Drawing transparent objects on top of deferred shading

I’m writing a deferred shading system in webgl.

My g-buffer is a framebuffer object with the following attachments:
-Color: Floating point texture, packing screenspace normals, colors, depth, etc.
-Depth: Renderbuffer

I pack the depth into the floating point texture because my system doesn’t support depth textures.

After filling the g-buffer, I do the light accumulation pass onto the default framebuffer and this works okay.

But after this, I want to draw some transparent objects using traditional forward shading.
But I want these transparent objects to be occluded by deferred shaded objects.
The problem is my default framebuffer’s depth buffer doesn’t really have useful information in it.

One solution is this:
While drawing the transparent objects, in the fragment shader, sample the g-buffer, compare the depth in the g-buffer with the depth of the pixel, and discard the pixel if it fails this depth test (by setting alpha = 0 or something).

This seems inefficient, because we’re sampling a vec4 from a float texture, just to read one value. Also, this precludes any early fragment depth testing.

What I would like to do is take my g-buffer’s depth renderbuffer and “attach it” to the default framebuffer, or something to that effect.

There is no way to attach the default framebuffer’s images to an FBO, nor is there a way to attach images to the default framebuffer.

The correct way to do this is to do your lighting pass into an FBO. At which point, your default framebuffer doesn’t need to even have depth buffer. You just do a blit from your lighting buffer into the default framebuffer.

Plus, if you decide later to employ HDR, you can do that easily enough; your blit just becomes a render call from your floating-point buffer to the default framebuffer.

Ah that makes sense. How do I “blit” though? Does it require glBlitFramebuffer? Because that appears to be unavailable in webgl

glBlitFramebuffer is the standard way. But you can instead render a full-screen quad to transfer a texture to the framebuffer. Or you can use glCopyTexSubImage2D to copy the framebuffer to a texture.