Lines do not render on an offscreen frame buffer with a completely black texture

Question

If I have a frame buffer which has a textured binded to it which is simply black with full alpha and I try to draw a line to it, even if the line has full alpha it wont render. I'm not stupid, so the lines definitely aren't black. If the texture is white instead the line suddenly render correctly as if the colour of the texture behind it effects the colour of the lines which is dumb. Only if the lines have transparency, should the colour behind them have effect.

I'm using line smoothing. I use the following blend function which is apparently the one to use,

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)

How do I fix this?

Lots of code:

For drawing lines:

def draw_line(a,b,c,w,antialias):
    if antialias:
        glEnable(GL_LINE_SMOOTH) #Enable line smoothing.
    c = [float(sc)/255.0 for sc in c] #Divide colours by 255 because OpenGL uses 0-1
    if len(c) != 4:
        c.append(1) #Add a value for aplha transparency if needed
    glMatrixMode(GL_MODELVIEW)
    glLoadIdentity() #Loads model matrix
    glColor4fv(c)
    glLineWidth(w)
    glBegin(GL_LINES)
    glVertex2fv(a)
    glVertex2fv(b)
    glEnd()
    if antialias:
        glDisable(GL_LINE_SMOOTH) #Disable line smoothing.

Setting up framebuffer object:

def setup_framebuffer(surface):
    #Create texture if not done already
    if surface.texture == None:
        create_texture(surface)
    #Render child to parent
    if surface.frame_buffer == None:
        surface.frame_buffer =  glGenFramebuffersEXT(1)
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, surface.frame_buffer)
    glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, surface.texture, 0)
    glPushAttrib(GL_VIEWPORT_BIT)
    glViewport(0,0,surface.surface_size[0],surface.surface_size[1])
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity() #Load the projection matrix
    gluOrtho2D(0,surface.surface_size[0],0,surface.surface_size[1])

def end_framebuffer():
    glPopAttrib()
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0)
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity() #Load the projection matrix
    gluOrtho2D(0,1280,720,0) #Set an orthorgraphic view

Creation of texture:

def create_texture(surface):
    surface.texture = glGenTextures(1)
    glMatrixMode(GL_MODELVIEW)
    glLoadIdentity() #Loads model matrix
    glBindTexture(GL_TEXTURE_2D, surface.texture) #Binds the current 2D texture to the texture to be drawn
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR) #Required to be set for maping the pixel data
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR) #Similar as above
    if surface.data == None:
        surf = pygame.Surface((1,1),SRCALPHA)
        surf.fill(surface.colour[:-1])
        surface.data = pygame.image.tostring(surf, "RGBA") * (surface.surface_size[0] * surface.surface_size[1])
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, surface.surface_size[0], surface.surface_size[1], 0, GL_RGBA,GL_UNSIGNED_BYTE, surface.data) #Put surface pixel data into texture

Function to draw lots of lines to the screen or to the texture of a Surface object with a frame buffer object:

def add_lines(surface, c, coordinates, w = 1, antialias = True):
    if surface.__class__ == Surface: #Only use a frame buffer if the line isn't being drawn to the screen.
        setup_framebuffer(surface)
    last = None
    for coordinate in coordinates: #Loop though the coordinates and draw the lines
        if last != None:
            draw_line(last,coordinate,c,w,antialias)
        last = coordinate
    if surface.__class__ == Surface: #Only use a frame buffer if the line isn't being drawn to the screen.
        end_framebuffer()

That's all I can see as important. Except maybe the initialisation code:

glutInit(sys.argv)
glutInitWindowPosition(0,0)
glutInitWindowSize(*game_size)
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA)
glutCreateWindow(title)
glutSetIconTitle(title)
glutReshapeFunc(self.reshaped)
glutKeyboardFunc(self.keydown)
glutKeyboardUpFunc(self.keyup)
glutSpecialFunc(self.specialdown)
glutSpecialUpFunc(self.specialup)
glViewport(0,0,self.first_screen[0],self.first_screen[1]) #Creates the viewport which is mapped to the window
glEnable(GL_BLEND) #Enable alpha blending
glEnable(GL_TEXTURE_2D) #Enable 2D Textures
glEnable(GL_POLYGON_SMOOTH) #Enable antialiased polygons
glHint(GL_POLYGON_SMOOTH_HINT, GL_NICEST)
glHint(GL_LINE_SMOOTH_HINT, GL_NICEST)
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
glMatrixMode(GL_PROJECTION)
glLoadIdentity() #Load the projection matrix
gluOrtho2D(0,1280,720,0) #Set an orthorgraphic view

Answer 1

There is at least one thing that looks suspicious: you're turning on Texturing in your init code, and forgetting about it.

So your lines are drawn with texturing on (and constant texture coordinates), presumably picking the texture that you're trying to write to.

This is likely not what you want (I don't remember what the fbo spec has to say on this, but it's not going to work). How about you turn off texturing when rendering the lines ?