Linux game programming

1
Linux game programming

• An introduction to the use of interval timers and
  asynchronous input notifications

2
Our prior animation demo

• We achieved the illusion of smooth and
  flicker-free animation, by synchronizing
  drawing-operations with Vertical Retrace
• But more is needed in a game thats fun
• Some deficiencies in our animate1 demo
• Ball movements tied to Vertical Retrace
• The viewer lacked any real-time control
• How can we overcome these limitations?

3
Decoupling move from draw

• Our program-loop had used logic like this
• do
• vsync() // delay until start of the next
  retrace
• hide_ball() // erase the ball
• move_ball() // adjust its location
• show_ball() // redraw the ball
• --count // decrement a counter
• while ( count gt 0 )

4
Linux provides interval timers

• include ltsys/time.hgt
• struct itimerval itval, itold
• itval.it_value.tv_sec 2
• itval.it_value.tv_usec 0
• itval.it_interval.tv_sec 0
• itval.it_interval.tv_usec 10000
• setitimer( ITIMER_REAL, itval, itold )
• (See the man page for additional details)

5
SIGALRM

• When timer expires our application gets
  notified, by being sent a signal from Linux
• Normally an application gets terminated if the
  SIGALRM signal is delivered to it
• But we can alter that default behavior, by
  installing a signal-handler that we design
• We can move-the-ball when SIGALRM is received,
  regardless of Vertical Retrace

6
Our signal-handler

• void on_alarm( int signum )
• // modify these global variables
• ball_xcoordinate xincrement
• ball_ycoordinate yincrement
• // The signal() function installs our handler
• signal( SIGALRM, on_alarm)

7
Main program-loop revised

• We can now omit ball-movement in main loop
• do
• vsync() // delay until start of the next
  retrace
• hide_ball() // erase the old ball
• oldx newx oldy newy // remember new
  position
• show_ball() // redraw the new ball
• --count // decrement a counter
• while ( count gt 0 )
• Ball-movement is managed by signal-handler

8
Giving the user control

• Linux supports asynchronous terminal i/o
• We can reprogram the terminal console so a SIGIO
  signal will be sent to our program whenever the
  user decides to press a key
• And we can install a signal-handler of our own
  design that executes if SIGIO arrives
• This will allow a user to control our game

9
Noncanonical terminal i/o

• We already learned how to reprogram the terminal
  to allow raw keyboard input
• include lttermios.hgt
• struct termios tty
• tcgetattr( 0, tty ) // get tty settings
• tty.c_lflag ( ICANON ECHO ISIG )
• tty.c_cc VMIN 1 tty.c_cc VTIME 0
• tcsetattr( 0, TCSAFLUSH, tty ) // install

10
Handling a key-press

• Heres a simple signal-handler that lets a user
  decide to terminate our program (by hitting the
  ltESCAPEgt-key) instead of the program itself
  deciding to quit when a counter reaches zero
• void on_input( int signum )
• int inch 0
• read( 0, inch, 4 )
• if ( inch ESCAPE_KEY ) done 1

11
Enabling asynchronous I/O

• Now we need to install our signal-handler,
  specify which program will receive SIGIO, then
  enable the delivery of these signals
• signal( SIGIO, on_input )
• fcntl( 0, F_SETOWN, getpid() )
• int flagval fcntl( 0, F_GETFL, NULL )
• flagval O_ASYNC // turn on flag-bit
• fcntl( 0, F_SETFL, flagval )

12
Program-loop revised again

• done 0
• do
• oldx xloc, oldy yloc // remember
• draw_ball() // use current location
• vsync() // await next retrace
• hide_ball() // erase previous ball
• while ( !done )
• // xloc, yloc, and done get changed by
  handlers

13
Enhancment more user-control

• In pong game the user moves a paddle
• The paddle can only be moved left or right
• Lower wall of the playing-court is removed
• Ball will escape unless it hits the paddle
• Keyboard has left and right arrow keys
• Our input signal-handler could move the paddle
  whenever a user hits an arrow-key

14
In-class exercise 1

• Before you remove the game-courts lower wall,
  see if you can implement the paddle-movements
  (left or right) in response to a users hitting
  the left-arrow or right-arrow
• You will need to investigate the numerical
  code-sequence that Linux generates when a user
  hits one of these arrow-keys
• Our rawtty.cpp application may be useful!

15
In-class exercise 2

• For brevity, our animate2 demo-program removed
  the capability of users controlling the speed of
  the ball-movements (with an argument supplied on
  the command-line)
• Can you devise a way for users to exert
  real-time control over the balls speed by
  pressing keys while the program is running (for
  example, the key and the - key)?