Perhaps just getting the current view as a PImage and then filtering that is even easier.

Code Example

1. import peasy.*;
2. PeasyCam cam;
3.  
4. void setup() {
5.   size(500, 500, P3D);
6.   cam = new PeasyCam(this, 100);
7.   cam.setMinimumDistance(1);
8.   cam.setMaximumDistance(100000);
9. }
10.  
11. void draw() {
12.   rotateX(-.5);
13.   rotateY(-.5);
14.   background(0);
15.   fill(255, 0, 0);
16.   box(30);
17.   pushMatrix();
18.   translate(0, 0, 20);
19.   fill(0, 0, 255);
20.   box(5);
21.   popMatrix();
22.  
23.   PImage screenshot = get();
24.   screenshot.resize(width/4, height/4);
25.   screenshot.filter(BLUR,2);
26.   cam.beginHUD();
27.   fill(255);
28.   rect(8,8,screenshot.width+3,screenshot.height+3);
29.   image(screenshot, 10, 10);
30.   cam.endHUD();
31. }

If you want faster blur, see this topic and other earlier ones. This has been discussed extensively with example algorithms much faster than Processing's built-in blur function (which is arguably the slowest function in the whole of Processing).

If you want to get really serious with gpu-accelarated filtering, know that the GLGraphics library facilitates GLSL shaders and has a convenient GLTextureFilter class to facilitate the use of shaders (for example a fragment shader) on a texture.

I didn't reply yet because I was trying to understand, which I still is pretty much a WIP, but I get this is the path I should follow...

I have a similar problem, I'd like to play around more with DOF and blur but then I cannot figure out how to apply the blur effect depending on the z coordinate for example or depending on the distance of each element with the camera.

Let's say I make 200 spheres with different z coordinate how can I adapt each of their blur? Shall I use GLModelEffect on each Model? This will make it very slow no? Is it even possible to apply a blur filter with GLModelEffect... On another hand if I apply filters on texture of each sphere with GLTextureFilter that's just going to blur the texture.

A suggestion on another post was to use a glsl fragment-shader, that runs a blur-kernel (size depending on the scene-depth) but I'm just scratching the surface of GLGraphics at the moment and I'm afraid creating such a shader is out of my league.

Alternatively could also use a "hand coded" blur filter but then it will too slow...

Any advice?