contentsLost() and
contentsRestored()) It looks like it has something to do with losing image
data due to the use of VolatileImage, but I'm wondering whether it's
necessary for my game: In most of my games, so much is going on screen at
any one point that I typically assume that the whole image is always lost
and redraw the whole frame from scratch. The game is implemented as a
state machine (not illustrated in the simplified version of the code I've
been posting), and any state which needs access to previous frame contents
(e.g. for a motion-blurring effect) needs to implement storing previous
image data itself.

Even a blit buffer strategy can use a VolatileImage as the back buffer.
  If you read the docs for VolatileImage you will see what I'm up to
there. That code is directly out of the docs.

    Here's yet another iteration of my design, taking into account your
advice of pulling the active-drawing code out of the EDT. I've had to
reintroduce the synchronize() statements, because otherwise it's possible
that the user might close the window (and thus invalidate the graphics
object) while I'm still using the graphics object to render the next
frame. I've added a Thread.sleep(1000) statement to increase the
likelihood of this race condition occurring for illustrative purposes.

    I've also used your WindowAdapter (and KeyAdapter) trick.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics2D;
import java.awt.Insets;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferStrategy;
import java.lang.reflect.InvocationTargetException;

import javax.swing.JFrame;

public class Test {

 private static final int DEFAULT_RENDERING_WIDTH = 640;
 private static final int DEFAULT_RENDERING_HEIGHT = 480;
 private static JFrame mainWindow;
  * Returns true if the game is in the process of shutting down and
  * If it's in the middle of a game-loop iteration, it'll finish that
  * iteration (potentially drawing 1 more frame of animation, and then
 private static boolean timeToQuit = false;
  * True if the player pressed the "action" key since the last iteration
  * through the game loop, false otherwise. In a real game, I'd probably
  * many of these booleans (e.g. an "up" key, a "down" key, etc., and move
  * into a seperate class for organization purposes.
 private static boolean pressedActionKey = false;

 public static void main(String[] args) throws InterruptedException,
   InvocationTargetException {
   * This first invokeAndWait initializes all the GUI components, and
   * registers the appropriate listeners to hook into the main game
   * engine.
  EventQueue.invokeAndWait(new Runnable() {
   public void run() {
    mainWindow = new JFrame("My game");
    mainWindow.addWindowListener(new WindowAdapter() {
     public void windowClosing(WindowEvent e) {
      timeToQuit = true;

You are already in the EDT here, invokeLater is not needed.

      EventQueue.invokeLater(new Runnable() {
       public void run() {
        synchronized (mainWindow) {
         mainWindow = null;
    mainWindow.addKeyListener(new KeyAdapter() {
     public void keyPressed(KeyEvent e) {
      if (e.getKeyCode() == KeyEvent.VK_SPACE) {
       pressedActionKey = true;
    mainWindow.setPreferredSize(new Dimension(
   * This while loop is the main game loop. It basically iterates through
   * 3 stages forever: getting the player input, reacting to it, and
   * drawing the results on screen.
  while (!timeToQuit) {
   synchronized (mainWindow) {
    if (mainWindow != null) {
     BufferStrategy strategy = mainWindow.getBufferStrategy();
     Graphics2D g = (Graphics2D) strategy.getDrawGraphics();
     Insets insets = mainWindow.getInsets();
       (double) (mainWindow.getWidth() - insets.right)
         / (double) DEFAULT_RENDERING_WIDTH,
       (double) (mainWindow.getHeight() - insets.bottom)
         / (double) DEFAULT_RENDERING_HEIGHT));

 public static void processGameLogic() {
   * Check if pacman touched a ghost, stuff like that. Doesn't touch any
   * Swing components.

 public static void getPlayerInput() {
   * Doesn't touch any Swing components.
  if (pressedActionKey) {
   /* make mario jump */
   System.out.println("Mario jumps.");
   pressedActionKey = false;

 public static void updateScreen(Graphics2D g) {
  g.fillRect(0, 0, 640, 480);
   * TODO: Draw all sorts of amazing eye-candy.

    - Oliver

Try it and see how it works. Post something that has some action!

If you are interested in writing some C timing code for me, let me know.


Knute Johnson
email s/nospam/knute/

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