Meaning of Aspect Ratio in the Save as Mesh dialog?

[graham.wideman]

In the Save As Mesh dialog, in the Refinement Settings area, there are several parameters, one of which is "Aspect Ratio".   What does this mean?

The dialog's "i" button leads to:  https://help.autodesk.com/view/fusion360/ENU/?guid=SLD-3D-PRINT, where we find:

• "Aspect Ratio: Specify ratio between the height and width of each face on the mesh body."

Yes, that's what Aspect Ratio would normally mean. But in a mesh, the triangles have a variety of aspect ratios, so a single value can't possibly apply. (Not to mention -- which is the height and which is the width for triangles in different orientations?)  Further, all the Refinement Setting presets set this value to 21.5, an extreme value that doesn't make sense to apply to all triangles.

[HughesTooling]

It's supposed to stop you getting long thin triangles. I tried a couple of shapes to show an example but didn't get what I expected testing on a simple box or cylinder. 

Here's an example using a cone that does actually work.

I have seen problems at times machining surfaces with subtle curvature where the long thin triangles leave a faceted surface after machining. There's no control over this in the manufacturing operations so I had to mesh the part in the design workspace using the aspect ratio to create more short edged triangles like the cone above.

[graham.wideman]

@HughesTooling

Thanks very much for exploring this.  From your experiment, do you think that the real meaning of the "Aspect Ratio" setting is actually "Maximum allowed mesh triangle Aspect Ratio"?  That's a constraint that would make a lot of sense.

If so, then the several docs with brief descriptions of this setting are incorrect.

While we're considering this setting there is another subtlety that should be mentioned in an accurate description. While Aspect Ratio in general is described by Width:Height (as in 16:9 computer monitor),  the constraint set by this Aspect Ratio setting presumably applies to each of the three directions. So for each edge of the triangle, treat that edge as the "base" (width), and form a ratio with the corresponding height.  We don't want triangles that are extreme in any of their directions.

[laughingcreek]

I the haze of my memory, I seem to recall that the aspect ratio  here refers to sides a and b of a right triangle, which would automatically constrain side c (the hypotenuse).

[graham.wideman]

@laughingcreek How does your answer relate to the current discussion? Virtually none of the triangles in a mesh will be right triangles. And we've already established what "aspect ratio" means in general.

[TrippyLighting]

You posted another question in response to @HughesTooling's post, which indicated that you still were unsure about what the exact aspect ratio means.

That question is what @laughingcreek tried to respond to.

Do you need more help in understanding what Aspect Ratio means? I'd be happy to tag one of the Fusion team members to help explain it.

[graham.wideman]

@TrippyLighting  You are right that we don't yet have official confirmation that HughesTooling's interpretation of the "Aspect Ratio" setting is indeed correct, though it seems likely.

More to the point, if that interpretation is correct, then the UI label and docs are incorrect. So tagging someone on the Fusion team to get those fixed at some point would be good.

[TrippyLighting]

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.99 5 of Fuiosn users.@graham.wideman wrote:
@..

While we're considering this setting there is another subtlety that should be mentioned in an accurate description. While Aspect Ratio in general is described by Width:Height (as in 16:9 computer monitor),  the constraint set by this Aspect Ratio setting presumably applies to each of the three directions. So for each edge of the triangle, treat that edge as the "base" (width), and form a ratio with the corresponding height.  We don't want triangles that are extreme in any of their directions.

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Triangles are always flat, and that "subtlety" of a 3rd dimension isn't needed.

I find that the documentation for tessellating a solid body describes this well enough. Any short written description, however, is likely falling short of describing the tessellation algorithm that does the actual work. I would think that describing that algorithm might be an interesting intellectual exercise, but would have very little practical value.

When I searched the documentation for the term "Aspect ratio" I came across this page for Mesh quality. That article is part of the documentation for FEM Simulation. The example shown looks like a .stl mesh, but that isn't the case. A .stl mesh only describes the surface of an object. In FEA, a mesh element is volumetric!

FEA meshing is tricky business!

[graham.wideman]

3rd dimension?

I'm not sure why you are discussing "a 3rd dimension isn't needed". I didn't mention any 3rd dimension.

Perhaps you were distracted by my phrase "each of the three directions"? I explained what that means, but let me try again, now armed with an additional resource:

The conventional term "aspect ratio" means ratio between width and height.  However, if indeed the Save as Mesh "Aspect Ratio" setting is constraining the "aspect ratio" of triangles, then presumably it would inspect width and height for each of 3 cases: treating each side as the base, compared to the height for that case.  All three of these cases should be constrained.  This is the "3 directions" I mentioned. Nothing to do with the 3 dimensions of space.

I now see that there are numerous different definitions for aspect ratio applied to triangles:

"How different simulation softwares calculate Aspect ratio for 2D Elements"

https://www.engmorph.com/2d-element-aspect-ratio-diff-simula

Some of these methods separately inspect the 3 cases, others have a single formula.  But ultimately all the algorithms provide a measure of the degree to which the shape of the triangle is "long and skinny" as opposed to closer to equilateral.

Changes advocated

I'm not advocating for any treatise on the tessellation algorithm. I'm simply advocating for the UI label and docs to not be actually wrong.  A minimal change would be for the UI label to change from "Aspect Ratio" to "Max aspect ratio" [edit: or maybe "Aspect Ratio Limit"], with perhaps the hint saying "Maximum allowed aspect ratio for mesh triangles."

It would be great if the documentation of this dialog at least did not claim that this setting sets the aspect ratio of the triangles in the mesh, which is false. Instead it should say that it constrains the meshing algorithm to aspect ratios less than this limit.

The docs could spell out why you might want such a limit, providing some clue to users how to reason whether they want a larger or smaller value.  Allowing them to actually use with dialog and setting deliberately.

[TrippyLighting]

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@graham.wideman wrote:

3rd dimension?

I'm not sure why you are discussing "a 3rd dimension isn't needed". I didn't mention any 3rd dimension.

I misread your post. Do you now understand what Aspect ratio means in the context of tessellating a solid body?

[graham.wideman]

> Do you now understand what Aspect ratio means in the context of tessellating a solid body?

If by "tessellating a solid body" you mean "tessellating the surface of the solid body", (and not your 3D FEA digression) then yes, the meaning of "aspect ratio" in general with respect to triangles has never been in doubt in this entire discussion (notwithstanding alternative ways to calculate such a measure that I linked.).

What HAS been in doubt is the meaning of the Save as Mesh dialog "Aspect Ratio" setting.  HughesTooling came up with a very plausible meaning: that it actually means "Maximum allowed Aspect Ratio".  This has yet to be confirmed by an official source. If confirmed, it warrants changing the label in the UI, and the docs.

[TrippyLighting]

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@graham.wideman wrote:

If by "tessellating a solid body" you mean "tessellating the surface of the solid body", (and not your 3D FEA digression) then yes, the meaning of "aspect ratio" in general with respect to triangles has never been in doubt in this entire discussion (notwithstanding alternative ways to calculate such a measure that I linked.).

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Yep, I am glad you understood that from the beginning.

I've been on this forum for 10 years, and I'd be hard pressed to point to another thread where this has been a problem.

Most users grasp what this means quickly (or it does not matter to them) and move on. So am I!

[graham.wideman]

You are addressing only the general meaning of the term "Aspect Ratio". You have not addressed the meaning of the Save as Mesh dialog Aspect Ratio setting.

Contrary to its name, that setting does not and cannot set the Aspect Ratio of the triangles in the tessellation. Therefore it is misnamed. 

The "i" button doc for this dialog links to page "Convert a solid body to mesh body" which describes this setting as:

• "Aspect Ratio: Specify the ratio between the height and width of each face edge on the mesh body."

This appears to be your understanding of the setting.  Again, this is false, it cannot set the aspect ratio of each face of the mesh, since they are all different.

So far as I can tell, you don't recognize that there is even a discrepancy between "setting the aspect ratio of each face" and "setting a limit on the aspect ratio of each face that the tessellation can produce".

[TrippyLighting · Accepted solution]

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@graham.wideman wrote:

This appears to be your understanding of the setting. 

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No, it isn't. Maximum aspect ratio would indeed be a better description of the menu item.