Why does it look so bad in Meshmixer?

[RogerInHawaii]

I start with one component of my design that looks like this when viewed in Fusion 360:

 I then select to do a Save As STL and in the dialog that pops up select to send it to Meshmixer. The Meshmixer program pops up and displays the component, and it looks like this:

 And it looks like it partly melted. Note the top, which in the original is a nice flat surface. The bolt holes along the bottom are rather "melty" as well. And the two sides are also rather messed up.

I then selected (without actually doing anything in Meshmixer to the component) to Export it out of Meshmixer as an STL file. I then bring up that exported STL file in a simple STL file viewer and it looks like this:

And it all looks good, pretty much identical to what the original component looks like in Fusion.

So, why does it look so horrible within Meshmixer? It seems that Meshmixer does indeed have the data all correct. It just doesn't display it properly. It's pretty disconcerting to see it all (apparently) warped and melted when working on it i Meshmixer. And it's not just this single component. I've done it on several other components and some look even worse than this.

[TrippyLighting]

Where do I start 😉

A CAD program represents surfaces as analytical surfaces (shperes, Cylinders and other simple shapes) and as NURBS surfaces. Those are mathematically precise descriptions of these objects and theoretically have infinite resolution down to the precision of the given CAD system.

These descriptions when viewed on a screen already have been simplified into a triangle mesh with a finite solution.

There are shading algorithms at play, that fool the eye into believing that there are actually smoothly curved surfaces when in reality there are only triangles. Such algorithms are Phong shading and Goraud shading. Without special provision for sharp edges, usually defined by some angle between surfaces, you'll get that second picture in your post.

With those provision you'll get the first picture. You can also change that to flat shading and then can discern the individual triangles, something I'd recommend using in Meshmixer.

In essence, however that same principle that applies to the graphics viewport also applies to .stl files. Those are triangulated meshes. That;s what you are dealing with in Meshmixer and basically is the form of 3D data sent to any available consumer 3D printer.

Edit: Little correction. Meshmixer shows the .stl mesh as one "cohesive" object as it does not naturally know what a sharp edge is. The CAD system can can discern between individual surfaces an render those as triangulated meshes, thus sharp edges are builtin.

[RogerInHawaii]

So, basically you're saying that Meshmixer doesn't do as good a job as Fusion or my simple STL viewer when it comes to shading.

You recommend to "change that to flat shading and then can discern the individual triangles, something I'd recommend using in Meshmixer."
I looked around on Meshmixer and don't see anything that lets you select "Flat shading". There's a "Shaders" button that brings up different colors and shading, but none of them appear to be "flat shading" and, in fact, clicking on any of them doesn't appear to actually do anything at all. Do you know exactly HOW to tell Meshmixer to use flat shading so that it won't look so "melted"?

[TrippyLighting · Accepted solution]

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@RogerInHawaii wrote:
. Do you know exactly HOW to tell Meshmixer to use flat shading so that it won't look so "melted"?

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Yep, it is explained here. 

[RogerInHawaii]

@TrippyLighting Thank  you, thank you, thank you. That looks sooo much better.

[TheCADWhisperer]

@RogerInHawaii 

I think  there are two issues here that might confuse:

1. Display of 3D geometry in 2D medium of computer monitor.

2. Mathematical geometry definition.

1. I order for say for example a cylinder to appear 3D on a 2D flat screen such as your monitor, the software breaks up the geometry into thousands of triangle and applies different levels of highlights and shading to the triangles.  But the underlying geometry is still defined by precise mathematics.  Without this highlight/shading a 3D object on a 2D medium would look flat and be difficult to discern as 3D.   I think @TrippyLighting has covered this portion of the issue and your original question was about display.  The software can also do the reverse - make a faceted surface look smooth through programming tricks.

2. But be aware that when you export a file as stl - you have changed the geometry.  It is now defined as only straight lines and planar triangular facets.  You can test this yourself.  One way is to select fewer triangles output when creating the stl.  Another way is to read an stl file that "looks smooth" back into Fusion 360 (or any true CAD software) and convert the mesh faceted geometry into B-Rep geometry.  You will see that you can now create a 2D sketch on any of the thousands of planar faces.  (see video demonstration below)

ba51b3d4-dae2-4639-bb93-c58df77b8ac0,640,620

[TheCADWhisperer]

https://autode.sk/2Of39ei

Hmm, well I don't see this Screencast in previous post, but when I Edit the post, it does appear embedded in the the post.