import processing.opengl.*;
import javax.media.opengl.*;
import javax.media.opengl.glu.*; //...

//...
GLU glu = new GLU();

void setup() {
  size(800, 600, OPENGL);
}

void draw() {
  PGraphicsOpenGL pgl = (PGraphicsOpenGL) g;
  GL gl = pgl.beginGL();
  
  gl.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT); //"replace" background() call;
  
  //these
  gl.glEnable( GL.GL_BLEND ); //to make blending possible !
  gl.glDisable(GL.GL_DEPTH_TEST);// to make them blended in every positions

  //...revove this bunch of code by taking OCD lib per example
  //LIB URL : http://www.gdsstudios.com/processing/libraries/ocd/
  gl.glMatrixMode( GL.GL_PROJECTION );
  gl.glLoadIdentity();
  glu.gluPerspective( 45.0, (float)width/(float)height, 0.1, 10000.0 );
  gl.glMatrixMode( GL.GL_MODELVIEW );
  gl.glLoadIdentity();
  glu.gluLookAt( 0.0, 1.0, 10.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0 );
  //
  
  //A TRIANGLE
  gl.glRotatef(millis()/10., 0, 1, 1);
  gl.glBegin(GL.GL_TRIANGLE_FAN);
    gl.glColor4f(1.0f, 0.0f, 0.0f, 0.7f); //glColor4f(r, g, b, alpha) instead of glColor3f(r, g, b)
    gl.glVertex3f( 0.0f, 1.0f, 0.0f);
    gl.glVertex3f(-1.0f,-1.0f, 1.0f);
    gl.glVertex3f( 1.0f,-1.0f, 1.0f);
  gl.glEnd();
    
  //ANOTHER TRIANGLE
  gl.glRotatef(millis()/10., 1, 0, 1);
  gl.glBegin(GL.GL_TRIANGLE_FAN);
    gl.glColor4f(0.0f,1.0f,0.0f, 0.7f); 
    gl.glVertex3f( 0.0f, 1.0f, 0.0f);
    gl.glVertex3f(-1.0f,-1.0f, 1.0f);
    gl.glVertex3f( 1.0f,-1.0f, 1.0f);
  gl.glEnd();
  
  pgl.endGL();
}