I will make an answer as vague as your question: check the trigonometric functions, like sine and cosine, to get coordinates on a circle.

ok thx i will search with the trigonometric fonction
scuse me if my question is vague I'll try to be clear

i want an ellipse sliding on a arc like red arrow on my picture
it's control the volume for the fid

I hope to have been clear now if not let me know
Thx
 

By vague, I meant some code as starting point...

Here is a starting point, for example.
I made a circle instead of an arc, but the logic is the same, just restrain the amplitude of the angle.

1. int SIZE = 50;
2.  
3. float angle;
4.  
5. void setup()
6. {
7.   size(300, 300);
8.   smooth();
9. }
10.  
11. void draw()
12. {
13.   background(#8080FF);
14.   noStroke();
15.   fill(#0000FE);
16.   ellipse(width / 2, height / 2, SIZE - 20, SIZE - 20);
17.  
18.   noFill();
19.   stroke(#DDE0FF);
20.   strokeWeight(2);
21.   ellipse(width / 2, height / 2, SIZE, SIZE);
22.  
23.   fill(#C0C0F8);
24.   stroke(#8080FF);
25.   float x = SIZE / 2 * cos(angle);
26.   float y = SIZE / 2 * sin(angle);
27.   ellipse(width / 2 + x, height / 2 + y, SIZE / 5, SIZE / 5);
28.   angle += PI / 360;
29. }

a ok i know what you mean
i want an ellipse who will slide on a arc
this ellipse control the volume of this fiducial and this ellipse will be controlled with a finger in the tcursor page
with you previous code i have a small idea to how to make this

1. void Tobject()
   {
     float obj_size = object_size*scale_factor;
     
    
     Vector tuioObjectList = tuioClient.getTuioObjects();
     for (int i=0;i<tuioObjectList.size();i++) {
        TuioObject tobj = (TuioObject)tuioObjectList.elementAt(i);
        noStroke();
       
        if (+tobj.getSymbolID() == 0)
        {
          pushMatrix();
          float a = map(tobj.getAngle(),0,6.30,1.5,4.8);
         
          fill (0,0,255);
          translate(tobj.getScreenX(width),tobj.getScreenY(height));
            pushStyle();
              noFill();
              stroke(255,255,255,130);
              strokeWeight(4);
              strokeCap(ROUND);
           
              arc(-obj_size/2+18,-obj_size/2+18,70,70,1.5,4.8);
              arc(-obj_size/2+18,-obj_size/2+18,70,70,1.5,(a)%4.8);
             
              //This is the arc that will have an ellipse
              arc(-obj_size/2+18,-obj_size/2+18,70,70,5.5,7);
             
                pushStyle();
                  noStroke();
                  fill(255,255,255,130);
                  ellipse(-obj_size/2+34,-obj_size/2+50,15,15);
                popStyle();
              popStyle();
          rotate(tobj.getAngle());
          bezierRect(-obj_size/2,-obj_size/2,obj_size,obj_size,2,2);
          popMatrix();
        }
         else if (+tobj.getSymbolID() == 1)
        {
          pushMatrix();
          fill (255,0,0);
          translate(tobj.getScreenX(width),tobj.getScreenY(height));
          rotate(tobj.getAngle());
          bezierRect(-obj_size/2,-obj_size/2,obj_size,obj_size,2,2);
          popMatrix();
        }
        else if (+tobj.getSymbolID() == 2)
        {
          pushMatrix();
          fill(255,255,0);
          translate(tobj.getScreenX(width),tobj.getScreenY(height));
          rotate(tobj.getAngle());
          bezierRect(-obj_size/2,-obj_size/2,obj_size,obj_size,2,2);
          popMatrix();
        }
        else if (+tobj.getSymbolID() == 3)
        {
          pushMatrix();
          fill(0,255,0);
          translate(tobj.getScreenX(width),tobj.getScreenY(height));
          rotate(tobj.getAngle());
          bezierRect(-obj_size/2,-obj_size/2,obj_size,obj_size,2,2);
          popMatrix();
        }
        else if (+tobj.getSymbolID() == 32)
        {
          pushMatrix();
          fill(0,0,255);
          translate(tobj.getScreenX(width),tobj.getScreenY(height));
          rotate(tobj.getAngle());
          octagon();
          popMatrix();
        }
       }
   }
   
   void bezierRect(float x, float y, float w, float h, float xr, float yr) {
    float w2=w/2f, h2=h/2f, cx=x+w2, cy=y+h2;
    beginShape();
    vertex(cx,cy-h2);
    bezierVertex(cx+w2-xr, cy-h2, cx+w2, cy-h2+yr, cx+w2, cy);
    bezierVertex(cx+w2, cy+h2-yr, cx+w2-xr, cy+h2, cx, cy+h2);
    bezierVertex(cx-w2+xr, cy+h2, cx-w2, cy+h2-yr, cx-w2, cy);
    bezierVertex(cx-w2, cy-h2+yr, cx-w2+xr, cy-h2, cx, cy-h2);
    endShape();
   }
   
   void octagon()
   {
     float obj_size = object_size*scale_factor;
   rect(-obj_size/2,-obj_size/2,obj_size,obj_size);
   rotate(PI/4.0);
   rect(-obj_size/2,-obj_size/2,obj_size,obj_size);
   }