Automatic retopology using Instant Meshes

In this article, I show you how to use Instant Meshes, a free software program, to automatically retopologize a 3D model with a large number of vertices, reducing the number of vertices.

What is retopology?

When you generate a 3DCG model using photogrammetry or sculpting, the number of vertices tends to increase, resulting in a heavy and unwieldy model. This is where a method called “retopology” is used to replace the model with a low-polygon model while maintaining the shape as much as possible.

Retopology can be done manually, but it can be done more efficiently with add-ons or software.

Example of retopology

Example of retopology

Download Instant Meshes

Instant Meshes is an open source software based on technology developed by a lab at ETH Zurich, and is available for Windows, Mac, and Linux.

The project page can be found at

https://igl.ethz.ch/projects/instant-meshes/

To download it, open the following GitHub page.

https://github.com/wjakob/instant-meshes

Scroll down to the bottom and you will find a section called Pre-compiled binaries, download the zip file according to the PC you are using.

After unzipping the zip file, click on the exe file to launch the software.

Instant Meshes screen

Instant Meshes screen

How to use

How to use Instant Meshes

We will be working with the following 3D scanned model.

Model before retopology

Model before retopology

The number of vertices in Blender is 49,998.

Check the number of vertices

Check the number of vertices

Click the folder icon that appears after clicking “Open Mesh” to load the 3D model. The supported formats are .ply and .obj.

Loading the obj file

Loading the obj file

The 3D model is now loaded. The loaded 3D model will not have a texture. After retopology, it is necessary to bake a new texture (we will publish this procedure in a later article).

Immediately after loading the model

Immediately after loading the model

From now on, all you have to do is to set up the menu on the left side in order from the top. First of all, from the “Mesh As” item, you can choose whether you want the finished product to be a triangle mesh (Triangles) or a quad mesh (Quads).

Specifying the shape of the mesh

Specifying the shape of the mesh

Specify the number of vertices after retopology from “Target vertex count”.

Specifying the number of vertices

Specifying the number of vertices

Click the Solve button in the Orientation field.

Solve

Solve

The flow of the mesh that will be generated will be displayed as a line, as shown below.

The rough flow of the mesh is now displayed.

The rough flow of the mesh is now displayed.

If you think the flow of the mesh is different, click on the comb icon and draw a line on the model with the mouse, and it will recalculate to point in that direction. (In the following example, the line is extended directly down from the neck)

Adjusting with a comb

Adjusting with a comb

Click the Solve button in the Position field.

Solve

Solve

Click the Solve button on the Position field to display the mesh that will be generated.

The mesh is now displayed

The mesh is now displayed

From this state, we can also go back to the previous step to adjust the flow of the mesh.

More comb lines.

More comb lines.


After Solve

After Solve

Once the adjustments are complete, we can output the results. Click “Export mesh” to display the output menu.

Export the model

Export the model

If you want to make all the polygons square, check the “Pure quad mesh” checkbox. Set the number of smoothing iterations in Smoothing iterations.

Click “Extract mesh”. The final mesh will be displayed as shown below.

Check the final mesh

Check the final mesh

Check in Blender

The original model and the model after retopology are loaded in Blender and compared as follows. You can see that the number of vertices has been reduced while the shape is almost the same except for some details.

Left: Original model / Right: After retopology

Left: Original model / Right: After retopology

The number of vertices in the model after retopology was 3,049.

The number of vertices has decreased

The number of vertices has decreased

If you switch to edit mode and view the model after retopology, you will see that all edges are now light blue. Sharpening has been applied to the edges.

Edges are sharpened

Edges are sharpened

Sharpening can be turned on by “Mark Sharpen” in the “Edges” menu, and turned off by “Clear Sharpen”.

Turning sharpening on and off

Turning sharpening on and off

Since I did not check the Pure quad mesh checkbox at the end of this tutorial, triangle and polygon polygons are also mixed in as shown below.

Meshes other than quadrilaterals are also mixed in

Meshes other than quadrilaterals are also mixed in

Comparison of retopology results

We tested how changing the options during the process changes the results.

Comparison of mesh shapes

I changed the “Mesh As” item. Here are the default output results for Triangles, Quad (2-RoSy, 4-Posy), and Quad (4-RoSy, 4-Posy) from the top.

Triangles

Triangles


Quad(2-RoSy, 4-Posy)

Quad(2-RoSy, 4-Posy)


Quad(4-RoSy, 4-Posy)

Quad(4-RoSy, 4-Posy)

Configuration details option

Next, let’s take a look at the configuration details options.

These settings are respectively

  • Extrinsic: Smoothing along the feature lines of the geometry.
  • Align to boundaries: If the input mesh is not closed, the boundaries will be preserved.
  • Sharp creases: If the normals of the input surface mesh are discontinuous, they will not be smoothed.

Align to boundaries.

Here are the results of turning “Extrinsic” on and off.

Left: Extrinsic on / Right: Extrinsic off

Left: Extrinsic on / Right: Extrinsic off

With “Extrinsic” turned off, you can see that the mesh does not follow the shape of the model, but is just drawn in a straight line.

Next, we show the results of turning Sharp creases on and off.

Sharp Creases off

Sharp Creases off


Sharp creases on

Sharp creases on

When Sharp creases are turned on, the mesh after retopology is placed exactly on the original shape without any smoothing correction.

Uploading to STYLY

For instructions on how to upload your retopologized 3D model to STYLY, please refer to the “Upload” chapter in the “Understanding the assets you can import” article below.

Bake textures on the 3D model after retopology

The model after retopology has no texture.
Since it will have a different UV placement than the original model, applying the original model’s texture as is will not work.

The following article explains how to bake the texture of the original model into the texture of the retopologized model using a function called baking.