[pdal] writers.gdal: counting all points in a cell

[Jim Klassen]

There is a comment in GDALGrid.cpp about counting all points if they are in a cell, ignoring the radius for that cell.

     // This is a questionable case.  If a point is in a cell, shouldn't
     // it just be counted?

My experience is that there are times when I want this behavior when I want to know every point is assigned to one and only one cell. Currently, the only options are to set radius so that the circle includes the whole cell (but also includes area outside the cell) or the circle only includes area in the cell, but also skips some area of the cell.

There are also times, I want radius to act exactly as it does now, viewing the cell as sampling an area around the centroid of the cell.  I can imagine cases (ex. large cell size with high point density) where a radius smaller than the cell size makes sense. This existing mode also seems to me like it is more representative of the physics of sampling a surface with a laser pulse which has a non-zero cross section than the "just count everything that falls in the cell" mode.

So, I want to be able to choose the behavior depending on the my current situation.

I propose choosing the "count all points that fall in the cell" mode if the user supplies a radius that is negative and maintaining the current behavior if the radius is 0 or positive?  I'm not sure a zero radius is useful, it is currently supported and potentially could select points exactly aligned with the center of the cell.  A negative radius currently would never select any points (since distance is always >= 0).

As a potential optimization, since radius is negative, the updateXXXQuadrant() calls don't select any cells, but do take minimal time in this case.  I'm not sure if there would be an overall benefit of guarding the updateXXXQuadrant() calls with a:
     if (m_radius >= 0)
for this new case vs the existing case. It probably depends highly on the CPU branch predictor.

The following patch implements this idea:

diff --git a/io/private/GDALGrid.cpp b/io/private/GDALGrid.cpp
index dade8f792..4f319f4ee 100644
--- a/io/private/GDALGrid.cpp
+++ b/io/private/GDALGrid.cpp
@@ -232,8 +232,9 @@ void GDALGrid::addPoint(double x, double y, double z)

      // This is a questionable case.  If a point is in a cell, shouldn't
      // it just be counted?
+    // Just count points in the cell if m_radius is < 0, otherwise act normally
      double d = distance(iOrigin, jOrigin, x, y);
-    if (d < m_radius &&
+    if ((m_radius < 0 || d < m_radius) &&
          iOrigin >= 0 && jOrigin >= 0 &&
          iOrigin < width() && jOrigin < height())
          update(iOrigin, jOrigin, z, d);