CODEWITHSUNDEEP
c++openglmfc
manatttta
manatttta

Reputation: 3124

OpenGL main loop drawing huge vector of points

I have a MFC application which main draw loop has to draw a set of huge points. Right now, this is done as follows:

void CmodguiView::OnDraw(CDC* /*pDC*/) {

  wglMakeCurrent(m_hDC, m_hRC);

  // Clear color and depth buffer bits
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  const std::vector<DensePoint> *pts;
  pts = getPts();

  if (pts) {
    glBegin(GL_POINTS);
    for (auto &&pt : *pts) {
        glColor3ub(pt.r, pt.g, pt.b);
        glVertex3f(pt.x, pt.y, pt.z);
    }
    glEnd();


    SwapBuffers(m_hDC);
  }

}

How do I optimize this? Can I avoid the for loop? Would it be possible, for instance, to rotate the points directly?

Upvotes: 0

Views: 690

Answers (1)

datenwolf
datenwolf

Reputation: 162164

The bottleneck are the countless calls of glVertex. This is called immediate mode, is denoted by glBegin…glEnd block and it has been deprecated for well over 10 years. Also immediate mode got obsolete with the introduction of vertex arrays over 15 years ago. So don't use it.

Instead you should use vertex arrays. You may combine them with buffer objects to further improve performance.

Anyway, assuming DensePoint is written as

struct DensePoint {
    GLubyte r,g,b;
    GLfloat x,y,z;
};

You can replace the glBegin…glEnd block with

std::vector<DensePoint> const * const pts = getPts();
if(pts) {
    glEnableClientState(GL_COLOR_ARRAY);
    glEnableClientState(GL_VERTEX_ARRAY);

    glColorPointer (3, GL_UNSIGNED_BYTE,
                    sizeof(DensePoint), &((*pts)[0].r));
    glVertexPointer(3, GL_FLOAT,
                    sizeof(DensePoint), &((*pts)[0].x));

    glDrawArrays(GL_POINTS, 0, pts->size());
}

Note that this still uses the old OpenGL-1.1 fixed function pipeline and points data stays in CPU/system memory, so this is will get not as much throughput as modern VBO based drawing operations. Also with modern OpenGL you get freely defined vertex attributes through shaders. But it's easy enough to go there starting from the code above.

Upvotes: 3

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