3D modeling is a complex business, with terms and terminology not often used in everyday life. One of the early terms that you may hear is that you need to correct your normals. But what does this mean and how do correcting normals affect your 3D models?
Normals in Blender represent lines that are created perpendicular to the geometry. The direction of the geometry determines the direction of the normal. Normals are used to determine the inward and outward sides of a face, edge, or vertex and can influence how geometry is rendered.
So a normal is created every time geometry is created and is used to determine which side of the geometry is the outward facing side. Let’s take a look at normals in more detail and find out why it is so important to understand normals.
How To Identify Normals?
There are numerous methods for being able to identify when a normal is facing the wrong direction. While not immediately obvious, the first opportunity to spot if a face is directed incorrectly is by its shading. when a face is directed the wrong way in Blender, the shade of the material can appear slightly darker or in the viewport.
Another method of identifying your normals is actually making the normals appear visible as colored lines. To do this first go into edit mode for a selected object. Then with the object in the correct mode access, the viewport overlays the menu, which is next to an icon of a filled and an empty circle in the top corner of the 3D viewport. A large menu will appear if you click on the down arrow, and towards the bottom, you will see the option to display your normals.
You can choose whether you want to display normals for your faces, edges, or vertices and then define the length of the normal lines. However, this creates a misconception about normals, as only a vertex can have a normal, yet you clearly have the option for edges and faces in the menu.
Face orientation is the most visible method of determining the direction of faces based on the vertex normals. This tool can also be found in the same viewport overlays menu. With this option active each face is assigned a color of either red or blue.
The color blue indicates the side of the face that the normal is directed from, in other words, the outward side. The color red indicates the inward side of the face. This makes it very easy to see which faces are correct and which are not. The goal here is to simply not be able to view any red faces in your scene.
What we may think of as a face normal is actually the average position of all vertex normals used to create that face. Take the side of a cube, for example, the normal of the face is located at the center as we would expect. However, if we add a vertex to this face using the subdivide tool, and then slide that vertex across an edge with the vertex slide tool. we will see the normal shifting towards the direction of that vertex.
This provides the basis though for another tool that is commonly used in Blender known as smooth shading.
The Effect Of Smooth Shading
Most beginner tutorials will normally introduce a tool at some point called the smooth shading tool. When you use this tool the surface of your 3D model becomes much smoother and realistic as if it was magic. You can no longer tell exactly where the individual faces are or the size of the faces on the model itself.
The tool here is manipulating the normals by using the smoothing attribute, which rotates the normal’s direction based on the viewport camera. If you were to apply smooth shading to a default cube, you would be able to see this effect as you navigate around the object.
Use caution when applying smooth shading to hard surface objects with flat faces, though, as this can create some weird shading artifacts on the surface of the model. Smooth shading is better suited for objects that have round edges and surfaces.
What Happens When Normals Are The Wrong Way Round?
Incorrect normals create several issues for 3D objects such as incorrect shading and behavior with lighting. The issue is much more severe though for objects that are to be used in game engines. A game engine will only render a face from the side that the normal is being directed. To save on resources, the inside of the face is not generated making it invisible from view.
If you ever play a game and move your camera through a character an object that asset may snap out of view. What is actually happening here is the camera is inside the asset in question but does not render the asset because it ignores the inward direction of all geometry. This is a part of a process known as backface culling, where the game determines what should and should not be rendered.
In Blender, we can actually preview this effect in the 3D viewport, and acts as an additional method for identifying incorrect normals. To activate the backface culling effect, you will find a drop-down button for the viewport shading menu in the very top corner of the viewport, next to the button for rendered shading.
Open up this menu and you will find the option for backface culling, just left click to enable it. Now if any faces are pointing in the wrong direction they will appear invisible if viewed from outside the object. Also if you bring the viewport camera into the object itself then it will disappear as the normals of the object’s faces are pointing in the opposite direction.
The Best Methods For Correcting Normals
Blender 3D makes it very easy to select and correct any normals that are incorrect and in need of fixing. First, we need to select the normals that we want to change or ‘flip’. We then access the normals menu by first going to the mesh menu In the viewport. Remember that we need to be in edit mode to do this as we are working on the geometry of the object and its behavior.
Within the mesh menu, there will be a host of options but the one we want will be clearly labeled as normals. Selecting this will allow us to control our normals in various ways. We can also access this same menu faster by using the hotkey combination of ‘Alt + N’.
If you have selected the correct geometry to be inverted, select the flip option which should be 1st in the menu. This will as the name suggests, flip the direction the normals are pointing. Make sure to use one of the methods for viewing the normals as you perform the function as it will make it easier to determine if the tools have worked correctly.
If you are not should that all the incorrect normals have been selected, you can instead select the entire model and then choose the recalculate outside option. This will have all the normals pointing out away from the center of mass. Note that this does not always work 100% of the time, so check your normals after using the tool and flip any that are still facing the wrong way.
The recalculate inside option is useful when you want to view your scene from the inside of an object. For example when creating a room. The other options are used to fine tune the direction of the normals via rotation, pole targets, etc, but are used more for artistic purposes rather than for fixing bad normals.
When Can Incorrect Normals Occur?
the most common time when incorrect normals can appear is when new geometry is created. For example, if you have a default cube and begin to extrude the edges then Blender may position the normals the wrong way round. If a vertex normal is incorrect any geometry that is created from that vertex may be incorrect as well.
Deleting and then adding geometry to a part of your model is another hotspot for creating incorrect normals. Any modeling tool can create these issues depending on how the object is being created. Blender itself is not always sure which direction you want the normals to go.
A good way to avoid incorrect normals is to avoid nonmanifold edges. A nonmanifold edge is one that cannot possibly exist in the real world. Going back to that default cube, the edge created by the extrusion is a nonmanifold edge. It has not thickness and basically hangs on the model without really impacting its structure.
So remember kids, always check your normals.
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