I just noticed that the transparency of the .png files(and maybe other transparencies too) sometimes just won't work correctly if you move the .png thru the Z axis.
To show my point this is the code:
//////////////CODE
import processing.opengl.*;
PImage t1,t2; int r=0; float x=0; void setup(){ //Asuntos de pantalla size(400, 400, OPENGL);
