> Compose the G-buffer, render the forward-pass objects directly onto it. There is no “combining”.
If you like to render the particles on a (downscaled) different resolution buffer, then you should render to a separate texture first, as long as DX11 doesnt support rendering to mutliple,different resoultion frame buffers (I'm not familiar with DX).
> What stops you from using the depth render target (or depth buffer) from the deferred pass?
Most likely different resolutions. Downsample the depth buffer (or the according part of your G-buffer) and read back the depth and use the depth in your shader for soft fading of particles near geometry.
> 1. Should I render the particles before or after tone mapping?
I get better result when excluding forward rendered (unlit) particles from tone mapping. As long as you don't lit your particles, you will have a hard time to get them integrated into your HDR for all cases, especially if you have lot of dark scenes.
> 2. Should I render the transparent meshes the same way as I'm rendering the particles now?
Depends, I prefer a stitches pattern for transparent surfaces and render them directly to the G-buffer to get proper lighting (see signature), but this will only work for some usecases. There's no real best solution for rendering a transparent surfaces with a deferred rendering approach.
