Dev - C++ - necesito ayuda con limites

#include <iostream>

#include "CImg.h"

using namespace cimg_library;

using namespace std;

int main(void) {

	int alto = 500, ancho = 500;

	int x0 = 0, y0 = 0, x1 = 50, y1 = 50;

	int azul_perro[3] = {0, 255, 255};

	int azul[3] = {0,0,255};

	int verde[3] = {0,255,0};

	int rojo[3] = {255,0,0};

	int cafe[3] = {128,0,64};

	int naranja[3] = {255,128,0};

	int amarillo[3] = {255,255,0};

	int rosa[3] = {255,128,128};

	int violeta[3] = {128,0,255};

	int gris[3] = {128,128,192};

	int negro[3] = {0,0,0};

	int blanco[3] = {255,255,255};

	CImg <int> imagen(ancho,alto,1,3,255);

	CImgDisplay ventana(ancho, alto, "Game",0);

		while(!ventana.is_closed()){

		if(ventana.is_keyARROWRIGHT()){

		x0++;

		x1++;

		imagen.fill(255);

		imagen.draw_rectangle(x0,y0,x1,y1,violeta);

		}

		if(ventana.is_keyARROWDOWN()){

		y0++;

		y1++;

		imagen.fill(255);

		imagen.draw_rectangle(x0,y0,x1,y1,rojo);

	}

		if(ventana.is_keyARROWLEFT()){

		x0--;

		x1--;

		imagen.fill(255);

		imagen.draw_rectangle(x0,y0,x1,y1,naranja);

	}

		if(ventana.is_keyARROWUP()){

		y0--;

		y1--;

		imagen.fill(255);

		imagen.draw_rectangle(x0,y0,x1,y1,cafe);

	}

		ventana.display(imagen);

		ventana.wait(10);

	}

}