v.to.rast: Fix crash when rasterizing points with the dense flag

vector/v.to.rast/dense_line.c

@@ -120,6 +120,18 @@ void plot_line_dense(double east1, double north1, double east2, double north2)
     }
 }
 
+/* dense point plotting, alternative to G_plot_point()
+ * east, north are map coordinates.
+ * Needed because dense mode never calls G_setup_plot(), so the move and cont
+ * routines G_plot_point() uses are not set. The coordinate is truncated to the
+ * cell that contains it, matching what dense_line() does with its endpoints:
+ * setup_plot() frames the region by cell edges, unlike G_setup_plot(), which
+ * frames it by cell centres and therefore rounds. */
+void plot_point_dense(double east, double north)
+{
+    st->dot((int)X(G_adjust_easting(east, &st->window)), (int)Y(north));
+}
+
 /* dense line plotting, alternative to G_bresenham_line()
  * x1, y1, x2, y2 are col, row numbers */
 void dense_line(double x1, double y1, double x2, double y2,

vector/v.to.rast/do_lines.c

@@ -7,7 +7,7 @@
 
 /* function prototypes */
 static int plot_line(double *, double *, int, int, int);
-static int plot_points(double *, double *, int);
+static int plot_points(double *, double *, int, int);
 static double v2angle(double[2], double[2], double, double);
 static double deg_angle(double, double, double, double);
 
@@ -131,7 +131,7 @@ int do_lines(struct Map_info *Map, struct line_pnts *Points,
             count++;
         }
         else if (type & GV_POINTS) {
-            plot_points(Points->x, Points->y, Points->n_points);
+            plot_points(Points->x, Points->y, Points->n_points, dense);
             count++;
         }
     }
@@ -199,11 +199,15 @@ static double deg_angle(double x0, double y0, double x1, double y1)
     return (v_ang * 360.0 / M_2PI);
 }
 
-static int plot_points(double *x, double *y, int n)
+static int plot_points(double *x, double *y, int n, int dense)
 {
     /* only plot the first point */
-    if (n > 0)
-        G_plot_point(*x, *y);
+    if (n > 0) {
+        if (dense)
+            plot_point_dense(*x, *y);
+        else
+            G_plot_point(*x, *y);
+    }
 
     return 0;
 }

vector/v.to.rast/local.h

@@ -30,6 +30,7 @@ int do_lines(struct Map_info *, struct line_pnts *, dbCatValArray *, int, int,
              struct cat_list *, int, double, int, int, int *, int);
 
 void plot_line_dense(double, double, double, double);
+void plot_point_dense(double, double);
 void setup_plot(double, double, double, double, int (*dot)(int, int));
 
 /* raster.c */

vector/v.to.rast/tests/conftest.py

@@ -0,0 +1,33 @@
+import os
+
+import pytest
+
+import grass.script as gs
+from grass.tools import Tools
+
+
+@pytest.fixture
+def xy_points_session(tmp_path):
+    """Active session in an XY project holding a small point map"""
+    project = tmp_path / "xy_test"
+    gs.create_project(project)
+    with (
+        gs.setup.init(project, env=os.environ.copy()) as session,
+        Tools(session=session) as tools,
+    ):
+        points = tmp_path / "points.txt"
+        # Deliberately off the cell boundaries: with a 1 unit resolution
+        # starting at 0, a whole-numbered coordinate falls on a cell edge,
+        # where rounding to the nearest cell and truncating to the containing
+        # cell agree. These do not, so they detect a half cell shift.
+        points.write_text("10.6|10.7|1\n20.2|20.3|2\n30.9|15.1|3\n")
+        tools.g_region(s=0, n=50, w=0, e=50, res=1)
+        tools.v_in_ascii(
+            input=str(points),
+            output="points",
+            separator="pipe",
+            x=1,
+            y=2,
+            cat=3,
+        )
+        yield session

vector/v.to.rast/tests/v_to_rast_dense_test.py

@@ -0,0 +1,31 @@
+"""Tests for rasterizing points with the dense flag
+
+Dense mode configures its own plotting routines and never calls G_setup_plot(),
+but points were always plotted with G_plot_point(), which uses the move and cont
+routines that only G_setup_plot() sets. Rasterizing a point map with -d
+therefore called a null function pointer and the module crashed
+(https://github.com/OSGeo/grass/issues/3105).
+"""
+
+from grass.tools import Tools
+
+
+def test_dense_points_match_plain_points(xy_points_session):
+    """Points rasterize into the same cells with and without the dense flag
+
+    The flag densifies lines, so for points there is nothing to densify and
+    both runs have to place every point in the same cell. The coordinates in
+    the fixture sit inside cells rather than on their edges, so a half cell
+    shift in either direction changes the row or the column and fails here.
+    """
+    tools = Tools(session=xy_points_session)
+
+    tools.v_to_rast(input="points", output="plain", use="cat", type="point")
+    tools.v_to_rast(input="points", output="dense", use="cat", type="point", flags="d")
+
+    plain = tools.r_stats(input="plain", flags="gn").text.splitlines()
+    dense = tools.r_stats(input="dense", flags="gn").text.splitlines()
+
+    # east, north and category of the centre of the cell each point landed in
+    assert sorted(plain) == ["10.5 10.5 1", "20.5 20.5 2", "30.5 15.5 3"]
+    assert sorted(dense) == sorted(plain)