Why does disabling writing to the depth buffer cause almost nothing to be drawn?

Question

Following SigTerm's suggestion, I render all my opaque polygons first, and then I disable the depth buffer (glDepthMask(GL_FALSE)) before I render my translucent polygons.

As soon as I disable the depth buffer though, it stops drawing anything to the screen. I can only see a glimpse of the world as I move the camera around (only little pieces show up for a second, and then disappear when I stop moving).

If I leave the depth buffer enabled the whole time, everything renders fine, it's just not as nice as I'd like (translucent objects obscure other translucent objects).

How do I get it to render properly?

My main render loop looks like this:

protected void RenderTerrain()
{
    GL.Enable(EnableCap.DepthTest);
    GL.Enable(EnableCap.CullFace);

    _bfShader.Use();
    _bfProjUniform.Mat4(_viewMat * _bfProjMat);
    _bfTex.Bind();

    GL.DepthMask(true);
    GL.Disable(EnableCap.Blend);
    _blendUniform.Set1(false);
    _bfVao.Bind();
    GL.DrawElementsInstancedBaseVertex(BeginMode.TriangleStrip, SharedData.FaceIndices.Length, DrawElementsType.UnsignedInt, IntPtr.Zero, _bfInstBuf.Length, 0);

    GL.DepthMask(false);
    GL.Enable(EnableCap.Blend);
    _blendUniform.Set1(true);
    _transVao.Bind();
    GL.DrawElementsInstancedBaseVertex(BeginMode.TriangleStrip, SharedData.FaceIndices.Length, DrawElementsType.UnsignedInt, IntPtr.Zero, _bfTransBuf.Length, 0);
}

---

The other half of the render loop:

protected override void OnRenderFrame(FrameEventArgs e)
{
    GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
    lock(_renderQueue)
    {
        while(_renderQueue.Count > 0)
        {
            _renderQueue.Dequeue().Invoke();
        }
    }
    RenderTerrain();
    RenderHUD();
    SwapBuffers();
}

Answer 1

Can you reenable the glDepthMask before calling glClear(), or at the end of your render loop?

Preventing writes to the depth buffer will prevent it from being correctly cleared.

http://www.opengl.org/wiki/FAQ#Masking