code for http://stackoverflow.com/q/24018479/1461210

add_sub_blend.py

from OpenGL import GL as gl
from OpenGL import GLUT as glut
from OpenGL.arrays import vbo
import numpy as np
from scipy.misc import lena

class TextureQuad2D(object):

    def __init__(self, texdata, rect=(-1., -1., 1., 1.)):

        # build the texture
        self.texture = gl.glGenTextures(1)
        gl.glBindTexture(gl.GL_TEXTURE_2D, self.texture)
        gl.glTexEnvf(gl.GL_TEXTURE_ENV,
                     gl.GL_TEXTURE_ENV_MODE, gl.GL_MODULATE)
        gl.glTexParameterf(gl.GL_TEXTURE_2D,
                           gl.GL_TEXTURE_WRAP_S, gl.GL_REPEAT)
        gl.glTexParameterf(gl.GL_TEXTURE_2D,
                           gl.GL_TEXTURE_WRAP_T, gl.GL_REPEAT)
        gl.glTexParameterf(gl.GL_TEXTURE_2D,
                           gl.GL_TEXTURE_MAG_FILTER, gl.GL_NEAREST)
        gl.glTexParameterf(gl.GL_TEXTURE_2D,
                           gl.GL_TEXTURE_MIN_FILTER, gl.GL_NEAREST)

        w, h = texdata.shape

        gl.glTexImage2D(
            gl.GL_TEXTURE_2D, 0, gl.GL_LUMINANCE, w, h, 0, gl.GL_LUMINANCE,
            gl.GL_FLOAT, texdata
        )

        # display list
        # --------------------------------------------------------------
        texlist = gl.glGenLists(1)
        gl.glNewList(texlist, gl.GL_COMPILE)

        gl.glBindTexture(gl.GL_TEXTURE_2D, self.texture)
        gl.glEnable(gl.GL_TEXTURE_2D)

        x0, y0, x1, y1 = rect

        gl.glBegin(gl.GL_QUADS)
        gl.glTexCoord2f(0, 1)
        gl.glVertex2f(x0, y1)
        gl.glTexCoord2f(0, 0)
        gl.glVertex2f(x0, y0)
        gl.glTexCoord2f(1, 0)
        gl.glVertex2f(x1, y0)
        gl.glTexCoord2f(1, 1)
        gl.glVertex2f(x1, y1)
        gl.glEnd()

        gl.glDisable(gl.GL_TEXTURE_2D)
        gl.glBindTexture(gl.GL_TEXTURE_2D, 0)

        gl.glEndList()
        # --------------------------------------------------------------

        self.texlist = texlist

        pass

    def draw(self):

        gl.glCallList(self.texlist)

if __name__ == '__main__':
    glut.glutInit()
    glut.glutInitDisplayMode( glut.GLUT_DOUBLE | glut.GLUT_RGB )
    glut.glutInitWindowSize( 500, 250 )
    glut.glutInitWindowPosition( 100, 100 )
    glut.glutCreateWindow( None )

    img = np.flipud(lena().astype(np.float32))
    img = (img - img.min()) / img.ptp()

    checker = np.zeros((6, 6), dtype=np.float32)
    checker[::2, ::2] = 1
    checker[1::2, 1::2] = 1

    background1 = TextureQuad2D(img, rect=(-1, -1, 0, 1))
    overlay1 = TextureQuad2D(checker, rect=(-1, -1, 0, 1))
    background2 = TextureQuad2D(img, rect=(0, -1, 1, 1))
    overlay2 = TextureQuad2D(checker, rect=(0, -1, 1, 1))


    def draw_scene():

        # set the viewport and projection
        gl.glViewport(0,0,500,250)
        gl.glMatrixMode(gl.GL_PROJECTION)
        gl.glLoadIdentity()
        gl.glOrtho(-1,1,-1,1,0,1)

        gl.glMatrixMode(gl.GL_MODELVIEW)
        gl.glClearColor(0,0,0,0)
        gl.glClear(gl.GL_COLOR_BUFFER_BIT)

        gl.glEnable(gl.GL_BLEND)

        glBendFuncSeparate(GL_SRC_ALPHA,GL_ONE,GL_ZERO,GL_ONE)

        # additive
        gl.glBlendEquation(gl.GL_FUNC_ADD)
        gl.glColor(1, 1, 1, 1)
        background1.draw()
        gl.glColor(1, 1, 1, 0.5)   # this looks OK
        overlay1.draw()

        # subtractive
        gl.glBlendEquation(gl.GL_FUNC_ADD)
        gl.glColor(1, 1, 1, 1)
        background2.draw()
        gl.glBlendEquation(gl.GL_FUNC_REVERSE_SUBTRACT)
        gl.glColor(1, 1, 1, 0.5)  # negative alpha values clearly won't work!
        overlay2.draw()

        gl.glDisable(gl.GL_BLEND)
        glut.glutSwapBuffers()

    glut.glutDisplayFunc( draw_scene )
    glut.glutIdleFunc( draw_scene )

    glut.glutMainLoop()