By default, Processing's 3-D coordinates have the origin placed are the top left corner of the screen, and the Y axis pointing down. Calling the cameraUp() function this setting is overriden by the more common in VR with the origin at the center of the screen, and the Y axis pointing up.
void setup() {
fullScreen(P3D);
cameraUp();
}
void draw() {
background(0);
box(300);
}