title: External function to render cloud of points
body: <p>I'm currently building a plug-in in C++ for the program Igor Pro. This plug-in takes three arrays of points (X, Y, Z) and returns a matrix with the pixel values of the data (float array[windowsize][windowsize]).</p>
  
  <p>I've already developed a solution in OpenGL (glew) by using VBOs but I feel that it's not the optimal solution. </p>
  
  <p>1) Do you think I should use offscreen rendering with FBOs or PBOs? I would like to make it run as fast as possible.</p>
  
  <p>2) Also, I'd like to run glutMainLoop only once. Should I change to FreeGLUT?</p>
  
  <pre><code>/*  
          RenderTriangle.c
  */
  #include "VC10\triangulation.h"
  #include "XOPStandardHeaders.h"         // Include ANSI headers, Mac headers, IgorXOP.h, XOP.h and XOPSupport.h
  #include "glui.h"
  
  // Prototypes
  HOST_IMPORT int main(IORecHandle ioRecHandle);
  
  // Global Variables
  const int windowSize = 512;
  GLuint ID;
  int size,el_size;
  float* waveX=NULL;
  float* waveY=NULL;
  float* waveZ=NULL;
  waveHndl waveH;
  
  // Custom error codes
  #define OLD_IGOR 1 + FIRST_XOP_ERR
  #define NON_EXISTENT_WAVE 2 + FIRST_XOP_ERR
  #define REQUIRES_SP_OR_DP_WAVE 3 + FIRST_XOP_ERR
  
  #pragma pack(2)     // All structures passed to Igor are two-byte aligned.
  typedef struct FitParams {
      waveHndl waveX; 
      waveHndl waveY;
      waveHndl waveZ;
      double result;
  } FitParams, *FitParamsPtr;
  #pragma pack()      // Restore default structure alignment
  
  // Prototypes of the OpenGL callbacks
  void init(void);
  void display(void);
  void idle(void);
  
  void init(void){
  
      float z_val=0;
      int point=0;
  
      //Loading data from the text files and applying the Delaunay algorithm
      triangulateio tri_data=triangulationWave(waveX,waveY,waveZ);
      vector&lt;float&gt;data_vector(tri_data.numberoftriangles*3*6); //3 points with 6 coordinates (position and color)
  
      size=data_vector.size();
  
      double zMin=tri_data.pointattributelist[N_ELEMENTS],zMax=tri_data.pointattributelist[N_ELEMENTS+1];
      for (int i_tri = 0; i_tri &lt; tri_data.numberoftriangles; i_tri++) {
  
          for (int i_point = 0; i_point &lt; 3; i_point++) {
  
              z_val=tri_data.pointattributelist[tri_data.trianglelist[i_tri * 3 + i_point]];
              z_val=1-((z_val-zMin)/(zMax-zMin)); //normalize and invertion
  
              point=6*(i_tri *3 + i_point);
  
              data_vector[point+0]=tri_data.pointlist[2*tri_data.trianglelist[i_tri * 3 + i_point]];
              data_vector[point+1]=tri_data.pointlist[2*tri_data.trianglelist[i_tri * 3 + i_point]+1];
              data_vector[point+2]=z_val;
              data_vector[point+3]=z_val;
              data_vector[point+4]=z_val;
              data_vector[point+5]=z_val;
          }
      }
  
      glewInit();
  
      glEnable(GL_DEPTH_TEST);
      glDepthMask(GL_TRUE);
      glDepthFunc(GL_LEQUAL);
      glDepthRange(-2.0f, 2.0f);
      glClearColor(0.0f, 1.0f, 0.0f, 0.0f);
      glShadeModel(GL_SMOOTH);
      glEnableClientState(GL_VERTEX_ARRAY);
      glGenBuffers(1,&amp;ID);
      glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ID);
      glBufferData(GL_ELEMENT_ARRAY_BUFFER,size*sizeof(float), &amp;data_vector[0], GL_STATIC_DRAW);
  
      el_size=6*sizeof(float);
  }
  void display(void){
  
      int zoom = 1;
      glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
      glMatrixMode(GL_PROJECTION);
      glLoadIdentity();
      glOrtho(-2*zoom, 2*zoom,2*zoom,-2*zoom, -5.0f, 5.0f);
      glPushMatrix ();
      glBindBuffer(GL_ARRAY_BUFFER, ID);
  
      // Pointer for the position of the points
      glEnableClientState(GL_VERTEX_ARRAY);
      glVertexPointer(3, GL_FLOAT, el_size, 0);
  
      // Pointer for the color of the points
      glEnableClientState(GL_COLOR_ARRAY);
      glColorPointer(3,GL_FLOAT, el_size,(void*)(el_size/2));
  
      //Draw callback
      glDrawArrays(GL_TRIANGLES,0,size/6);
  
      glPopMatrix ();
      glFlush();
  }
  
  // Idle callback function for OpenGL API
  void idle(void){
  
      float* pixels = (float*)malloc(windowSize*windowSize*sizeof(float));
      glReadPixels(0, 0, windowSize, windowSize, GL_DEPTH_COMPONENT, GL_FLOAT, pixels);
  
      //
      //char waveName[MAX_OBJ_NAME+1]; 
      //CountInt dimensionSizes[MAX_DIMENSIONS+1]; 
      //float* wp;
      //strcpy(waveName, "wave0");
      //MemClear(dimensionSizes, sizeof(dimensionSizes));
      //dimensionSizes[ROWS] = windowSize*windowSize; // Make 1D wave 
      //wp = (float*)WaveData(waveH); // Get a pointer to the wave data 
  
      //for(int i = 0;i &lt; windowSize*windowSize; i++){    
      //  *wp++ =pixels[i];                 
      //}
  
      //glDeleteBuffers(1,&amp;ID);
  }
  
  // Main function for the XOP
  extern "C" int
  RenderTriangle(FitParamsPtr p)
  {
      waveX=(float*)WaveData(p-&gt;waveX);
      waveY=(float*)WaveData(p-&gt;waveY);
      waveZ=(float*)WaveData(p-&gt;waveZ);
  
      char *myargv [1];
      int myargc=1;
      myargv [0]=strdup ("RenderTriangle");
      glutInit(&amp;myargc,myargv);
      glutInitDisplayMode(GLUT_SINGLE | GLUT_DEPTH | GLUT_RGB);
      glutInitWindowSize(windowSize,windowSize);
      glutCreateWindow("OpenGL");
      init();
      glutDisplayFunc(display);
      glutIdleFunc(idle);
      glutMainLoop();
      p-&gt;result = 1;
  
      return 0;
  }
  
  static XOPIORecResult
  RegisterFunction()
  {
      int funcIndex;
  
      funcIndex = (int)GetXOPItem(0);         // Which function invoked ?
      switch (funcIndex) {
          case 0:                                 // y = RenderTriangle(w,x) (curve fitting function).
              return (XOPIORecResult)RenderTriangle;  // This function is called using the direct method.
              break;
      }
      return 0;
  }
  
  /*  XOPEntry()
  
      This is the entry point from the host application to the XOP for all
      messages after the INIT message.
  */
  extern "C" void
  XOPEntry(void)
  {   
      XOPIORecResult result = 0;
  
      switch (GetXOPMessage()) {
          case FUNCADDRS:
              result = RegisterFunction();    // This tells Igor the address of our function.
              break;
      }
      SetXOPResult(result);
  }
  
  /*  main(ioRecHandle)
  
      This is the initial entry point at which the host application calls XOP.
      The message sent by the host must be INIT.
      main() does any necessary initialization and then sets the XOPEntry field of the
      ioRecHandle to the address to be called for future messages.
  */
  HOST_IMPORT int
  main(IORecHandle ioRecHandle)
  {   
      XOPInit(ioRecHandle);                           // Do standard XOP initialization.
      SetXOPEntry(XOPEntry);                          // Set entry point for future calls.
  
      if (igorVersion &lt; 600) {
          SetXOPResult(OLD_IGOR);
          return EXIT_FAILURE;
      }
  
      SetXOPResult(0);
      return EXIT_SUCCESS;
  }
  </code></pre>