Bad downsampling! Bad!

downsample.cpp

void Canvas::downsample(float *pixels, unsigned side)
{
  unsigned nside = side / 2;
  for (unsigned i = 0; i < nside * nside; i++) {
    unsigned row = i / nside;

    unsigned x = (i * 2) + (row * side);
    unsigned y = x + 1;
    unsigned z = x + side;
    unsigned w = z + 1;

    unsigned count = 0;
    if (!isBlack(pixels[(3 * x)], pixels[(3 * x) + 1], pixels[(3 * x) + 2])) {
      count++;

      if (x != 0) {
        pixels[(3 * i)]     += pixels[(3 * x)];
        pixels[(3 * i) + 1] += pixels[(3 * x) + 1];
        pixels[(3 * i) + 2] += pixels[(3 * x) + 2];
      }
    }

    if (!isBlack(pixels[(3 * y)], pixels[(3 * y) + 1], pixels[(3 * y) + 2])) {
      count++;

      pixels[(3 * i)]     += pixels[(3 * y)];
      pixels[(3 * i) + 1] += pixels[(3 * y) + 1];
      pixels[(3 * i) + 2] += pixels[(3 * y) + 2];
    }

    if (!isBlack(pixels[(3 * z)], pixels[(3 * z) + 1], pixels[(3 * z) + 2])) {
      count++;

      pixels[(3 * i)]     += pixels[(3 * z)];
      pixels[(3 * i) + 1] += pixels[(3 * z) + 1];
      pixels[(3 * i) + 2] += pixels[(3 * z) + 2];
    }

    if (!isBlack(pixels[(3 * w)], pixels[(3 * w) + 1], pixels[(3 * w) + 2])) {
      count++;

      pixels[(3 * i)]     += pixels[(3 * w)];
      pixels[(3 * i) + 1] += pixels[(3 * w) + 1];
      pixels[(3 * i) + 2] += pixels[(3 * w) + 2];
    }

    if (count > 0) {
      pixels[(3 * i)]     /= count;
      pixels[(3 * i) + 1] /= count;
      pixels[(3 * i) + 2] /= count;
    }
  }
}

void Canvas::draw()
{
  for (size_t i = 0; i < _curves.size(); i++) {
    if (_arePointsVisible) {
      _curves[i]->drawControlPoints();
    }
    _curves[i]->drawCurve();
  }
  glFlush();

  if (_isFinalized) {
    _isFinalized = false;

    for (unsigned size = _width; size > 2; size /= 2) {
      std::ostringstream oss;
      oss << "test_" << size << ".bmp";

      screenshot(oss.str(), size, size);

      float *pixels = new float[size * size * 3];
      glReadPixels(0, 0, size, size, GL_RGB, GL_FLOAT, pixels);

      downsample(pixels, size);

      glDrawPixels(size / 2, size / 2, GL_RGB, GL_FLOAT, pixels);
      glFlush();

      delete[] pixels;
    }
  }
}