click mouse to go to next screen
you mustn’t use background() at the begin of a state but use fill(.....); rect(0,0,width,height); I think
Chrisir 
// carousel with a transition between the screens (states)
//
// states
final int stateGame = 0; // consts
final int statePause = 1;
final int stateC = 2;
int state = stateGame; // current
// for transition
int statePrevious=stateGame;
boolean transitionFlag=false;
float transitionAmt=0; // for lerp
// -------------------------------------------------
void setup() {
// init (runs only once)
size(800, 600);
} // func
void draw() {
// runs on and on
if (transitionFlag) {
// show the transition
manageTransition();
}//if
else {
// show the screen (transition is over)
showState(state);
text("press mouse to go on ", 120, 16);
//
}//else
//
} // func
// functions for states - called from draw() -------------------------------
void manageTransition() {
float xprevious=0, xcurrent=0;
xprevious = lerp( 0, -width, transitionAmt);
xcurrent = lerp( width, 0, transitionAmt);
pushMatrix();
translate(xprevious, 0);
showState(statePrevious);
popMatrix();
pushMatrix();
translate(xcurrent, 0);
showState(state);
popMatrix();
//
transitionAmt+=0.025; // for lerp
// Is the transition finished?
if (transitionAmt>=1) {
// yes, reset
transitionAmt=0;
transitionFlag=false;
}
}
void showState(int state) {
switch (state) {
case stateGame:
// Game
handleStateGame();
break;
case statePause:
// Pause
handleStatePause();
break;
case stateC:
// Pause
handleStateC();
break;
default:
// error
println("Error number 939; unknown state : "
+ state
+ ".");
exit();
break;
} // switch
}//func
void handleStateGame() {
// Game
fill(11);
rect(0, 0, width, height);
fill(244, 3, 3); // red
//
noStroke();
fill(255, 2, 2) ;
rect(100, 100, 100, 100);
fill(255, 2, 255) ;
// rect(300, 100, 100, 100);
} // func
void handleStatePause() {
// Pause
fill(255);
rect(0, 0, width, height);
fill(244, 3, 3); // red
text ("Pause.... "
+ " ... ", 210, 313);
} // func
void handleStateC() {
//
fill(255);
rect(0, 0, width, height);
fill(0);
text("state C", 111, 122);
}
// ----------------------------------------
// keyboard input
void keyPressed() {
switch (state) {
case stateGame:
// Game
keyPressedForStateGame();
break;
case statePause:
// Pause
keyPressedForStatePause();
break;
case stateC:
//
//
break;
default:
// error
println("Error number 1039; unknown state : "
+ state
+ ".");
exit();
break;
} // switch
} // func
// ----
void keyPressedForStateGame() {
state++;
} // func
void keyPressedForStatePause() {
if (key == CODED)
{
if (keyCode == UP) {
//
} else if (keyCode == DOWN) {
// none
}
if (keyCode == LEFT) {
//
} else if (keyCode == RIGHT) {
//
} else {
// do nothing
} // else
} // if (key == CODED) {
else
{
// not CODED ----------------------
if (key == 'p') {
//
state = stateGame;
} else {
// do nothing
} // else
} // else not CODED
} // func
// -------------------------------------------------------
// Mouse input
void mousePressed() {
//
statePrevious=state;
transitionFlag=true;
transitionAmt=0; // for lerp
switch (state) {
case stateGame:
// Game
mousePressedForStateGame();
break;
case statePause:
// Pause
mousePressedForStatePause();
break;
case stateC:
// C
mousePressedForStateC();
break;
default:
// error
println("Error number 1139; unknown state : "
+ state
+ ".");
exit();
break;
} // switch
//
} // func
void mousePressedForStateGame() {
//
state++;
//
} // func
void mousePressedForStatePause() {
//
state++;
} // func
void mousePressedForStateC() {
//
state=0;
} // func
// =========================================