This post is mainly about using the Shrinkwrap modifier for modelling, which is below, but there's also a quick update on some of my projects right at the end.
My Favourite Modifier
Before I started the 'Heartbreaker' project I probably wouldn't have said that the Shrinkwrap modifier is one of my favourite modifiers in Blender (not that anyone had actually asked me, or likely ever would). Maybe in the top 10, but only just. I would probably have gone for one of the classics, like the Subsurf or Mirror, you just can't go wrong with those two. However, that's all changed. If anyone ever asks me*, I will say my new favourite modifier, at least regarding modelling - which is what I'm doing most of the time - is the Shrinkwrap. It has become my go-to, problem solving, reliable friend.
*Which they wont.
Are You Insane? And What Does The Shrinkwrap Modifier Even Do?
No, I am not. A valid question (the second one), thank you (me) for asking. In its simplest form, the Shrinkwrap modifier is tasked with snapping the current object onto the surface of another object. It also has the ability to only snap specific vertices if you specify a vertex group.
Here we see a simple subdivided plane being shrinkwrapped to the surface of a sphere:
I think the Shrinkwrap modifier was probably first created as a retopology tool, the snapping allowing you to easily create new, low-poly geometry, over your high-res model, without having to constantly think about manual snapping. Considering this, I'm not sure that 'Shrinkwrap' is actually the best name for it; maybe 'Snap' would have been better. The Snap modifier has a ring to it. But then, who am I to start renaming things? Sure, I was technically the best renamer in the area of Greater London in the years 1993 - 1998*, but I have no official certification for that, so I'll leave actual naming and any subsequent renaming to those that do.
*I retired from the gruelling world of competitive renaming undefeated and vowed never to return, due to the physical stress it caused my body.
How Does Retopology Tie-In To Hard-Surface Modelling? Are You Sure You're Not Insane?
It's not so much that retopology fits into hard surface modelling it's more that some hard-surface modelling scenarios and retopology share some common needs. Also, please stop asking if I'm insane.
There are two scenarios that can be very time-consuming when modelling:
This is very similar to what retopology requires and these two problems both happen to be the Shrinkwrap modifier's strength: conforming vertices to a specific surface.
That's All Well And Good, But Show Me Some Specific Examples
Please don't be so demanding. I've got some examples from 'Heartbreaker', the project I just literally won't shut up about.
The 'Heartbreaker' Iron Man suit has a tendency to have many separate panels that all conform to the same profile. Below, on the left, is the forearm, which is made of many pieces. They all have the same bulge and crease going through them, which would be time consuming to model manually. Instead, I built one continuous surface to describe the surface I want my mesh to conform to, shown on the right. All the pieces on the left conform to the profile of the mesh on the right (the Shrinkwrap target):
Heartbreaker also has many examples of detailing cut into curved panels. Cutting into curved surfaces is notoriously difficult as any sharpening edge loops on those details end up causing undesirable pinching, particularly at corners. It's also very difficult to perfectly maintain a curved surface while adding in new geometry.
Here, the head remains perfectly curved despite having cut details into it, thanks to good ol' Shrinkwrap:
Here is a wider view of the top of the head on the left, with its Shrinkwrap counterpart on the right:
The 'eyebrows' in the above image were excluded from the Shrinkwrap by adding all vertices apart from the eyebrow to a vertex group and selecting it on the Shrinkwrap modifier.
*If I've already started modelling something, but decide I need a Shrinkwrap, I will sometimes duplicate the object I'm modelling, simplify it, and use it as the Shrinkwrap object.
I also like to change the 'Maximum Draw Type' to 'Wire' in the 'Display' panel of the 'Object' tab in the 'Properties Editor' for the Shrinkwrap object so you can see the object you are modelling as well (shown below). Also, you may find it useful to turn on 'Draw All Edges', also in the 'Display' panel and 'Optimal Display', on the Surbsurf modifier, if you're using one.