applying a "filter" - a square with different blending functions is drawn on top of the rendered scene. The first form fades everything in/out; the second multiplies everything by a color; the third creates a negative image

filter.py

import time, math
from pyglet.gl import *

window = pyglet.window.Window()


class BlendFilter:
    def __init__(self, srcFunc, destFunc, color):
        self.srcFunc = srcFunc
        self.destFunc = destFunc
        self.color = color
        self.vlist = pyglet.graphics.vertex_list(4, ('v2f', [-1,-1, 1,-1, -1,1, 1,1]))

    def draw(self):
        glMatrixMode(GL_PROJECTION)
        glPushMatrix()
        glLoadIdentity()
        glMatrixMode(GL_MODELVIEW)
        glPushMatrix()
        glLoadIdentity()

        glEnable(GL_BLEND)
        glBlendFunc(self.srcFunc, self.destFunc)
        glColor4f(self.color[0],self.color[1],self.color[2],self.color[3])
        self.vlist.draw(GL_TRIANGLE_STRIP)

        glPopMatrix()
        glMatrixMode(GL_PROJECTION)
        glPopMatrix()
        glMatrixMode(GL_MODELVIEW)


class TexAlphaSquare:
    def __init__(self, width, height, xpos, ypos, heading, texturefile, color):
        self.xpos = xpos
        self.ypos = ypos
        self.heading = heading
        self.color = color
        if texturefile:
            img = pyglet.image.load(texturefile)
            self.texture = img.get_texture()
        else:
            self.texture = None
        verts = [-width/2.0, -height/2.0,  width/2.0, -height/2.0,  -width/2.0, height/2.0,  width/2.0, height/2.0]
        texcoords = [0,0, 1,0, 0,1, 1,1]
        self.vlist = pyglet.graphics.vertex_list(4, ('v2f', verts), ('t2f', texcoords))
    def draw(self):
        glEnable(GL_BLEND)
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
        if self.texture:
            glEnable(GL_TEXTURE_2D)
            glBindTexture(GL_TEXTURE_2D, self.texture.id)
        glColor4f(self.color[0], self.color[1], self.color[2], self.color[3])
        glPushMatrix()
        glTranslatef(self.xpos, self.ypos, 0)
        glRotatef(self.heading, 0, 0, 1)
        self.vlist.draw(GL_TRIANGLE_STRIP)
        glPopMatrix()
        if self.texture:
            glBindTexture(GL_TEXTURE_2D, 0)
            glDisable(GL_TEXTURE_2D)
        glDisable(GL_BLEND)

objects = [ TexAlphaSquare(640, 480, 0, 0, 0, 'hst9904a.jpg', [1,1,1,1]),
            TexAlphaSquare(192, 192, 192, 0, 0, 'rocket.tif', [1,1,1,1]) ]

thefilter = BlendFilter(GL_ZERO, GL_SRC_ALPHA, [1,1,1,0.5])
#thefilter = BlendFilter(GL_ZERO, GL_SRC_COLOR, [1,0,0.5,1])
#thefilter = BlendFilter(GL_ONE_MINUS_DST_COLOR, GL_ZERO, [1,1,1,1])


@window.event
def on_draw():
    glClearColor(0, 0, 0, 0)
    glClear(GL_COLOR_BUFFER_BIT)
    
    glLoadIdentity()
    glTranslatef(320, 240, 0)

    for o in objects:
        o.draw()
    thefilter.draw()


def update(dt):
    global objects
    angle = (time.time() % 10) * 36
    objects[1].xpos = 192 * math.sin(angle * math.pi/180.0)
    objects[1].ypos = 192 * math.cos(angle * math.pi/180.0)
    objects[1].heading = 270 - angle
    thefilter.color[3] = (1.0+math.sin(time.time()))/2.0


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