// Configuramos el modo gráfico
REG_DISPCNT = MODE_5_2D | DISPLAY_BG2_ACTIVE;
VRAM_A_CR = VRAM_ENABLE | VRAM_A_MAIN_BG;
BGCTRL[2] = BG_BMP_BASE(0) | BgSize_B8_256x256;

// convertir = 1 -> aplicarle el filtro sobel
// convertir = n -> quitarle el filtro sobel
void imagenSobel(u8* bmp, u16* paleta, int longitud, int convertir) {
	int i, j;
	int x, y;
	u8 p1, p2, p3, p4, p6, p7, p8, p9, linea[256];
	u8 R, G, B, gris;
	u16* aux = (u16*)malloc(longitud/2);
 
	for(i = 0; i < longitud/2; i++) {
		R = paleta[i] & 0x1F;
		G = (paleta[i] >> 5) & 0x1F;
		B = (paleta[i] >> 10) & 0x1F;
		gris = div32(30*R, 100) + div32(59*G, 100) + div32(11*B, 100);
		paleta[i] = RGB15(gris, gris, gris);
	}
 
	if(convertir == 1) {
		for(i = 0; i < 256; i++) {
			for(j = 0; j < 256; j++) {
				p1 = (i-1)*256+(j-1); 	        // (x-1, y-1)
				p2 = (i-1)*256+j;		// (x, y-1)      -------------- 
				p3 = (i-1)*256+(j+1); 	// (x+1, y-1)  | p1  p2  p3 |
				p4 = i*256+(j-1);		// (x-1, y)      | p4 |p5| p6|
				p6 = i*256+(j+1);		// (x+1, y)     | p7  p8  p9 |
				p7 = (i+1)*256+(j-1); 	// (x-1, y+1)  --------------
				p8 = (i+1)*256+j;		// (x, y+1)
				p9 = (i+1)*256+(j+1); 	// (x+1, y+1)
 
				x = (p1+(p2+p2)+p3-p7-(p8+p8)-p9);
				y = (p3+(p6+p6)+p9-p1-(p4+p4)-p7);
				aux[i*256 + j] = sqrt((x*x) + (y*y));
			}
		}
	}
 
	for(i = 0; i < 256; i++) {
		paleta[i] = aux[i];
	}
}

nuevo_valor_pixel_p5 = SQRT((X*X) + (Y*Y))
X = (p1+(p2+p2)+p3-p7-(p8+p8)-p9)
Y = (p3+(p6+p6)+p9-p1-(p4+p4)-p7)