Mesh manipulation in programming

Motivation

Apart from manual mesh manipulation of mesh in apps and tool like netfabb Studio, it is also possible to manipulate mesh with code - creating programs, that manipulate meshes in some more ro less interesting manner. It would be sad not to examine this topic as well being on the Faculty of Information Technology.

ADMesh

ADMesh is open-source CLI tool for STL mesh manipulation and simple repairs. Apart from CLI, there is also C library API.

Short example follows. This loads model.stl (binary or ASCII) and rewrites is as ASCII or binary STL - it converts binary to ASCII and vice versa.

#include <stdlib.h>
#include <admesh/stl.h>

int main(void) {
  stl_file stl_in;
  char *filename = "model.stl";

  printf("Opening %s\n", filename);
  stl_open(&stl_in, filename);
  stl_exit_on_error(&stl_in);

  if (stl_in.stats.type == binary) {
    printf("Writing ASCII file %s\n", filename);
    stl_write_ascii(&stl_in, filename, "ADMesh");
    stl_exit_on_error(&stl_in);
  } else {
    printf("Writing binary file %s\n", filename);
    stl_write_binary(&stl_in, filename, "ADMesh");
    stl_exit_on_error(&stl_in);
  }

  stl_close(&stl_in);
  return EXIT_SUCCESS;
}

Notice that:

all functions accepts pointer to instance of stl_file as a first argument
there is stl_exit_on_error() after critical statements (eg. reading and writing to/from filesystem) to avoid segfaults when something went wrong
it is good manner to call stl_close() when the work is finished

Compile the source with:

gcc source.c -ladmesh -o myapp

Eventually, if your ADMesh installation is not in your standard PATH:

gcc -L/dir/with/so -I/dir/with/h source.c -ladmesh -o myapp

And run the app:

./myapp

On some systems, you’ll need to use:

LD_LIBRARY_PATH=/usr/local/lib ./myapp

We don’t have any specific Windows instructions, but there are screenshots for NetBeans (provided by a student).

All structs and function can be found in the header file admesh/stl.h. Unfortunately, there is no documentation.

Task

To get 3 points, program C, C++, Python or Ruby tool, that will determine best possible rotation around Z axis for the given file (as first command line argument). Best rotation in this case is such rotation, when the area of smallest rectangle with sides parallel to X and Y axis, that can embrace the 3D model, is smallest.

To install Python admesh module on school computer, run the following:

pip3 install --user admesh

Look at the bottom side of green cuboid (only corners are drown) - that’s our rectangle.

The app will print the best rotation (in degrees) to it’s standard output. It will also save a rotated version of the STL to a second file (second command line argument) - it will be saved in the same format (binary/ASCII) as was the input file loaded (i.e. not as in the example). Rotated version will be rotated by the exact same angle as the tool printed.

Best angle can be determined by brute force with angles dividable by 5° (5°, 10°, 15°, …).

Useful functions and data:

stl_rotate_z(&stl_in, angle_in_degrees) - rotates around the Z axis
stl_file.stats.number_of_facets is number of facets/triangles in the mesh
stl_file.facet_start is a pointer to the first facet (can be used as an array)
facets are saved in stl_facet structs that have vertex array with 3 items, each of them is stl_vertex struct with 3 floats: x, y and z.
interesting stats (might be useful) are present in stl_file.stats

Need some STL file? Use this one, but keep in mind that the tool shall work with any valid STL file.

hellskull.stl