straightforward click drag to rotate camera around object

rotatecamera.pde

float theta;
float phi;
float r;

void setup(){
  size(640, 360, P3D);
  smooth();
  theta=0;
  phi=0;
  r=100;
}

void mouseDragged(){
  theta += (mouseX-pmouseX)*0.01;
  phi -= (mouseY-pmouseY)*0.01;
}

void draw(){
  
  float x = cos(phi)*cos(theta)*r;
  float z = cos(phi)*sin(theta)*r;
  float y = sin(phi)*r;
  
  lights();
  background(0);
  
  float up = floor((PI/2-phi)/PI)%2==0 ? 1 : -1;
  
  camera(x, y, z, // eyeX, eyeY, eyeZ
         0.0, 0.0, 0.0, // centerX, centerY, centerZ
         0.0, up, 0.0); // upX, upY, upZ
  
  noStroke();
  beginShape();
  vertex(10,0,0);
  vertex(0,0,10);
  vertex(0,10,0);
  vertex(10,0,0);
  endShape();
  
}