3) shift that image -x, so that the freshly drawn circle  appears in the same place and trace goes left.

I believe the variable x, and consequently coord, refer to circle's diameter; not its canvas position?

Here, in Processing v1.5.1, I was able to see some vanishing trace effect while circle is deflating.

A slightly different effect when anti-aliasing is turned on. Check it out for yourself:

final static short side = 800, center = side >> 1;
final PGraphics pg = createGraphics(side, side, P2D);

void setup() {

  size(side, side, P2D);
  frameRate(50);

  pg.beginDraw();

  pg.noSmooth();
  //pg.smooth();  <--- experiment uncomment this line! 
  pg.ellipseMode(RADIUS);
  pg.stroke(-1);

  pg.endDraw();
}

void draw() {

  float radius = cos( radians(frameCount) ) * center;

  pg.background(0, 10);
  pg.ellipse(center, center, radius, radius);

  image(pg, 0, 0);
}

I think i just gave a wrong snippet(
What i got now is a circle moving from left to right and leaving the trace.
But i want it to be drawn right in the centre of the screen every time, and the trace go left, like the cam is following the circle.

Here's my code

1. PGraphics pg;
   int x = 0,side = 400;
   
   void setup() {
   
     size(side, side,P2D);
   
     pg = createGraphics(side, side, P2D);
     pg.fill(255); 
   }
   
   
   void draw() {
   
     
     pg.beginDraw();
   
     pg.background(0,5);
     x+=1;
     if(x >= width)
      x = 0;
     
     float noise = noise(x)*50 + 10;
     pg.ellipse(x,width/2,noise,noise);
     pg.endDraw();
     println(x);
     image(pg, 0, 0);
    
   }
   

Your description is very unclear.

At first I thought you meant this...

Code Example 1

1. PGraphics pg;
2.  
3. void setup() {
4.   size(400, 400);
5.   pg = createGraphics(width/5, height/5, JAVA2D);
6.   pg.beginDraw();
7.   pg.smooth();
8.   pg.noStroke();
9.   pg.ellipse(pg.width/2, pg.height/2, pg.width-5, pg.height-5);
10.   pg.endDraw();
11.   imageMode(CENTER);
12.   fill(0, 5);
13. }
14.  
15. void draw() {
16.   rect(0, 0, width, height);
17.   image(pg, mouseX, mouseY);
18. }

But then I thought maybe you meant something like this...

Code Example 2

1. int numCircles = 15;
2.  
3. void setup() {
4.   size(400, 400);
5.   noStroke();
6.   smooth();
7. }
8.  
9. void draw() {
10.   background(0);
11.   for (int i=numCircles-1; i>=0; i--) {
12.     fill(255-float(i)/numCircles*255);
13.     float rad = 100 - i * 1.5;
14.     ellipse(width/2 + i * sin(frameCount*0.1)*12, height/2, rad, rad);
15.   }
16. }

I am really sorry, i'll do my best to make clear what i need.
So, i am making an audio visualisation sketch using Minim library. In the original code the radius of circle depends on apmlitude of audio signal. For snippet i gave above i replaced that with simple noise function.
So i got a sketch (in my prev message) that draws a cricle that shifts from left to right and leaves a trace - as clear as i could explain.
But i need it to be drawn always in the centre of the screen! It should look like circle is moving from left to right, it leaves a trace and camera is following a circle.

I thought if i shift a circle by X and shift an image by -X that would be it. But when i do that, trace and circle are drawn in the same place, i mean nothing moves at all. Like this:

May I propose an optimized function shiftImage(), caching some variables?  
It helps a little w/ a higher resolution!  

void shiftImage(int dX, int dY) {
  PImage source = pg.get();

  int w = pg.width;
  int h = pg.height;
  int l = pg.pixels.length;

  for (int y=0; y<h; y++)
    for (int x=0, iS; x<w; x++) {
      iS = w*(y+dY) + x+dX;

      if (iS >= 0 & iS < l)
        pg.pixels[w*y + x] = source.pixels[iS];
    }

  pg.updatePixels();
}

Thank you everyone!) Problem is solved and you can check out some acid audio visualisation i got so far)

1. import ddf.minim.*;
2. import ddf.minim.signals.*;
3. import ddf.minim.analysis.*;
4. import ddf.minim.effects.*;
5. int deathTime;
6. Minim minim;
7. AudioPlayer song;
8. AudioMetaData meta;
9. PGraphics pg;
10. float x,size;
11. int hue;
12. void setup() {
13.   size(800,600);
14.   pg = createGraphics(width, height);
15.   pg.beginDraw();
16.   pg.colorMode(HSB,360,100,100);
17.   pg.smooth();
18.   pg.endDraw();
19.   minim = new Minim(this);
20.   song = minim.loadFile("Your-audio-file.mp3");
21.   meta = song.getMetaData();
22.   song.play();
23.   x = 0;
24.  
25. }
26. void draw() {
27.  if(x > 360) x = 0;
28.  pg.beginDraw();
29.  pg.fill(0,10);
30.  pg.rect(0,0,width,height);
31.  pg.fill(x++,100,60);
32.  size = song.mix.get(0)*800;
33.  if(size>640) size =size*0.8;
34.    pg.ellipse(width/2,height/2,size,size);
35.  pg.endDraw();
36.  
37.  shiftImage(1,0);
38.  
39.  if(size>300) {
40.  pg.beginDraw();
41.  pg.stroke(random(360),100,50);
42.  pg.translate(width/2,height/2);
43.  for(int i = 0; i < 3; i++) {
44.  pg.rotate(random(360));
45.  pg.line(0,0,width/2,height/2);
46.  }
47.  pg.endDraw();
48.  }
49.  image(pg,0,0);
50.  
51. }
52.  
53.   void shiftImage(int dX, int dY) {
54.   PImage source = pg.get();
55.   int w = pg.width;
56.   int h = pg.height;
57.   int l = pg.pixels.length;
58.   for (int y=0; y<h; y++)
59.     for (int x=0, iS; x<w/2; x++) {
60.       iS = w*(y+dY) + x+dX;
61.       if (iS >= 0 & iS < l)
62.         pg.pixels[w*y + x] = source.pixels[iS];
63.     }
64.    
65.     for (int y=0; y<h; y++)
66.     for (int x=w/2, iS; x<w; x++) {
67.       iS = w*(y+dY) + x-dX;
68.       if (iS >= 0 & iS < l)
69.         pg.pixels[w*y + x] = source.pixels[iS];
70.     }
71.   
72.   pg.updatePixels();
73. }
74. void stop()
75. {
76.   song.close();
77.   minim.stop();
78.   super.stop();
79. }
80. void mousePressed() {
81.  song.close();
82. }
83. void keyPressed() {
84.   stop();
85. }

Über nice, realisfake!

Hey! I did some other tweaks in the shiftImage() function:

• Merged the 2 paired y,x for loops there into 1 pair only
• Made the light beams be drawn after the pg backbuffer
• A pause feature, etc

I uploaded the sketch on OpenProcessing with reference to you, guys)
http://www.openprocessing.org/sketch/90878