This is a basic tutorial on how to use Instant Meshes, a free and open-source program that allows you to perform retopology and reduce the polygon count of 3D models.
Video Transcript
Hello everyone! This is a basic tutorial on how to use Instant Meshes, a free and open-source program that allows you to perform retopology and reduce the polygon count of 3D models. One of its most interesting features is the ability to manually define certain topology flows to guide the creation of low-poly topologies.
To show you a practical example, I’ll be using a 3D model obtained through photogrammetry. I reconstructed this model with Meshroom, using the same workflow I explained in my basic series on Turntable photogrammetry. As a result, the model has a high vertex count and poor topology.
Instant Meshes can be downloaded either as source code to compile or as a precompiled executable from the link in the description. It’s a standalone program that accepts models in OBJ or PLY format.
I’ve cleaned up the model obtained from photogrammetry a bit. Now I’m exporting it in OBJ format so I can import it into Instant Meshes.
After launching Instant Meshes, click the Open Mesh button to import the 3D model in OBJ format. You can rotate the 3D view using the left mouse button. Zooming is done with the mouse scroll wheel. You can pan the view with the right mouse button.
In the Advanced panel, there are several display modes available, which allow us, for example, to view the model in Wireframe mode, to show the numerical IDs of vertices and faces, and more. Another button at the top is Remesh As, where we can specify the type of geometry we want as output. The default option is Quads, meaning only quadrilateral faces, and I’ll keep it set that way.
The Target Vertex Count parameter has a somewhat misleading name, because it doesn’t represent the number of vertices the final mesh will have. It’s actually a guiding parameter for the density of the grid that will be generated. It serves as an ideal target for the algorithm, not a strict limit or a guaranteed result. Instant Meshes will suggest a value we shouldn’t exceed, because the original mesh might not have enough resolution. What we’re mainly interested in is a retopology with good simplification, so we might actually want to lower the value suggested by Instant Meshes. Therefore, the program isn’t ideal if we need an exact polycount, but we can always simplify the geometry later in Blender or another program.
Now, we can click the first of the Solve buttons, located in the Orientation Field section, to have Instant Meshes calculate some initial topology flows. After clicking Solve, the initial flows will appear on the object. In some cases, they’ll work fine, such as on the legs or other elongated parts, but in others they’ll need to be adjusted to obtain different topologies.
To inspect and modify the initial topology, in the Orientation Field section we have three tools available that allow us to adjust the flow in certain areas. In this same section, there’s also a field called Singularities, where Instant Meshes tells us the number of critical points where the orientation flow changes abruptly. These singularities often result in vertices with 3, 5, or more connected edges.
Let’s use the first tool, called Comb, which allows us to define custom flows by clicking and dragging on the geometry with the left mouse button. To rotate the view, you'll need to temporarily deactivate the Comb tool and then reactivate it. Lines can be deleted by clicking the X icon next to each new line. Notice how each stroke affects the number of Singularities, sometimes improving and sometimes worsening the result.
To visualize the Singularities of the Orientation Field, we can enable their display in the Advanced tab, which we briefly looked at earlier. Here you can also adjust the icon size for these singularities. Another way to visualize singularities is by activating the Attractor button, which has a magnet icon. In any case, we can try redefining the flow lines using the Comb tool. The flow is recalculated automatically, so clicking Solve again shouldn’t cause visible changes. Keep in mind that some singularities are nearly impossible to eliminate, such as those on faces or, in the case of the little wolf on this statue, near the ears. For instance, I tried defining a new topology flow to fix an issue, but I actually ended up introducing two new singularities.
In any case, the Orientation Field is the first of two steps. The second is the Position Field, where we deal with areas where the quadrangular topology shows discontinuities in the distribution of vertices. Let’s click the Solve button in the Position Field section to jump right into it. As you can see, we now have the lines of the new topology, generated from the flows we defined in the Orientation Field. However, even here some problem areas are clearly visible. These are also referred to as Singularities.
It’s easy to spot some Position singularities, as they often appear in areas where a line ends abruptly and converges into another edge flow. You can view the Position Field Singularities using the corresponding checkbox in the Advanced tab.
In this case too, we have a Comb tool that lets us modify the flow slightly to try and reduce singularities. However, some singularities simply cannot be removed. At best, they can be moved closer together or relocated to less noticeable or less problematic areas, such as flat surfaces or parts of the mesh that won’t need to be animated.
Once we’re done with our edits, all that’s left is to export the result. By clicking Export Mesh, we can set a few options for the output geometry, such as Pure Quad Mesh to generate geometry made up of quadrilateral faces only, and optionally apply some smoothing iterations. Then, we need to click Extract Mesh, which will have Instant Meshes generate the final geometry for export, and we’ll be able to preview it in the 3D view. Finally, we need to click Save to specify the name and location of the OBJ file. Make sure to manually include the OBJ file extension, or Instant Meshes won’t save the output file!
Now we can import the OBJ file into Blender and compare it with the original high-poly version. Right away, we’ll notice that the vertex count is much higher than the value we set in the Target Vertex Count field in Instant Meshes. As I mentioned earlier, that parameter is more of a density reference than a precise target. The topology, however, is much cleaner than the original model and will allow us to simplify or modify the model more easily. The appearance is very faceted, both because all the edges are marked as Sharp Edges, and because I have the Cavity option enabled. To get a smoother look, I remove the Sharp Edges from the Edges menu in Edit Mode and turn off the Cavity option. The overlap with the high-poly model is good, which means we’ll be able to easily bake the Base Color and Normal Map from the high-poly version, but that’s a topic for another time.
