checkpoint

content/sundries/a-very-digital-artifact/index.md

@@ -54,22 +54,44 @@ shop:
 >
 > *Me*: That's totally fair! I'll get you some files in a few days.
 
-As you can see, I leaned even harder into the bluff; my next communication with the shop was nearly
-four weeks later. But that's getting ahead of things.
+"I'll get you some files in a few days," was an even harder lean into the bluff; my next
+communication with the shop was nearly four weeks later. But that's getting ahead of things.
 
 # Meshes and solid bodies
 
 First off, let's talk about file formats and how to represent shapes with a
 computer.[^math-computers] I said I could provide an *STL
 file*. [STL](https://en.wikipedia.org/wiki/STL_(file_format)) is a pretty bare-bones format that
-describes the outside surface of a shape as a set of many, many triangles, each of which is described
-by three 3D points. This format is popular with 3D printers, which is how I became familiar with
-it.
+describes the outside surface of a shape as a set of many, many triangles, each of which is
+described by three 3D points, forming a "mesh" in the shape of the thing you're modeling. This
+format is popular with 3D printers, which is how I became familiar with it.
 
-This type of representation is easy to create and read, but it's not great for manipulation.
+This format is simple to implement and easy for a computer to read, but if you have a model in that
+format that you need to manipulate, you won't have a good time. In order to actually do things like
+change the shape of the model, it needs to be converted into a CAD program's native representation
+of a "solid body", which is pretty much what it sounds like: a shape made of a finite volume of
+"stuff", instead of an infinitesimally thin shell enclosing an empty volume, which is what the mesh
+is.
+
+In order for the CAD program to convert a mesh into a solid body, the mesh must be *manifold*,
+meaning, no missing faces (triangles), and with a clearly-defined interior and exterior (all
+triangles are facing in one direction relative to their interior). When there are no missing faces,
+it's called "water tight". You can still have "holes" in a mesh, like if you have a model of a
+donut, but the surface of the donut can't have any missing faces.
+
+The CNC shop had requested a model in a format called
+[ST**P**](https://www.fastradius.com/resources/everything-you-need-to-know-about-step-files/). `.stp`
+is the extension for a "STEP" file; STEP is supposed to be short for "standard for the exchange of
+product data", so someone was playing pretty fast and loose with their initialisms, but I
+digress. But the main thing about STEP files is that CAD programs can really easily convert them
+into their native internal solid body representation, which allows easy manipulation.
+
+But so far, I had nothing at all. Time to get some data and see if I can turn it into a model.
 
 # Public data
 
+
+
 ## From space?
 
 ## Thanks, California state!