These are probably artifacts because of antialiasing on the borders. It is apparently slightly better by using noStroke() and removing the stroke() call. Also better by drawing the erasing circle with r+1. Can't get rid entirely of the artifacts with my quick tests. That's probably why the most common idiom in Processing is just to erase the whole background in draw() and just redraw everything. Which won't work with threads, of course.

Hello phi.lho,
nice to meet you again !

yes i already tried to erase circle with r+1, i tried too to use background but with "blinding"... and i told me "no it's not normal to do it it must be a good reason, a mistake in my code".
So according to you the code is good it'd be a processin broblem!

Actually, I was mistaken... Like you, I had a naive view on the way threads work (my experience with them is rather limited, I must admit).

I had an epiphany once I saw that the black circle had a partial green ghost (once I got rid of stroke): we forgot that fill() and other similar commands are global to the PApplet, so if you change it, you change it for all running threads!
I changed the code to use synchronized (on g, the global PGraphics of the PApplet, shared by the threads), so when doing an atomic change, the other threads are blocked, doesn't try to work on the same graphics.

1. Balle b,b1,b2;
   
2. int fond=155;
3. color rouge=color(255,0,0);
4. color vert=color(0,255,0);
5. void setup() {
6.   size(200,300);
7.   background(fond);
8.   ellipseMode(RADIUS);
9.   noStroke();
10.   noSmooth();
11.  
12.   b=new Balle(0,0,10,10,0);
13.   b.start();
14.   b1=new Balle(0,0,5,2,rouge);
15.   b1.start();
16.   b2=new Balle(0,0,8,5,vert);
17.   b2.start();
18. }
19. 
    
20. void draw(){
21. }
22. 
    
23. class Balle extends Thread{// Ball
    
24.   int X;
25.   int dx;
26.   int Y;
27.   int dy;
28.   int r;
29.   int v;
30.   color couleur;
31.  
32. Balle (int x,int y,int ra,int vi,color c){
33.   X=x;
34.   Y=y;
35.   dx=1;dy=1;
36.   r=ra;
37.   v=vi;
38.   couleur=c;
39. }
40. 
    
41. void start(){
42.   super.start();
43. }
44.  
45. void run(){
46.   while (X<2000){
47.     Efface(X, Y);
48.     Deplace();
49.     Dessine(X, Y);
50.     try {
51.         sleep((long)(100/v));
52.       } catch (Exception e) {
53.       }
54.   }
55. }
56. 
    
57. void Efface(int x, int y) { // Erase
    
58.   synchronized (g) {
59.     fill(fond);
60.     ellipse(x,y,r,r);
61.   }
62. }
63. 
    
64. void Dessine(int x, int y) { // Draw
    
65.   synchronized (g) {
66.     fill(couleur);
67.     ellipse(x,y,r,r);
68.   }
69. }
70.  
71. void Deplace(){ // Move
    
72.   X += dx;
73.   Y += dy;
74.   if (X+r>width)dx=-1;
75.   if (X<r)dx=1;
76.   if (Y+r>height)dy=-1;
77.   if (Y<r)dy=1;
78. }
79.  
80. }

I split the operation in two (well, three), so the other threads get a change to do something (more or less complex, so more or less time consuming) specific to themselves (not changing the global state). But here, the synch could be done on a higher level:

1. void run(){
2.   while (X<2000){
3.     synchronized (g) {
4.       Efface(X, Y);
5.       Deplace();
6.       Dessine(X, Y);
7.     }
8.     try {
9.         sleep((long)(100/v));
10.       } catch (Exception e) {
11.       }
12.   }
13. }
14. void Efface(int x, int y) {
15.     fill(fond);
16.     ellipse(x,y,r,r);
17. }
18. void Dessine(int x, int y) {
19.     fill(couleur);
20.     ellipse(x,y,r,r);
21. }

 

This scales:

1. Balle[] balles = new Balle[100];
2. int fond=155;
3. 
   
4. void setup() {
5.   size(200,300);
6.   background(fond);
7.   ellipseMode(RADIUS);
8.   noStroke();
9.   noSmooth();
10. 
    
11.   for (Balle ball : balles) {
12.     ball = new Balle(0, 0, int(random(5, 20)), int(random(2, 20)),
13.         color(random(0, 255), random(0, 255), random(0, 255)));
14.     ball.start();
15.   }
16. }

Thanks phi.lho, I learnt a lot from this example!

Thanks phi lho,

I never heard about synchronized so i seach explanations and understand that it is very important for Threads !!

see here

your help was great!!!