Using main depth buffer

I’m porting some code from Direct3D to OpenGL and I’m a bit lost on how to approach the following:

  • Render objects to main depth and color buffers (window system provided)
  • Create a new render target (application created)
  • Turn off depth writes
  • Write new geometry to new render target using main depth buffer for depth tests

I was looking into FBOs, but it seems like this is just not possible with them. Any suggestions? Which extensions should I be looking at?

  • Render objects to main depth and color buffers (window system provided)
  • Copy depth values from main framebuffer to a depth texture
  • Create a new render target & attach it to fbo
  • Attach depth texture to fbo
  • Turn off depth writes
  • Render new geometry

… that should do the job, though there might be a faster way I didn’t think.