In this lesson video we are going to discuss normal direction, why it is important and how to fix mesh problems related to normal direction. We already discussed briefly about normal direction before. Basically the normal direction is the direction perpendicular to one of the sides belong to a face plane. Let's create a plane object and move this to the side and up a bit. So it doesn't overlap with the ground floor display object which consists only of a single face element has a normal direction which is up towards the z axis. This cube however, consists of six faces.

Each of these faces has its own normal direction. This face for example, its normal direction is up towards the z axis just like the face on the plane object. But this phase normal direction is this way towards the x axis, this phase normal direction Is this way towards the negative or axis, you get the idea. Now a face can be considered to have two sides. The side that exists aligned with the normal direction is called the front face side. This is the default side that we want to see.

The other side, which is the opposite of the normal direction is called the back face side. This is the side you don't want to see. For the plane object. This is the front face, and if we rotate it, this is the back face. If we want to see the back faces in this cube, however, you cannot see them by default. Because they are all facing inside the cube.

You can go to the edit mode, select this face at the top and then lated now you can see the back faces of this cube object. Currently it is hard to tell which are the front faces and which are the back faces to help us to see them better. Blender provides a special a viewport. Oh feature called the face orientation, you can access it via this viewport overlays option, and then choose face orientation here. What this overlay feature does is basically colorize. All of the front faces with blue color and the back faces with red color, we can see that the bottom part of display an object has a red color.

And the inner part of this cube object has a red color also, normally, you always want to have everything that is visible to the camera to have a blue color. Okay, let me turn off this face orientation option four. Now, you might be wondering, why is it knowing this normal direction is important? Well, there are at least three answers for this. First, if you are planning to export your 3d model to game engines, you should know that most game engines use back face culling by default. We already discussed about breakfast calling before but just to reiterate or memory.

Back face culling is basically a rendering method to optimize performance by not rendering the back faces. So all of the big faces will be invisible as if there were no faces at all. We can see the effect by going to the solid mode viewport options here, and then turn on this back face culling, we can see the big faces now become invisible. Again, you need to remember that all 3d game engines have back face culling on by default. So you always want to check whether your model has back face culling problem or not in render before exporting to the external game engines. The second reason why knowing the normal direction is important because later we are going to learn about the normal map.

Essentially normal map is a special texture that uses RGB colors to bend the normal direction of a surface giving an illusion of complex depth or bumpiness on their surface. The third reason why knowing normal direction is important is because there are modifiers in Blender that rely on the normal direction information. If the normal directions in your model flipped randomly or incorrectly, then these modifiers will output unexpected wrong results. We will see an example of this issue later in the next project video. For now, let's focus on how to fix normal problems that may occur in our geometry. when importing 3d objects from other people or from other applications, or doing some complex Point to Point modeling, you may occasionally encounter objects that have flipped normals in some parts of it.

In render, we can fix this issue using the manual method or by using the automatic method. Let me turn on the face orientation option again, to flip certain faces normal direction manually, you need to be in the face mode, then You need to select the face you want to flip and then press ALT n. This will open up the normals sub menu, we can see there are so many things we can do with the normal direction. to flip it, we can select this flip command here, we can see now this face normal is flipped, it is no facing inside. If you forget the shortcut, you can always open the mesh menu up here, and then go to normals, we can see the normals sub menu here, if you find an object that has some of the faces flipped, and some are not, there is an easier way or an automatic method to fix this kind of issue.

First, you need to be interface mode, then press A to select all and then press Shift n. By doing this blender will calculate each of the phases and then set the normal directions to be facing outside. The opposite method of this is by pressing Shift control And with this blender will calculate each of the faces like before, but then set the normal directions to be facing inside. This is useful for doing an interior scene or for creating a game level that happens inside a room or a dungeon. For example, if you forget each of these shortcuts, you can also press ALT n first, and you can see shift n here for recalculate outside and Shift Ctrl N here for recalculate inside