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Retopology Best Practices for Faster Mobile Game Development

Ahmad Merheb spoke about retopology techniques, discussed the potential problems artists may face during retopology and strategies to overcome them, and introduced his personal retopology technique called Reverse Topology.

What Is Retopology? 

The term "retopology" came into existence following the rising popularity of sculpting software, with the first, ZBrush, being released back in 2007. Artists started creating incredible 3D sculpts that looked detailed and appealing (even a bit too much sometimes). However, the question came up: what could be done with these sculpts? Although they looked amazing, how could we animate them or incorporate them into a game project? Were they simply for 3D printing?

So, in a nutshell, the need to transform high-resolution sculpts into lower-resolution models led to the advent of retopology. Retopology involves creating a new topology (mesh geometry) over a high-resolution sculpt, creating a 3D model that can be animated and integrated into different workflows.

Importance of Retopology in Mobile Games

In mobile games, there are different workflows: one that utilizes Retopology and another that relies on highly detailed hand-painted textures. Which approach you choose largely depends on the art style you aim to achieve.

However, most workflows typically involve sculpting a high-resolution mesh, performing retopology, then baking the high-resolution sculpt onto the low-resolution one and exporting normal maps, specular maps, glossiness maps, etc. which is often referred to as PBR workflow.

In this article, I will explain retopology techniques and the potential problems one may encounter, along with strategies to overcome them. Additionally, I will present my personal technique for retopology called "Reverse Topology," which enables faster completion and reduces the guesswork associated with retopology.

Without further ado, let's dive in.

Understanding Retopology

Manual Retopology

Manual Retopology involves creating topology poly by poly by laying out vertices and edges and polygons on top of the high-resolution mesh. This technique is typically used for objects that will be animated and require well-defined and clean topology to enable smooth deformation with the mesh.

In the mobile games industry, Manual Retopology is extensively used because we are bound by a strict TRC (Technical Requirement Checklist) received from our tech team, which we as the art team must adhere to. This TRC often includes the maximum poly count and texture size, and sometimes, certain loop requirements.

Here are some tools commonly used for Manual Retopology:

  • Topogun: My all-time favorite standalone software;
  • 3D Coat;
  • Blender: Specifically the addon Retopoflow that can be added to Blender.

Automatic Retopology

Automated Retopology involves creating a mesh topology with the click of a button or moving some sliders before clicking a button. This method may seem faster and easier than the manual retopology process. However, it cannot work in every situation. While it may work for some props or small objects, it's usually unsuitable for most organic meshes that will be animated.

The most well-known Automatic Retopology tool is ZRemesher in ZBrush. It's amazing how many people use it for quick results and testing. Even some outsourcers use it for organic meshes. However, I don't recommend relying on this tool since the work required afterward to fix and remove spiral loops is more involved than doing manual retopology.

There is also an addon for Blender called QuadRemesher, which can provide decent results (better than ZRemesher) by defining the target poly count and clicking a button.

Retopology Techniques for Mobile Games

Definition of Ngons and Stars

Under normal circumstances, each vertex should only contain four edges going through it (see the image above). This is how a healthy normal vertex looks like and this is how you get perfect deformations when subdivided and when animated. 

However, what happens when there are more or fewer than four edges? Well, we start experiencing problems. Over time, these scenarios were given names: Ngons and Stars. Let's take a more in-depth look at these issues.

What is a Ngon?

Ngons occur when a single vertex has more than four edges going into it (see the image above). This problem arises because a vertex with more than four edges creates a stress point, which causes tension and impacts the surface during animation. This creates unwanted visual stress on the mesh, leading to further problems.

What is a Star?

Stars occur when a vertex has only three edges going into it (see the image above). In this case, we have a lower edge normal than the normal one of four. This situation creates a stress point that will be blatantly visible in animations or if the mesh is subdivided.

The Impact of Ngons and Stars in Mobile Games

Do Ngons and stars matter in mobile games? Well, it's a bit of both. Let me explain.

In mobile games, Ngons and Stars aren't a significant problem because we typically do not need to subdivide our meshes. The meshes remain in a low-poly state at runtime, and some game engines even triangulate all meshes upon importing. This eliminates the problem of subdividing and seeing stress points. However, animation remains an issue. Ngons and Stars create stress points on the mesh where deformation occurs. Even though we are producing a low-poly mesh, we should still be aware of Ngons and Stars.

Solution for Ngons and Stars

Well, the first step we need to take is to acknowledge that every organic mesh has Ngons and Stars unless you're dealing with a quad-plane character. Instead of attempting to remove them entirely and fighting a losing battle, we must plan where they will be on the mesh.

A good retopology will have Ngons and Stars placed in specific locations away from deformation areas, such as the lips on the face or the elbow on the body. They should also be placed on flat areas that won’t be moving much, such as the cheek area or the forearm area. If we follow this approach, Ngons and Stars are less likely to cause problems, whether we plan to subdivide or animate the mesh.

Reverse Retopology Method 

After years of performing manual retopology and designing hundreds of characters, I started to notice a pattern every time I did a retopology. I found myself constantly struggling in areas with Ngons or Stars. Once I completed the retopology process and compared it with a previous mesh design, I noticed something very interesting.

The Ngons and Stars were always in the same location in every mesh. This wasn't something I intentionally planned; I just followed the same routine during each retopology process. This concept sparked an idea in my mind. What if we skip the guesswork and retopologize differently, starting from a reverse perspective? What if I start by placing the Ngons and Stars right from the beginning?

After all, Ngons and Stars represent a change in direction (see the image above) within the mesh, so these areas mark great and important areas in the mesh that will define how the cage will end up. 

In other words, if you place the Ngons and Stars right from the start what you have to do next is just fill in the loops, and there will be no more guesswork and no more fighting.

To demonstrate this I have made a video about this method check it out below:

Then, in order to place those Ngons and Stars in the right location each time, I have created a map that I open each time I do a character's head. I have included this map below so you can use it as well and try out the Reverse Retopology method. From my experience, this method reduced the amount of time spent by 40%, which is precisely what I needed.

I am currently working on maps for the body, hands, and feet as well. If you are interested, keep an eye on my blog as I will probably post it there for people to benefit from, once it's done.

Future of Retopology 

I have always believed that retopology should be an automatic process as, in my opinion, it does not involve any creative input from artists, it's very technical and monotonous. I have tried to script tools myself in order to make this process easier. 

Ultimately, there should be a tool that allows the artist to specify the location for Ngons and Stars and then computes the rest, so that there would be minimal input from the artist's side, and the rest would be just processing.

As long as there will be sculpting, retopology will always be needed. Some use ready-made base meshes up to certain limitations and then after hitting those limitations they switch to retopology. 

Hope this article was useful to you and you learned something new. If you have any questions, feel free to reach out to me through my website. I'm always more than happy to help.

Ahmad Merheb, Art Lead/Tutor

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