you could repeatedly render the same object to screen and create an array to store "old" positions as the trail and decrement the colour or increase the alpha value. I will try to post some code tomorrow to illustrate what i mean.

the easiest way (and maybe the fastest one):
instead of erasing the whole screen with
background(255) every frame, draw a semi transparent rect:
Code:

void draw(){
 fill(255,64);
 rect(0,0,width,height);

 //draw your stuff
}
 

this makes older frames fade out slowly. example
doing something like background(grayvalue, alpha) doesn't work...

regards,
postpop

the only trouble with rendering a transparent image on screen is that everything will blur. On my project SADI I used a structure that created an object for each character which would render it and and store/modify such information as the font, position, skew and alpha value. Unfortunately this was written directly in Java (and 1.5 to boot) so is unlikely to port well to processing.

The central idea is that there was an IF block that dealt with  adjusting the alpha value, working on time elapsed since initialized although you could do this by frame count as well.
Code:

//fade out function
public void updateFade(){
if(this.startFrame>this.minFullFrames){



this.alphaVal=1f-((float)(this.startFrame-this.minFullFrames)/((float)this.minFullFrames*(float)this.fadeOutSpeed));
}
//catch out of bounds
if(this.alphaVal<0){
this.fadeVal=0;//makes sure that fadeVal is min 0 
} else if(this.alphaVal>1){
this.alphaVal=1;//limists to maximum of 1
}
}
 

this would then be called by the parent object so that:
Code:

//when drawing
int deadObjects=0;
for(i=0;i<array.length;i++){

array[i].updateFade;
if(array[i].alphaVal<=0){
deadObject++;
}
//remove the dead objects
drawnObjects newArray= new drawnObjects[array.length];
for(int i=deadObject;i<array.length;i++){
newArray[i-deadObject]=array[i];
}
array = newArray;
//draw objects
for(int i=0;i<renderedObjectsNumber;i++){
array[i].draw;
}
 

I hope this makes sense. I realise it's not as computationally effective as the other version but allows the rest of the screen not to be blurred.

I'm trying to get postpop's suggestion working in OpenGL mode.  Here's draw  my code:

import processing.core.*;
import processing.opengl.*;
import damkjer.ocd.*;

public class MBlurTest extends PApplet {

//time management stuff
private long tO;//time at last frame
private long tN;//time at this frame
private long tT;//total time since start
private long dt;//delta time (new time - old time)

//camera stuff
private Camera myCamera;

//trig object
private FastTrig ft = new FastTrig(720);

public void setup() {
size(800,600,OPENGL);
framerate(30);
noStroke();

//time management initialization stuff
tO = System.currentTimeMillis();
tT = 0;

//myCamera = new Camera(this,0,0,-300);
myCamera = new Camera(this,0,0,-600);
myCamera.feed();
background(0);
}

public void draw() {
updateTime();
lights();
pushMatrix();
fill(0,0,255,255);
float x = (float)(100 * ft.sine(tT/40));
float y = (float)(100 * ft.cosine(tT/40));
translate(x,y,(float)0);
sphere(15);
popMatrix();

/*
fill(255,0,0);
beginShape(QUADS);
vertex(-50,-50,-295);
vertex(50,-50,-295);
vertex(-50,-50,-295);
vertex(-50,50,-295);
endShape();*/


pushMatrix();
fill(255,0,0,8);
translate(0,0,-290);
box(100);
popMatrix();


}

private void updateTime(){
tN = System.currentTimeMillis();
dt = (tN - tO);
tO = tN;
tT += dt;
}
}

FastTrig is a utility class I wrote that just calculates sine, cosine, and tan to a given resolution (720 ticks in a circle, in this example) and stores it in an array for faster trig calculations.

What happens is that, if you comment out the line that draws the transparent box in the foreground, the sphere moves around in a circle, tracing out a path (but not fading, obviously).  If you leave the box(100); line in, the sphere is drawn in its' initial position and then the box is drawn over top of it; the box slowly becomes more opaque and obscures the sphere, but you don't see the sphere's motion beyond that first point.

I tried the rect(0,0,width,height) solution from the example you posted, but that didn't work for me either.. thoughts?

In that code, I'd given the box a non-black color so that I could see where it was being drawn. The behavior is the same whether it's black or some other color.

The behavior is also the same in P3D and OpenGL.  I will try with a white background and see if that makes a difference, althoough in the application I'm eventually intending, I need a black bg.

Cheers,
Ryan

Hmm, it's still behaving oddly.

I think I may take the code from the example that was posted earlier, strip out the bits related to drawing the 3d network, and see if I can't work forward from there..

Thank you very much for your help!

Ryan

Edited:


It works for me too. This is quite strange -- the only significant difference I can see between your code and my most recent attempt is using the JOGL object to clear the depth buffer, rather than the PGraphics3

Anyhow, thank you so much for spending what looks like a substantial chunk of time saving my rear.  If you're ever in Ithaca, NY, I seriously owe you a drink or three!

Cheers,
Ryan