glm::closestPointOnLine

static float z = 3.f;
{
  z -= 0.1;

  glm::vec3 p(1, z, 0); //どこか頂点
  glm::vec3 a(0, -2, 0);//線分の始点
  glm::vec3 b(0, 2, 0); //線分の終点

  // 点に最も近い直線上の点を求めるのに必要
  // #include <glm/gtx/closest_point.hpp>
  glm::vec3 kk = glm::closestPointOnLine(p, a, b);

  glLineWidth(3);//線分
  glBegin(GL_LINES);
  glVertex3fv(glm::value_ptr(a));
  glVertex3fv(glm::value_ptr(b));
  glEnd();

  glPointSize(7);//どこか頂点
  glColor3d(1, 0, 0);
  glBegin(GL_POINTS);
  glVertex3fv(glm::value_ptr(p));
  glEnd();

  glPointSize(7);//線分上の点
  glColor3d(0, 1, 0);
  glBegin(GL_POINTS);
  glVertex3fv(glm::value_ptr(kk));
  glEnd();
}