a scanline rasterizer derivation

raster.cpp

void span( float x0, float y0 )
{
  // blah
}

void raster( vec2 point[3] )
{
  std::array<mut::vec2, 3> p = {
    point[0], point[1], point[2]
  };

  // sorting network for the vertices
  if (p[2].y < p[1].y) std::swap(p[2], p[1]);
  if (p[1].y < p[0].y) std::swap(p[1], p[0]);
  if (p[2].y < p[1].y) std::swap(p[2], p[1]);

  // triangles will be of either form:
  //
  //    0             0
  //                     1
  //  1
  //      2          2
  //
  // test point 1 on line 0->2 ?
  //

  // find the gradients (dx/dy)
  const std::array<float, 3> d = {
    (p[1].x - p[0].x) / (p[1].y - p[0].y),
    (p[2].x - p[1].x) / (p[2].y - p[1].y),
    (p[0].x - p[2].x) / (p[0].y - p[2].y),
  };

  // find y crossings
  const float miny = floor(p[0].y + .5f) + .5f;
  const float midy = p[1].y;
  const float maxy = p[2].y;

  // top half
  draw::colour( 0x00FF00 );

  float x0, x1;
  float y = miny;

  // calculate top half starting points
  x0 = p[0].x + (y - p[0].y) * d[0];
  x1 = p[2].x + (y - p[2].y) * d[2];

  // top half
  for (; y < midy; y += 1.f) {
    span(x0, x1);
    x0 += d[0];
    x1 += d[2];
  }

  // calculate bottom half starting points
  x0 = p[1].x + (y - p[1].y) * d[1];
  x1 = p[2].x + (y - p[2].y) * d[2];

  // bottom half
  for (; y < maxy; y += 1.f) {
    span(x0, x1);
    x0 += d[1];
    x1 += d[2];
  }
}