fading an object in and out by changing its alpha value over time

fade.py

import time, math
from pyglet.gl import *

window = pyglet.window.Window()

class AlphaSquare:
    def __init__(self, color):
        self.xpos = 0
        self.ypos = 0
        self.color = color
        self.vlist = pyglet.graphics.vertex_list(4, ('v2f', [-128,-128, 128,-128, -128,128, 128,128]))
    def draw(self):
        glEnable(GL_BLEND)
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
        glColor4f(self.color[0],self.color[1],self.color[2],self.color[3])
        glPushMatrix()
        glTranslatef(self.xpos, self.ypos, 0)
        self.vlist.draw(GL_TRIANGLE_STRIP)
        glPopMatrix()
        glDisable(GL_BLEND)


solidSquare = AlphaSquare([1, 1, 1, 1])
solidSquare.xpos = 320
solidSquare.ypos = 240

transpSquare = AlphaSquare([1, 0, 0, 0.5])
transpSquare.xpos = 320
transpSquare.ypos = 240


@window.event
def on_draw():
    global transpSquare, solidSquare
    glClearColor(0, 0.3, 0.5, 0)
    glClear(GL_COLOR_BUFFER_BIT)
    glLoadIdentity()
    solidSquare.draw()
    transpSquare.draw()


def update(dt):
    global transpSquare
    transpSquare.xpos = 320 + 128 * math.sin(time.time())
    transpSquare.ypos = 240 + 128 * math.cos(time.time())
    transpSquare.color[3] = (1+math.sin(time.time()))/2.0

pyglet.clock.schedule_interval(update,1/60.0)
pyglet.app.run()