Empathy List Archives

RW

Rogier Wolff

Sun, Jan 7, 2024 6:13 PM

Hi,

I tried cutting a "from the internet" STL in half by using openscad.

But as usual, if you import the STL, difference with the part you
don't want, and try to render you get CGAL errors and "there is no top
level to render". (or something like that).

So I decided to write an STL checker. I first wrote the read function.
(I found a library, but learning a new computer language to be able to
use it was "out of scope" for this weekend). The STL format is simple
enough. Then I wrote an "write STL" function from my internal
datastructures. I get the identical STL back! Great!

Then I wrote my first check function: Process the edges and check that
all of them are traversed exactly twice and once in each
direction. (actually that was the intention, for now I just print the count
and directions seen, and I used grep to verify that there were no not-expected
results.)

My testcube (openscad: cube (10);) works perfectly.

Then the 42Mb STL from the internet and.... Checks out! All edges
traversed exactly once in each direction!

My conclusion is that there cannot be any holes. Absolutely NOT.
A hole would have an edge that is traversed only once.

This means that OPENSCAD (and Slic3r) must be mangling the STL on
import.

Let me say that I consider this "bad form": You guys have convinced me
over the years that many STL files downloaded from the internet are
"bad", so I falsely accused the given STL of being "bad".

I suspect that it's the "snap-to-grid" that is causing this. Is there
a way to disable this without recompiling openscad ?

If I am going to recompile openscad, does anybody have a pointer to
where this is happening in the source code?

Is there any workaround so I can just cut my STL in half and get it
printed?

I'm considering multiplying all coordinates by 1000 in my program for
re-export towards Openscad, and see what happens then....

Roger.

--
** R.E.Wolff@BitWizard.nl ** https://www.BitWizard.nl/ ** +31-15-2049110 **
** Delftechpark 11 2628 XJ Delft, The Netherlands. KVK: 27239233 **
f equals m times a. When your f is steady, and your m is going down
your a is going up. -- Chris Hadfield about flying up the space shuttle.

Hi, I tried cutting a "from the internet" STL in half by using openscad. But as usual, if you import the STL, difference with the part you don't want, and try to render you get CGAL errors and "there is no top level to render". (or something like that). So I decided to write an STL checker. I first wrote the read function. (I found a library, but learning a new computer language to be able to use it was "out of scope" for this weekend). The STL format is simple enough. Then I wrote an "write STL" function from my internal datastructures. I get the identical STL back! Great! Then I wrote my first check function: Process the edges and check that all of them are traversed exactly twice and once in each direction. (actually that was the intention, for now I just print the count and directions seen, and I used grep to verify that there were no not-expected results.) My testcube (openscad: cube (10);) works perfectly. Then the 42Mb STL from the internet and.... Checks out! All edges traversed exactly once in each direction! My conclusion is that there cannot be any holes. Absolutely NOT. A hole would have an edge that is traversed only once. This means that OPENSCAD (and Slic3r) must be mangling the STL on import. Let me say that I consider this "bad form": You guys have convinced me over the years that many STL files downloaded from the internet are "bad", so I falsely accused the given STL of being "bad". I suspect that it's the "snap-to-grid" that is causing this. Is there a way to disable this without recompiling openscad ? If I am going to recompile openscad, does anybody have a pointer to where this is happening in the source code? Is there any workaround so I can just cut my STL in half and get it printed? I'm considering multiplying all coordinates by 1000 in my program for re-export towards Openscad, and see what happens then.... Roger. -- ** R.E.Wolff@BitWizard.nl ** https://www.BitWizard.nl/ ** +31-15-2049110 ** ** Delftechpark 11 2628 XJ Delft, The Netherlands. KVK: 27239233 ** f equals m times a. When your f is steady, and your m is going down your a is going up. -- Chris Hadfield about flying up the space shuttle.

FH

Father Horton

Sun, Jan 7, 2024 6:20 PM

Use Meshmixer for stuff like this.

On Sun, Jan 7, 2024 at 12:14 PM Rogier Wolff via Discuss <
discuss@lists.openscad.org> wrote:

Hi,

I tried cutting a "from the internet" STL in half by using openscad.

But as usual, if you import the STL, difference with the part you
don't want, and try to render you get CGAL errors and "there is no top
level to render". (or something like that).

So I decided to write an STL checker. I first wrote the read function.
(I found a library, but learning a new computer language to be able to
use it was "out of scope" for this weekend). The STL format is simple
enough. Then I wrote an "write STL" function from my internal
datastructures. I get the identical STL back! Great!

Then I wrote my first check function: Process the edges and check that
all of them are traversed exactly twice and once in each
direction. (actually that was the intention, for now I just print the count
and directions seen, and I used grep to verify that there were no
not-expected
results.)