3DSMAX: Cleaning up a photogrammetry model

Question

I'm attempting to build a model of a car fender. The goal is to modify this in software, CNC route a mold and fabricate the resulting part.

The original part is fairly complex, with a bunch of compound curves and things that look perpendicular but aren't quite. I do not have access to a 3D scanner. So, I thought I'd try AutoDesk ReCap Photo to generate a model, clean it up and go from there. This is what I got from ReCap:

Getting rid of the background trash, etc. was straightforward. What I'm trying to do now is clean up the mesh itself, where there are a lot of bumps (see the bottom of the fender in the image, and areas around the vent towards the top of the piece).

I'm currently waiting on ReCap to generate a new model from much higher-res images of a much cleaner part. That will hopefully reduce the amount of noise in the model.

The Question: What techniques should I consider to smooth the surface to get rid of bumps, dips, and generally remove noise from the mesh, or should I be using a different technique to generate the model?

I considered creating a lower-res quad patch and using the Cloth modifier to drop it onto the fender. That (perhaps obviously) didn't work very well. :/

Answer 1

Photogrammetry is statistical estimation which tries to find the most probable reason for the given set of photos. That includes reasoning what's the object, where the camera has been and what error the camera causes.

Photogrammetry can work only if the light is stable during the session, the surface isn't glossy (=all reflections are diffuse) and there's enough unique surface details with high enough contrast. As important is to prevent all changes in the camera between the shots. If moving the camera affects the light in the room or the camera does automatic adjustments or intelligent postprocesing to make the image better, you are out of luck.

The surface must not be glossy and it must have high contrast details. Stone and wood are often ok as is, but metal, plastic, glass and painted surfaces generally must be prepared. A common way is to have matt water based grey paint and a black fine, but still well visible pattern on it. If there's glossy areas, they appear as holes or unexpected bumps.Lack of details causes the same.

Having a dense high contrast pattern doesn't prevent surface errors because photo pixels still makes everything a little random. But the pattern can prevent large bumps, so it's more probable to get usable cloud decimation result.

It's far too easy to forget the near environment. It must also be traceable to prevent mixing it with the target.

Proper (=diffuse shadowless and stable) light is very difficult task. I have seen tutorials where the beginner is guided to shoot outdoors in a cloudy day and to work fast to have consistent light. It worked for me, too.

Search for quidance how to prepare the object and how to shoot the photos. Start from this: https://www.3dscanstore.com/blog/3d-scanning-reflective-objects.