ZBrushCentral

Smoothing Groups? Smooth UV's?

ZBrush Usage Questions
#1

My search for greater understanding on the topic of smooth UV’s began last weekend when some FBX files I created in Zbrush returned the following error message when imported into Unreal Engine 4: “Warning: No smoothing group information was found in this FBX…”.

Since that time I have been searching the net and trying to understand just what exactly smoothing groups are and have come across countless threads about how there is no such equivalent feature in Zbrush to Max’s smoothing groups, posts about how Maya has some kind of feature for it and how it differs to Max’s and posts where Zbrush users are told to use Blender.

What I cannot find is anything which answers clearly these key questions:
• What is a smoothing group?
• Are Smooth UV’s the same thing?
• Should I be using/worrying about them?
• How does Zbrush handle them?
• Do I need to separate piece of software to create/assign them?

Sorry if this is really basic stuff! Unfortunately I have not been able to train or study in 3d art anywhere so have been learning how to use Zbrush alone over the last year.

#2

Unreal Engine likes Smoothing Groups which is why you see that message but it will import the FBX without them, or you can turn on the SNormals option for the FBX plugin and then smoothing groups will be generated and you won’t see the message.

A smoothing group is a way to tell an app what bits of your model you want to appear smoothed and what bits you want to appear faceted. ZBrush doesn’t use them - it relies on subdivision and creasing to get the same effect. (Dynamic Subdivision is the nearest thing in ZBrush to the smoothing applied in game engines but they won’t necessarily look the same.)

Smooth UVs is entirely different and not related to how smooth a model looks. UVs are the coordinates by which a 2D image is mapped to a 3D model’s surface so that it can be displayed with color or texture. Smooth UVs are when the UV coordinates are adjusted to suit a higher resolution model, such as one that has been subdivided in ZBrush. If you are not yet familiar with UVs then I wouldn’t worry about Smooth UVs just yet - ZBrush’s default settings should work fine.

HTH,

#3

What is a smoothing group?

https://www.youtube.com/watch?v=PMgjVJogIbc

This video should explain how a vertex’s normals will influence how the object gets shaded. Once you understand that, smoothing groups should be easy to understand. Most programs automatically create their own smoothing group based on angles, smoothing groups are just an additional way to customize this smooth/hard look. This image will also give you a basic example of smoothing groups in action.

How does Zbrush handle them?

As marcus_civis said, zbrush doesn’t use smoothing groups. Every polygon just gets shaded independently, creating a faceted look.

Should I be using/worrying about them?

Unless you’re going for an unlit material/texture-only look, then you usually should consider them when working on a game asset.
If you’re not using normal maps then they are really important to getting nice shading, and if you are using tangent-based normal maps then you’ll want to use smoothing groups (and uv islands) around sharp edges so that the gradient doesn’t have to work around such extreme angles. Here’s an old example showing how softer vertex normals and smoothinggroups+matching uv islands can be important for normal maps:
http://www.svartberg.com/tutorials/article_normalmaps/normalmaps.html

If you’re creating a lowpoly asset in zbrush and want to get the affect of smoothing groups, you’ll probably want to use polygroups in their place. Prior to exporting you could then use Subtool: Group Split to split each polygroup into its own subtool, and then merge them back together without the weld option. Then export with snormals.