Editing NMG

[[category:tutorials]]

{|align="right"
 |[[File:nmg-wiki-tutorial-screenshot.png|thumb|256px|Example NMG
object created with "put"]]
 |}

BRL-CAD contains a powerful Non-Manifold Geometry (NMG) mesh
primitive.  NMG objects are usually created through geometry
importers, or through commands like "facetize" that create NMG from
other geometry.  However, using the low-level database commands "get"
and "put", it is possible to manually create and edit NMG objects.

BRL-CAD's internal representation is similar to the Radial Edge data
structure described in http://www.scorec.rpi.edu/REPORTS/1986-1.pdf.
However, the textual representations that can be created and viewed
with "get" and "put" are somewhat simpler; this representation deals
with vertices, faces, and loops.  Loops, in effect, are closed curves
that form boundaries for faces, and a face can have multiple loops.
For example, a face with a hole in it might have two loops, with one
defining each boundary: one for the outer "edge" and one for the
inner.

To obtain a sample of the way that nmgs are created, these commands
create a ramp structure with another primitive and then convert it
with the "facetize" command:

<br clear=all />
 mged> make ramp_arb arb6
 mged> facetize -n ramp_nmg ramp_arb
 mged> get ramp_nmg
 nmg V { { 1.500499999999999944932938 -0.5003749999999999031885523 1.500375000000000014210855 } { 1.500499999999999944932938 1.500624999999999875655021 1.500375000000000014210855 } { -0.500499999999999944932938 0.5001250000000000417443857 1.500375000000000014210855 } { 1.500499999999999944932938 -0.5003749999999999031885523 -0.5006249999999998756550212 } { -0.500499999999999944932938 0.5001250000000000417443857 -0.5006249999999998756550212 } { 1.500499999999999944932938 1.500624999999999875655021 -0.5006249999999998756550212 } } F { { 3 5 1 0 } } F { { 3 0 2 4 } } F { { 5 4 2 1 } } F { { 3 4 5 } } F { { 0 1 2 } }

or, formatted, more nicely,

 nmg
 V {
   { 1.500499999999999944932938 -0.5003749999999999031885523 1.500375000000000014210855 }
   { 1.500499999999999944932938 1.500624999999999875655021 1.500375000000000014210855 }
   { -0.500499999999999944932938 0.5001250000000000417443857 1.500375000000000014210855 }
   { 1.500499999999999944932938 -0.5003749999999999031885523 -0.5006249999999998756550212 }
   { -0.500499999999999944932938 0.5001250000000000417443857 -0.5006249999999998756550212 }
   { 1.500499999999999944932938 1.500624999999999875655021 -0.5006249999999998756550212 }
 }
 F { { 3 5 1 0 } }
 F { { 3 0 2 4 } }
 F { { 5 4 2 1 } }
 F { { 3 4 5 } }
 F { { 0 1 2 } }

The "nmg" at the beginning is the type of what follows; the actual
information is just a list of vertices and a number of face
statements.  Each face is represented by a list that contains any
number of loops; each loop is a list of vertices.  The vertices are
referred to by their index into the V list.

To create a new nmg structure, use "put" instead of "get"; the first
argument to "put" should be the name of the new object.  For example,
to create a similar ramp structure named "ramp_nmg2" but with
different coordinates, you'd do this (it appears that the vertices must be specified in clockwise viewed from the outside order within each loop; otherwise, mged crashes):

<code>
mged> put ramp_nmg2 nmg V { { 0 0 0 } { 1 0 0 } { 0 1 0 } { 1 1 0 } { 0 1 1 } { 1 1 1} } F { { 0 2 3 1 } } F { { 3 2 4 5 } } F { { 0 1 5 4 } } F { { 0 4 2 } } F { { 1 3 5 } }
</code>

As a slightly more complex example, consider making a box.  The box should have a vertex V for each of its corners, so the V-list should look like <code>{ { 0 0 0 } { 0 1 0 } { 1 1 0 } { 1 0 0 } { 1 0 1 } { 0 0 1 } { 0 1 1 } { 1 1 1 } }</code>.  It should have six faces, one for each face of the cube, but these faces are simple ones with only one loop, since none of the faces have holes.  The bottom face is composed of <code>{ { 0 1 2 3 } }</code>, the top one of <code>{ { 4 5 6 7 } }</code>, and the sides are given by <code>{ { 0 1 6 7 } }</code>, <code>{ { 1 2 5 6 } }</code>, <code>{ { 2 3 4 5 } }</code>, and <code>{ { 3 0 7 4 } }</code>; this means that the final command to create it is:

<code>
mged> put box_nmg nmg V { { 0 0 0 } { 0 1 0 } { 1 1 0 } { 1 0 0 } { 1 0 1 } { 0 0 1 } { 0 1 1 } { 1 1 1 } } F { { 0 1 2 3 } } F { { 4 5 6 7 } } F { { 0 1 6 5 } } F { { 1 2 7 6 } } F { { 2 3 4 7 } } F { { 0 5 4 3 } }
</code>

This yields the following object: (an MGED wireframe, from the az35,el25 view setting):
[[File:nmg-wiki-tutorial-box-mged-screenshot.png|none|1024px|The MGED wireframe of the box from above]]

Another more complex example is to create a box with a hole through it.  Much of the command is the same as the one above for creating a plain box; the first thing to do is to add the necessary vertices to the <code>V</code> list.  For the lower face, <code>{ 0.25 0.25 0 }</code>, <code>{ 0.75 0.25 0 }</code>, <code>{ 0.75 0.75 0 }</code>, and <code>{ 0.25 0.75 0 }</code> are the necessary vertices; for the upper face, these are <code>{ 0.25 0.25 1 }</code>, <code>{ 0.25 0.75 1 }</code>, <code>{ 0.75 0.75 1 }</code>, <code>{ 0.75 0.25 1 }</code>.  Changing the first two face statements from above to <code>F { { 0 1 2 3 } { 8 9 10 11 } } F { { 4 5 6 7 } { 12 13 14 15 } }</code> makes the faces have the necessary cutouts.  Four faces must be added to form the sides of the hole through the box; these are <code>F { { 8 12 13 11 } }</code>, <code>F { { 11 13 14 10 } }</code>, <code>F { { 10 14 15 9 } }</code>, and <code>F { { 12 8 9 15 } }</code>.  The final command with all of this is:

<code>
mged> put box_with_hole_nmg nmg V { { 0 0 0 } { 0 1 0 } { 1 1 0 } { 1 0 0 } { 1 0 1 } { 0 0 1 } { 0 1 1 } { 1 1 1 } { 0.25 0.25 0 } { 0.75 0.25 0 } { 0.75 0.75 0 } { 0.25 0.75 0 } { 0.25 0.25 1 } { 0.25 0.75 1 } { 0.75 0.75 1 } { 0.75 0.25 1 } } F { { 0 1 2 3 } { 8 9 10 11 } } F { { 4 5 6 7 } { 12 13 14 15 } } F { { 0 1 6 5 } } F { { 1 2 7 6 } } F { { 2 3 4 7 } } F { { 0 5 4 3 } } F { { 8 12 13 11 } } F { { 11 13 14 10 } } F { { 10 14 15 9 } } F { { 9 15 12 8 } }
</code>
@@ Line 71: / Line 71: @@
 This yields the following object: (an MGED wireframe, from the az35,el25 view setting):
-[[File:nmg-wiki-tutorial-box-mged-screenshot.png|none|1024px|The MGED wireframe of the box from az35,el25]]
+[[File:nmg-wiki-tutorial-box-mged-screenshot.png|none|1024px|The MGED wireframe of the box from above]]
 Another more complex example is to create a box with a hole through it.  Much of the command is the same as the one above for creating a plain box; the first thing to do is to add the necessary vertices to the <code>V</code> list.  For the lower face, <code>{ 0.25 0.25 0 }</code>, <code>{ 0.75 0.25 0 }</code>, <code>{ 0.75 0.75 0 }</code>, and <code>{ 0.25 0.75 0 }</code> are the necessary vertices; for the upper face, these are <code>{ 0.25 0.25 1 }</code>, <code>{ 0.25 0.75 1 }</code>, <code>{ 0.75 0.75 1 }</code>, <code>{ 0.75 0.25 1 }</code>.  Changing the first two face statements from above to <code>F { { 0 1 2 3 } { 8 9 10 11 } } F { { 4 5 6 7 } { 12 13 14 15 } }</code> makes the faces have the necessary cutouts.  Four faces must be added to form the sides of the hole through the box; these are <code>F { { 8 12 13 11 } }</code>, <code>F { { 11 13 14 10 } }</code>, <code>F { { 10 14 15 9 } }</code>, and <code>F { { 12 8 9 15 } }</code>.  The final command with all of this is:
@@ Line 78: / Line 78: @@
 mged> put box_with_hole_nmg nmg V { { 0 0 0 } { 0 1 0 } { 1 1 0 } { 1 0 0 } { 1 0 1 } { 0 0 1 } { 0 1 1 } { 1 1 1 } { 0.25 0.25 0 } { 0.75 0.25 0 } { 0.75 0.75 0 } { 0.25 0.75 0 } { 0.25 0.25 1 } { 0.25 0.75 1 } { 0.75 0.75 1 } { 0.75 0.25 1 } } F { { 0 1 2 3 } { 8 9 10 11 } } F { { 4 5 6 7 } { 12 13 14 15 } } F { { 0 1 6 5 } } F { { 1 2 7 6 } } F { { 2 3 4 7 } } F { { 0 5 4 3 } } F { { 8 12 13 11 } } F { { 11 13 14 10 } } F { { 10 14 15 9 } } F { { 9 15 12 8 } }
 </code>
-This produces the following wireframe when drawn in MGED using the az35,el25 view setting:
-[[File:nmg-wiki-tutorial-box-with-hole-mged-screenshot.png|none|1024px|The MGED wireframe of the box with a hole from az35,el25]]