Interestsing, the problem is actually not the screen fading code. that works well.

The problem is that nothing gets drawn anymore as soon as you call updatePixels.
Not sure if i miss something here or if it is a bug. But i couldnt find any information about load/updatePixels not working with opengl.

1. import processing.opengl.*;
2. void setup() {
3.   size(600,400,OPENGL);
4.    background(255);
5. }
6. void draw() {
7.  ellipse(mouseX,mouseY,10,10);
8.  loadPixels();
9.  //updatePixels();
10. }

hmm but the fading doesnt work then... strange

this must be some kind of bug

maybe an already known issue, not all renderer work the same and have the same features. But i couldnt find any information about this problem.

if you're working on opengl render a rect on top of everything and change alpha from 0 to 1. that will do it and it will be alot faster than accessing the pixel data as you're doing right now

i thought about recommending it. but if you dont use an alpha that is at least 50 and more drawn stuff stays there and leaves some trails.

1. import processing.opengl.*;
   void setup() {
     size(600,400,OPENGL);
     background(0);
   
   }
   void draw() {
   fill(0,6);
   rect(0,0,width,height);
   fill(255,0,0);
   ellipse(mouseX,mouseY,10,10);
   }
   
   
   

here's a fade in/out:
you will clear the background anyhow

import processing.opengl.*;


float startTime, time;
float dir = 2;
float alpha = 0;
void setup()
{
  size(600,400,OPENGL);

    startTime = millis();
}


void draw()
{
    time = (millis() - startTime) * 0.001f;
   
    background( 0 );

    fill(255, 0, 0);
    ellipse( width*0.5+cos(time)*200, height*0.5+cos(time*4)*200, 100,100 );
   
   
    alpha += dir;
    fill( 0, alpha );
    rect( 0, 0, width, height );
   
    if( alpha < 0 || alpha > 255 ) dir *= -1;
}

Ah ok. now i got your idea. Clever way to use the fade.

Well I tried the code pixelnerve wrote above, but it's not fading the whole screen, but the ellipse itself.

What I want is fading the whole screen gradually....

If you run the very first code I uploaded in P2D you'll see.

And yes, I know to paint a slightly transparent rect before anything is drawn,

but most of the time it leaves ghostly residue....

Anyone has some idea?

Ok i should get some kind of gold medal for this.
Problem solved. As it turns out, your code simple needed more cowbell so i added some.

1. import javax.media.opengl.*;
2. import processing.opengl.*;
3. void setup() {
4.   size(600,400,OPENGL);
5.   background(255);
6. }
7. void draw() {
8.   cowbell();
9.   ellipse(mouseX,mouseY,10,10);
10.   fade();
11. }
12. void cowbell(){
13.   PGraphicsOpenGL pgl = (PGraphicsOpenGL) g;
14.   GL gl = pgl.beginGL(); 
15.   gl.glClearColor(0, 0, 0, 0);
16.   gl.glClear(GL.GL_DEPTH_BUFFER_BIT);
17.   pgl.endGL();
18. }
19. void fade() {
20.   int r,g,b;
21.   loadPixels();
22.   for (int i=0;i<pixels.length;i++) {
23.     r=pixels[i]>>16&255;
24.     g=pixels[i]>>8&255;
25.     b=pixels[i]&255;
26.     pixels[i]=color(r-1,g-1,b-1);
27.   }
28.   updatePixels();
29. }

but remember, after calling fade you can not draw anything onto the screen untill you've called cowbell()

Yes! It works!

Thanks a lot, bjordal.

You surely deserve a gold medal for this.  :-)

I quite don't get your code yet, but I'll study more.

Thanks!

ofcourse mine works.
with alphablend mode on and rendering a fullscreen quad on top of everything works fine. and it's faster than loadPixels/updatePixels

my process:
renderScene()
drawFadeoutQuadOnTopOfEverything();


you process:
renderScene()
grab pixel data from opengl framebuffer
loop thru all pixel buffer and update pixel
write it back to opengl framebuffer

Still, if it works for you it's fine, just keep the other method in mind when you need the extra power spent in that effect