The Art of Computer Science Part II

1. The Art of Computer SciencePart II Bill Klinger – RVCC Computer Science Dept.

2. The Art of Computer Science • Architecture • structure of the processes • interaction of the processes • Algorithms • clever • efficient • Making the electrons dance • We will examine the “art” of a computer game

3. Sockets and Threads in Action

4. The Art of Computer SciencePart II • Art in Algorithms • real-time problems and solutions • Examples • collaborative Pong • collaborative Driving

5. Homework • How do we keep users in synch when the timing intervals are much smaller than potential variances? • In a driving game, we move ahead by advancing frames based upon our speed. Given: n = frame drawings per second m = max speed of car (in mph) s = current speed of car (in mph) What algorithm should we use to draw the frames?

6. Problems in Real-time Games • Need to keep the action going • But • there is network delay • PCs run at different speeds • may be other activity on a PC • PCs will get out of synch

7. Multi-user Pong

8. Dead Reckoning • Definition • A method of estimating the position of an craft without astronomical observations, as by applying to a previously determined position the course and distance traveled since. • Predictive calculation based on inference; guesswork

9. Dead Reckoning in Pong • Predict future position based upon last position and action • know ball x, y, dx, dy • know paddle y, dy • bounce off walls and paddles • Adjust, if necessary, when get new info • Problem – how to do this when listening to server and user? • another thread

10. The Electrons Dancing

11. Let’s Try It • Keep in mind • Threads that get your input • Threads that listen to the server • Threads that move • Sockets • Pong Exercise • login with guest name and password • go to Academic Departments • go to Computer Science • click on William Klinger faculty link • click on Participant Material on lower left side of page • click on Pattern Exercise

12. Art in Algorithms - 3 • Problem: how to advance the frames to show the speed of a car?

13. Art in Algorithms - Speed • We update the graphics n times per second • But we don’t advance the frames on each update • if at half speed, update every other frame • if at quarter speed, update one cycle, skip three cycles • if at three-fourths speed, update three cycles, skip one

14. Speed % of #CYCLES #CYCLES MAXSPEEDADVANCINGNOT MOVING 0 0 all 20% 1 4 25% 1 3 33% 1 2 50% 1 1 66% 2 1 75% 3 1 80% 4 1

15. How Long to Wait? • Let: c = cycles per second (game loops) s = speed m = max speed w = number of cycles not moving • Then w = total cycles waiting / total cycles moving w = [ c * (1 – (s / m ) ) ] / [ c * (s / m) ] w = (1 – (s / m ) ) / (s / m) w = ( m / s ) - 1 c’s cancel multiply num and denom by m / s

16. The Speed Formula • If speed < .5 max speed max speed # wait cycles = speed - 1 • If speed >= .5 max speed 1 # adv cycles = # wait cycles Note this is independent of the # cycles per second!

17. Speed Algorithm countCycles++; if( speed < (MAXSPEED / 2 )) { // the algorithm is to wait waitcycles times waitCycles = (MAXSPEED / speed) - 1 ; if( countCycles > waitCycles ) { // waited enough cycles now advance the car countCycles = 0; countMoves = 1; trackLoc++; } // end if - no else - don't advance the car } // end moving if .5 maxspeed

18. More Speed // the speed is .5 maxspeed domoves = 1.0 / ((MAXSPEED / speed) - 1 ); if( countmoves <= domoves) { // advance the car countmoves++; countcycles = 0; trackLoc++; } // end if advancing else { // pause a frame and reset countcycles = 0; countmoves = 0; }

19. I Feel the Need for Speed • Think about • threads listening to your input • threads listening to the server • threads moving the car • server threads sending moves • Driver Exercise • go back to Participant Material • click on Driver Exercise

20. The Art of Computer Science • You can admire the graphics • You can read the code • Some art lives where you can’t see it