Understanding good topology using grids
Good topology is a mesh that deforms the way you want and makes the shape you want with the fewest number of triangles. The easiest way to get clean topology is by using a quad-based workflow. A quad is a face that contains four edges and four vertices. Although it is considered a single face, it is technically two triangles combined. This can be seen in Figure 2.1. Because you can manipulate the vertices individually, it must contain two triangles.
Figure 2.1 – Quad made up of two triangles
If you were to take multiple quads and string them together to form a plane, you would get a grid as shown in Figure 2.2.
Figure 2.2 – A grid (the perfect topology for a plane)
If what you were planning to make was a plane, this would be an example of perfect topology. Unfortunately, objects and characters are often a bit more complex. Thankfully, there are a few useful tools we can use to help us check out topology using the grid as our example.
The most useful tool we have to check what our topology is doing is the Loop Cut tool. The Loop Cut tool adds a loop of edges through a row of connected faces. To use this tool, go through the following steps:
- To begin a loop-cutting operation, press Ctrl + R.
- After entering the operation, you can hover over a face and a line will appear along a loop of faces designating where the loop cut will take place – this is illustrated in Figure 2.3.
Figure 2.3 – Loop cut going through the plane
- To finalize the loop cut, press LMB, and to cancel it, press RMB. The finalized loop is shown in Figure 2.4.
Figure 2.4 – Finalized edge loop
- To add more loops to the cut, scroll up on the mouse wheel. To add fewer, scroll down, or you could press the number of loops you wanted on the number line and number pad.
For the purpose of checking the topology, we will mostly be using the guiding loop to see where our loops are going, so we will cancel the actual loop cuts. If we run a loop cut along one of the rows of faces shown in Figure 2.5, a few things are apparent.
Figure 2.5 – Loop cut for checking topology
- First, the loop cut is actually going through the faces, showing us that our grid is properly made of quads
- The second is that the line is going straight through the mesh, terminating on either end into the void
- We can repeat this loop on every part of our grid, from every direction, and this is still true
However, one thing may cause issues here if you have started an extrusion on one of the points and then canceled it by pressing RMB. Because RMB does not remove the geometry created from the extrusion, you may be left with overlapping edges. In Figure 2.6, you can see what this might look like. Although it appears as though it is a clean grid, there is a duplicate edge blocking the loop cut.
Figure 2.6 – Broken loop cut caused by an extrusion
This issue is usually solved by merging by distance. Merging by distance, also called Remove Doubles, merges the vertices of a selected mesh by the desired distance. To perform a Merge by Distance operation, see the following:
- Start by selecting the mesh with A.
- Press M to open the Merge tab.
- Then, select By Distance from the dropdown.
You can see the menu in Figure 2.7.
Figure 2.7 – Merge menu
Another thing that can stop a loop from going through is a face flipped the wrong way. A face’s direction is determined by its normal. Normals are used to determine the direction in which the light will bounce off of a surface. A face has a front face and a back face determined by this normal. You can see a face normal direction by applying some settings in the Viewport Overlays tab shown in Figure 2.8. These settings are different in Object Mode and Edit Mode. For determining whether a face is flipped or not, check the Face Orientation checkbox in the Object Mode overlays tab. This will make all of the front faces blue, and all of the back faces red.
Figure 2.8 – Face Orientation checkbox
You can see the face that is flipped on this mesh in Figure 2.9 with Face Orientation enabled.
Figure 2.9 – Face Orientation revealing a flipped face
To change the orientation of a face, there are two options. You can press Alt + N to open up the Normals tab displayed in Figure 2.10.
Figure 2.10 – Normals menu
In this tab, you can flip a selection of faces by selecting Flip at the top, recalculate faces on the outside of the mesh, or recalculate them on the inside. The most common option used here is Recalculate Outside. This option will fix flipped faces most of the time. The hotkey to recalculate the normals on the outside is Shift + N. In Figure 2.11, the face is fixed, and the loop goes through all of the faces as desired.
Figure 2.11 – Fixed normals
By now, you should have a basic understanding of how a grid works, and how we can check the topology of that grid to judge whether it constitutes good topology. However, to work on more complex objects and manipulate grids to improve the topology, we need a stronger understanding of the three rules of topology.
