Enabling depth test causes nothing at all to appear

Question

I have a depth buffer and am rendering a cone on the screen. It works, but then I realized I had forgotten the standard enabling of depth testing, and when I turned it on, now nothing appears at all.

I only have this one object, a cone, in my scene, so nothing else is in front of it.

The depth buffer is setup fine:

glFramebufferRenderbuffer(GL_FRAMEBUFFER,
                              GL_DEPTH_ATTACHMENT,
                              GL_RENDERBUFFER,
                              m_depthRenderbuffer);

The I bind the render buffer:

glBindRenderbuffer(GL_RENDERBUFFER, m_renderbuffer);

Then I render the cone:

(My experiments with this messy cone are the subject of another question I shall ask shortly).

But, as soon as I add this next line after creating the depth buffer and binding the render buffer:

glEnable(GL_DEPTH_TEST)

...then my cone magically disappears. There is nothing, not even a little dot, on the screen.

Why would that be?

Answer 1

When using GL_DEPTH_TEST,

glClear(GL_DEPTH_BUFFER_BIT) has to be called before rendering so that the depth buffer is initialized correctly.