Archive for December, 2016

Plot a bunch of edges in matlab with per-edge color data

Monday, December 19th, 2016

Suppose you have a list of vertices V (#V by dim) and a list of edges E (#E by 2) indexing V, then one hacky to draw all of these edges with per-vertex color data DV (#V by 1) is to use trisurf:

trisurf(E(:,[1 1 2]),V(:,1),V(:,2),V(:,3),'CData',DV,'EdgeColor','interp');

However, if you try to use this trick to draw per-edge color data DE (#E by 1), you’ll find that 'EdgeColor','flat' does not work. Instead you can explode you compact “mesh” of edges into individual edges and repeat you edge color data for each edge point:


Command line program to view 3d meshes from files and piped input

Thursday, December 15th, 2016

Here’s a little C++ program to directly render meshes in 3D viewer from the command line.

This let’s you write little test programs without worrying about linking to a 3D viewer. You just need to output a mesh in a standard format. For example, here’s a tiny program that outputs a cube in an .off format:

#include <igl/read_triangle_mesh.h>
#include <Eigen/Core>
#include <iostream>

int main(int argc, char * argv[])
  using namespace Eigen;
  MatrixXd V(8,3);
  MatrixXi Q(6,4);
    "OFF\n"<<V.rows()<<" "<<Q.rows()<<" 0\n"<<
    V.format(IOFormat(FullPrecision,DontAlignCols," ","\n","","","","\n"))<<
    (Q.array()).format(IOFormat(FullPrecision,DontAlignCols," ","\n","4 ","","","\n"));
  return EXIT_SUCCESS;

Compile this into cube_off then issue:

./cube_off | view mesh

**Update: ** And here’s a funny, little one-liner you can call from matlab to display a mesh via the .obj format:

system(sprintf('echo \"%s%s\" | /usr/local/bin/viewmesh',sprintf('v %0.17f %0.17f %0.17f\n',V'),sprintf('f %d %d %d\n',F')))

Background computation threads with igl::viewer::Viewer

Sunday, December 4th, 2016

Here’s a minimal example showing how to launch background computation threads for each mesh in a list of meshes. Meanwhile the main thread runs a mesh viewer with all meshes concatenated into one huge multi-component mesh. Whenever a computation thread signals that an update to the mesh needs to be made, the main thread will re-concatenate the meshes and update the viewer. In this example, the “computation” is determining how much to move a clock “hand” (random mesh).

Here’s the program running on the cow, cheburashka and knight models:

// Tiny example to demonstrate spawning a background computation thread for
// each mesh in a list and update the viewer when computation results in a
// change to (one of) the meshes.
// In this example, three meshes are read in and interpreted as "hands" of a
// clock. The "computation" is just a busy-wait until the hand should move
// (after one second, one minute, one hour). This is, of course, a terrible way
// to implement a clock.
// ./test libigl/tutorial/shared/{,,}
#include <igl/read_triangle_mesh.h>
#include <igl/point_mesh_squared_distance.h>
#include <igl/combine.h>
#include <igl/viewer/Viewer.h>
#include <Eigen/Geometry>
#include <thread>
int main(int argc, char * argv[])
  using namespace std;
  using namespace Eigen;
  // Read in k meshes (<=3 will be used)
  std::vector<Eigen::MatrixXd> VV(argc-1);
  std::vector<Eigen::MatrixXi> FF(argc-1);
  for(int i = 1;i<argc;i++)
    VV[i-1].col(0).array() -= VV[i-1].col(0).mean();
    VV[i-1].col(1).array() -= VV[i-1].col(1).minCoeff();
  bool continue_computing = true;
  // Flag to redraw and mutex to guard it
  bool redraw = false;
  std::mutex m;
  // After waiting `tic` seconds, rotate `V` by `deg` degrees
  const auto rot = [&continue_computing,&redraw,&m](
    const double tic, const double deg, Eigen::MatrixXd& V)
      // Let's watch this at 500x: Real clocks are slow.
      V *= Eigen::AngleAxisd(deg/180.*igl::PI,
        std::lock_guard<std::mutex> lock(m);
        redraw = true;
  // Launch background "computation" threads for each "hand" of the clock
  // std::ref is important, otherwise std::bind passes by copy
  std::vector<std::thread> threads;
    case 3:
    case 2:
    case 1:
    default: break;
  igl::viewer::Viewer v;;
  // Helper function to view k meshes
  const auto & set_meshes = [](
    const std::vector<Eigen::MatrixXd> & VV,
    const std::vector<Eigen::MatrixXi> & FF,
    igl::viewer::Viewer & v)
    Eigen::MatrixXd V;
    Eigen::MatrixXi F;
  // Continuous draw loop. TODO: trigger draws using glfwPostEmptyEvent
  v.core.is_animating = true;
  // Before every draw check if the meshes have changed. 
  v.callback_pre_draw = 
    [&redraw,&m,&VV,&FF,&set_meshes](igl::viewer::Viewer & v)->bool
          std::lock_guard<std::mutex> lock(m);
          redraw = false;
      return false;
  // Tell computation threads to stop
  continue_computing = false;
  // Join with computation threads: return to serial main thread.
  for(auto & t : threads) if(t.joinable()) t.join();

mesh clock igl viewer threads

This is pretty hacky, but it will allow me to use the standard libigl viewer for making comparisons of mesh-editing methods whose performances are very different.