Deferred Rendering Skybox OpenGL

I've just implemented deferred rendering and am having trouble getting my skybox working. I try rendering my skybox at the very end of my rendering loop and all I get is a black screen. Here's the rendering loop:

    //binds the fbo
    gBuffer.Bind();

    //the shader that writes info to gbuffer
    geometryPass.Bind();

    glDepthMask(GL_TRUE);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_DEPTH_TEST);
    glDisable(GL_BLEND);

    //draw geometry
    geometryPass.SetUniform("model", transform.GetModel());
    geometryPass.SetUniform("mvp", camera.GetViewProjection() * transform.GetModel());

    mesh3.Draw();

    geometryPass.SetUniform("model", transform2.GetModel());
    geometryPass.SetUniform("mvp", camera.GetViewProjection() * transform2.GetModel());
    sphere.Draw();

    glDepthMask(GL_FALSE);
    glDisable(GL_DEPTH_TEST);

    glEnable(GL_BLEND);
    glBlendEquation(GL_FUNC_ADD);
    glBlendFunc(GL_ONE, GL_ONE);

    glBindFramebuffer(GL_FRAMEBUFFER, 0);
    glClear(GL_COLOR_BUFFER_BIT);

    //shader that calculates lighting
    pointLightPass.Bind();
    pointLightPass.SetUniform("cameraPos", camera.GetTransform().GetPosition());

    for (int i = 0; i < 2; i++)
    {
        pointLightPass.SetUniformPointLight("light", pointLights[i]);
        pointLightPass.SetUniform("mvp", glm::mat4(1.0f));
        //skybox.GetCubeMap()->Bind(9);
        quad.Draw();
    }

    //draw skybox
    glEnable(GL_DEPTH_TEST);
    skybox.Render(camera);

    window.Update();
    window.SwapBuffers();

The following is the skybox's render function

glCullFace(GL_FRONT);
glDepthFunc(GL_LEQUAL);

m_transform.SetPosition(camera.GetTransform().GetPosition());
m_shader->Bind();

m_shader->SetUniform("mvp", camera.GetViewProjection() * m_transform.GetModel());
m_shader->SetUniform("cubeMap", 0);

m_cubeMap->Bind(0);
m_cubeMesh->Draw();

glDepthFunc(GL_LESS);
glCullFace(GL_BACK);

And here is the skybox's vertex shader:

layout (location = 0) in vec3 position;

out vec3 TexCoord;

uniform mat4 mvp;

void main()
{
    vec4 pos = mvp * vec4(position, 1.0);
    gl_Position = pos.xyww;
    TexCoord = position;
}

The skybox's fragment shader just sets the output color to texture(cubeMap, TexCoord). As you can see from the vertex shader, I'm setting the position's z component to be w so that it will always have a depth of 1. I am also setting the depth function to be GL_LEQUAL so that it will fail the depth test. Should this not only draw the skybox in places where other objects weren't already drawn? Why does it result in a black screen?

I know I have set up the skybox correctly because if I just draw the skybox by itself it shows up just fine.

I can briefly see for a split second the geometry that should be drawn before the skybox is drawn on top of everything.

Since you're using double buffering, seeing different things must be due to a different frame being drawn. The depth buffer in the default framebuffer isn't being cleared, which I believe is the cause of the temporal instability at least.

In your case, you want the default depth buffer to be the same as the GBuffer when you draw the skybox. A quick way to achieve this is with glBlitFramebuffer, also avoiding the need to clear it:

glBindFramebuffer(GL_READ_FRAMEBUFFER, gbuffer);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBlitFramebuffer(..., GL_DEPTH_BUFFER_BIT, ...);

Now to explain the black screen when the skybox fills the screen. Without the depth test, of course the skybox just draws. With the depth test, the skybox still draws on the first frame, but shortly after the second frame clears only the colour buffer. The depth buffer still contains stale skybox values so it does not get re-draw for this frame and you're left with black...

However your geometry pass draws without depth testing enabled, so this should still be visible even if the skybox isn't. Also this would only happen with GL_LESS and you have GL_LEQUAL. And you have glDepthMask false, which means nothing should write to the default depth buffer in your code. This points to the depth buffer containing other values, perhaps uninitialized, but in my experience it's initially zero. Also this still happens when the skybox doesn't fill the screen, drawn as a cube away from the camera, which blows away that argument. Now, perhaps if the geometry failed to draw in the second frame that would explain it. For that matter blatant driver bugs would too, but I'm not seeing any problems in the given code.

TLDR: Many unexplained things, so **I tried it myself and can't reproduce your problem...

Here's a quick example based on your code and it works fine for me...

(green sphere is the geometry, red cube is the skybox)

gl_Position = pos:

Note the yellow from additive blending even if the skybox is drawn over the top. I would have thought you'd be seeing this too.

gl_Position = pos.xyww:

Now for the code...

//I haven't enabled back face culling, but that shouldn't affect anything

//binds the fbo
fbo.bind();

//the shader that writes info to gbuffer
//geometryPass.Bind(); //fixed pipeline for now

glDepthMask(GL_TRUE);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_DEPTH_TEST);
glDisable(GL_BLEND);

glColor3f(0,1,0);
fly.uploadCamera(); //glLoadMatrixf
sphere.draw();

glDepthMask(GL_FALSE);
glDisable(GL_DEPTH_TEST);

glEnable(GL_BLEND);
glBlendEquation(GL_FUNC_ADD);
glBlendFunc(GL_ONE, GL_ONE);

fbo.unbind(); //glBindFramebuffer(GL_FRAMEBUFFER, 0);
glClear(GL_COLOR_BUFFER_BIT);

//shader that calculates lighting
drawtex.use();
//pointLightPass.SetUniform("cameraPos", camera.GetTransform().GetPosition());
drawtex.set("tex", *(Texture2D*)fbo.colour[0]);

for (int i = 0; i < 2; i++)
{
    //pointLightPass.SetUniformPointLight("light", pointLights[i]);
    //pointLightPass.SetUniform("mvp", glm::mat4(1.0f));
    //skybox.GetCubeMap()->Bind(9);
    drawtex.set("modelviewMat", mat44::identity());
    quad.draw();
}

drawtex.unuse();

//draw skybox
glEnable(GL_DEPTH_TEST);

glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBlitFramebuffer(0, 0, fbo.size.x, fbo.size.y, 0, 0, fbo.size.x, fbo.size.y, GL_DEPTH_BUFFER_BIT, GL_NEAREST);
glBindFramebuffer(GL_FRAMEBUFFER, 0);

//glCullFace(GL_FRONT);
glDepthFunc(GL_LEQUAL);

//m_transform.SetPosition(camera.GetTransform().GetPosition());
skybox.use();

skybox.set("mvp", fly.camera.getProjection() * fly.camera.getInverse() * mat44::translate(1,0,0));
//m_shader->SetUniform("mvp", camera.GetViewProjection() * m_transform.GetModel());
//m_shader->SetUniform("cubeMap", 0);

//m_cubeMap->Bind(0);
cube.draw();
skybox.unuse();

glDepthFunc(GL_LESS);
//glCullFace(GL_BACK);

//window.Update();
//window.SwapBuffers();