the up values must be
0, 1, 0)

That's the default UP of the camera, see
http://www.processing.org/reference/camera_.html

Thank you for the prompt reply...

no, it's not a limitation.

The default values are camera(width/2.0, height/2.0, (height/2.0) / tan(PI*60.0 / 360.0), width/2.0, height/2.0, 0, 0, 1, 0).

Try 0, 1, 0) for UP, at the moment you have 0,0,1

Okay, I just tried that and the screen is entirely black. I'm fairly certain it is because the up vector is now set as the same direction as where the camera is pointing, which results in an error...

maybe look at

https://forum.processing.org/topic/i-don%E2%80%99t-understand-the-camera#25080000001364377

or

1. void setup() {
2.   size(640, 360, P3D);
3.   fill(0);
4.   strokeWeight (5);
5. }
6. void draw() {
7.   background(0);
8.  
9.   camera(50.0, -45.0, 230, // eyeX, eyeY, eyeZ
10.   0.0, 0.0, 0.0, // centerX, centerY, centerZ
11.   0.0, 1.0, 0.0); // upX, upY, upZ
12.  
13.   //
14.   stroke(255, 0, 0); // red - right
15.   line(-100.0, 0, 0, 200.0, 0, 0);
16.  
17.   stroke(0, 0, 255);  // blue - down
18.   line(0, -100, 0, 0, 200, 0);
19.  
20.   stroke(0, 255, 0);  // green - depth
21.   line(0, 0, -100.0, 0, 0, 200.0);
22. }

I also think the 3d camera is upside down; I have to declare camera(.... 0, 0, -1) for my sketch to behave the way I think it will. I recently ported a processing sketch over to openGL, and lo and behold I had to put gluLookAt(....., 0, 0, 1) instead of -1. I think that it might have to do with the fact that Processing is a left handed coordinate system instead of a right.