1. import processing.opengl.*;
   
   void setup() {
     size(600, 600, OPENGL);
   }
   
   void draw() {
     background(255);
     translate(width/2, height/2, 0);
   
     float rotX = map(mouseY, 0, height, radians(-90), radians(90));//look up/down
     float rotY = map(mouseX, 0, width, radians(90), radians(-90));//look left/right
   
     rotateX(rotX);
     rotateY(rotY);
   
     sphere(50);
   
     translate(0, 0, -50);
     stroke(0, 0, 255);
     box(50/4, 50/4, 50);
   }
   
   

Vino: you are experiencing a situation called gimbal lock
Vino: which is a penalty of using euler angles
Vino: which is what you're doing
Vino: here's what you need to do
Vino: you can convert a vector (x, y, z) coordinates in global space
Vino: to a euler angle
Vino: so you have
Vino: if you want the mouse to look top-left, (1, 1, 0)
Vino: or bottom right (-1, -1, 0)
Vino: that's just an example who knows what the real values would be
Vino: but then you say, Vector(1, 1, 0).ToEuler()
Vino: and you pass those euler values into rotatex(euler.x)
Vino: rotatey(euler.y)
Vino: and that way you know he's always looking in the right place