Removing Interior Faces and Gaining Insight into Mesh Interiors: Step-by-step Guide

Interior faces in 3D modeling refer to the polygons or faces that are concealed inside a three-dimensional object's framework, away from the surface that can be seen on the outside. These faces are contained within the boundaries of the object, adding to its overall geometry without changing the way it appears from the outside. Consider them as the boundaries that separate an object's internal volume, such as the walls inside a hollow bottle or cup.

Interior faces emerge when designing detailed 3D models using different modeling techniques like extrusion, Boolean operations, or subdivision. These faces may appear unintentionally, especially when working with complex or asymmetric geometry. They may not be visible in the final render or visualization, but they still affect the model's effectiveness, performance, and general quality.

Why You Need to Get Rid of Interior Faces 

While interior faces do not directly affect the model's outward appearance, they can have an impact on a number of 3D modeling and rendering processes:

• Performance: Because rendering engines must process every face, including interior faces, this can delay final renders or real-time interactions. Performance may be enhanced by removing interior faces.

• File Size: A model's file size is increased by adding extra interior faces. When sharing or distributing the model, this could be a problem, especially if it results in larger and more challenging-to-manage file sizes.

• Render Times: Lighting, shading, and reflections are calculated for each face by rendering software. Longer render times can result from an increase in computational load caused by interior faces that are hidden.

• Realism: Realistic shading and highlights are produced by the interaction of light and shadow with faces. Even though they are not immediately visible, interior faces can interfere with these interactions and reduce the render's overall realism.

• Topology and geometry: Interior faces may contribute to non-manifold geometry, which may result in gaps, holes, or incorrect normals in the model's structure.

How Do You Get Rid of Interior Faces and How to See Inside a Mesh in SelfCAD

In this section, we are going to explore inner faces while designing a Laurel wreath. 

At first glance, you cannot imagine this design has inner faces. Specifically, we will see how you can perfectly connect these objects and avoid the inner faces emerging. Secondly, if you already have that problem, this section will also help you to understand how you will get rid of them. Lastly, you will also learn how you can detect the inner faces. 

Explaining the Inner Faces issue

The basic shapes used in designing this laurel wreath are two tori connected to each other, as shown below:

When you hide one of the tori, for example, mesh 8, you can see that mesh 4 has an inner face as shown:

Therefore, both the meshes have inner faces at the point of their connection. 

3 Ways You Can Remove the Inner Faces

The first option is to remove the faces of the toruses before connecting them. The second option is as follows:

On the toolbar, in the Utilities category, select Combine and Merge. Then most importantly, activate the Remove Inner Faces option. 

Then when you click the tick button to finalize, you will realize that 16 faces and 10 vertices have been removed:

The third option of removing inner faces is again going to the Utilities category in the toolbar and choosing the Combine >Merge option. The difference between option two and this third option is that we are not activating the Remover Inner Faces option. 

When finalized, only 8 vertices were removed. The faces were left inside. 

In this case, on the toolbar on the Utilities category, select Geometry Clean and activate the Remove Inner Faces option:

Click finalize. This action removed 16 vertices, 2 faces, and 2 polygons. 

That is how you can simply remove the inner faces. This applies only when you are aware that there is a problem, and that inner faces have been formed in your operations. The process of removing the inner faces was a bit easy because the faces where the merging took place were alike and coplanar. Therefore, they were perfectly aligned. 

How to Remove Inner Faces Using Stitch & Scoop

Assume one of the faces at the point of merging was not coplanar. For example, hide mesh 4, and then select mesh 8 face as shown below and extrude it (Modify> Extrusion). 

Use the Scale tool in the toolbar to scale it a little bit as follows:

When you unhide mesh 4 and mesh 8, the merging seems perfect and it is hard to notice that there is a problem as shown below:

However, when you turn on the wireframe, you realize that there is a problem:

To solve this, go to the toolbar, choose Stitch and Scoop tool and select Union and then finalize it:

When you turn on the wireframe mode, you can see that it perfectly removed everything:

How to Select Faces Inside a Mesh

Sometimes, you may just need to select the inner faces alone for various reasons, such as to delete them. Simply clicking on the faces on the mesh will not help because only the outer faces will be selected. 

To select the inner faces, you will have to turn on the X-ray mode. 

Then activate the Polygon Selection. Using marquee selection, drag your cursor on the region of interest, as shown:

You realize that this action selects everything including the inner faces;

To have only the inner faces selected, turn off the x-ray mode, then hold on to the control key in your keyboard, and use marquee selection to deselect the outer faces. You can rotate the mesh as you deselect the faces. 

To prove that you have selected the inner faces alone, go to the toolbar in the Utilities category and select the Split tool then hide our object (Union 2) and you remain with the inner faces selected. 

The selected inner face selected looks like this:

Once you have selected the inner faces, you can simply hit the delete button and this will remove the inner faces. All these steps of removing the inner faces are successfully removed because we knew there was a problem existing. 

Using Flip Normal and Backface Culling to Work with Inner Faces

The "Flip Normal" tool is used to reverse or invert the direction of a surface or face normal on a 3D object. Normals are vectors that point outward from the surface of a polygon or 3D model and are essential for determining how light interacts with the surface for shading and rendering purposes. The tool helps in flipping the normals outward, from the inside. 

On the other hand, the "Backface Culling" tool selectively displays only the front-facing polygons of a 3D model and discards the ones that are facing away from the viewer. Turning it off also makes the front-facing polygons disappear. The combination of these two tools also plays a key role in removing the inner faces of your mesh. 

When we go back to our object, with an inner face, you can prove this by turning on the wireframe mode and you see the inner face:

When you click on the Flip Normal tool in the Utilities category, you realize that the operation won’t happen because the geometry is not manifold. 

In this section we want to figure out how to see these inner faces. To do this, select only the top. On the right panel, activate the Polygon Selection and on the settings, activate the Deep selection option and set Tolerance to 5. Then select the top faces as shown below:

Go to the Utilities category, select Flip Normals, and ensure the Back Face culling is on. This helps you to see all through to the inside. 

Once the inner faces are visible, you can just click and highlight them.

Then simply click the Delete button to remove them. The inside is now completely empty:

Turn off the Back Face Culling:

Deselect everything. On the Utilities category select the Flip Normals and your object is fixed back.

The normals have been fixed back perfectly because it is now manifold. 

Identifying, and even removing the inner faces of your 3D model has been made easy when using user-friendly software like SelfCAD. It has an easy-to-use interface and tools by both 3D artists and beginners as well. Removal of interior faces and subsequent interior visualization are essential steps in 3D modeling to produce a high-quality, realistic result. Modelers are given the tools they need to optimize models, streamline workflow, and highlight intricate internal details thanks to the methods covered in this article, which range from identifying interior faces to using sophisticated retopology methods. By mastering these techniques, 3D artists can increase the complexity and realism of their works. The potential for creating immersive, visually stunning 3D models is only constrained by the artist's creativity and skill as technology develops.

Interior faces, those hidden polygons nestled within the core of 3D objects, may appear unnoticed at first, but they have a significant impact. As we've seen, removing them is critical for optimizing performance, managing file sizes, and ensuring the final model's integrity. The thorough selection of a model's geometry goes beyond mere aesthetics, and it shapes the very foundation of effective 3D modeling. In 3D modeling, removing interior faces and the subsequent visualization of interiors are important steps in achieving a quality realistic outcome. 

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