We all run into it at one point or another - our Sketchup model has tons of reversed faces, and isn't rendering or displaying properly. Whenever this has happened to me, I've scoured the web to see if any new solutions or plugins have come out that might help batch-reverse faces. I found the FrontFace tool, which is great but time-consuming. The only other viable option I've found is to simply go through the model, clicking and reversing faces as I find them. Until recently. I found a solution that literally saved me an entire day of work. What once took me 8 hours was reduced to around an hour of work, and I want to share this fix with you. First, let me show you what my problem looked like:
First off, what is this thing you ask? It's a plastic hanging tunnel for some playground equipment I've been working on. I have exported it as an .OBJ file from Sketchup, and imported it into a program called Flare3D, which is a Flash configurator. This is where I first noticed the problem happening. You can see that many of the areas on the model look dark, and they shouldn't be since I have set my model shadows to "off". The dark areas are actually reversed faces, often called "flipped normals". In some rendering and modeling programs, the textures show up incorrectly, and the faces have to be reversed. This can be done in Sketchup by right-clicking the face and choosing "reverse faces". This works, but when a model has thousands of faces, it gets old fast.
There's a better way. First, in Sketchup I need to see which faces are reversed. My model looks like this:
Everything looks just fine, but this is deceiving. To see which faces are reversed, I need to do to View > Face Style > Monochrome:
Now look. Just look. Hundreds if not thousands of reversed faces! They are the darker, purple-gray areas of the model:
First, I'll show how to reverse just a single face. I right-click on the reversed face (if there are groups, double-click within the group until you are able to select the faces individually) and select "Reverse Face":
Here is the result:
You can see that the face has become white, meaning it has correctly been reversed. However, because my geometry is not connected, half of the inside of the tunnel is still reversed. This often happens when models are imported from another program, which is what I did (my model was originally created in Inventor).
There is a better way, which is to use the Orient Faces command. This command matches all faces in a connected mesh to face the same way. in my model, many of the connected meshes have some faces that are facing correctly, and some that are reversed. I start by selecting the mesh, and then right-click on a correctly oriented face, and choose "Orient Faces":
Sketchup walks through the model searching for reversed faces in the connected mesh, and automatically reverses them. This operation happens quickly, and is really effective:
Check that out! Imagine if I had to manually reverse all of those faces myself.
This helped me a lot, but I did run into situations where an entire connected mesh was reversed. In this situation, I could not use the Orient faces command, since there were no correctly oriented faces to use. I found a simple workaround, which was to manually reverse a single face and then use that to orient the rest. Allow me to walk you through this process. First, I select a single face in the mesh, and right-click to select "Reverse Faces":
Here is the result:
Now that I have one correctly oriented face, I can right-click on that face and choose "Orient Faces" to reverse the remaining faces in the connected mesh! Here is the result:
After having gone through this process, my model looks much better:
And, it looks much better in Flare3D:
The process of reversing faces in my model took about an hour, which is pretty reasonable. I hope it can help you work faster and better as well.