From 580ec38ab85aff795429a8a74b5286b1fd63444c Mon Sep 17 00:00:00 2001
From: Javier Goizueta <jgoizueta@gmail.com>
Date: Sat, 23 Apr 2016 15:07:06 +0200
Subject: [PATCH 1/3] Adjust overview clustered point to pixel centers

Fixes #240
---
 scripts-available/CDB_Overviews.sql | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/scripts-available/CDB_Overviews.sql b/scripts-available/CDB_Overviews.sql
index 699ca5c..a12c31b 100644
--- a/scripts-available/CDB_Overviews.sql
+++ b/scripts-available/CDB_Overviews.sql
@@ -652,7 +652,9 @@ AS $$
     overview_rel TEXT;
     reduction FLOAT8;
     base_name TEXT;
+    pixel_m FLOAT8;
     grid_m FLOAT8;
+    offset_m FLOAT8;
     aggr_attributes TEXT;
     attributes TEXT;
     columns TEXT;
@@ -678,8 +680,10 @@ AS $$
 
     SELECT * FROM _cdb_split_table_name(reloid) INTO schema_name, table_name;
 
-    -- compute grid cell size using the overview_z dimension...
-    SELECT CDB_XYZ_Resolution(overview_z)*grid_px INTO grid_m;
+    -- pixel_m: size of a pixel in webmercator units (meters)
+    SELECT CDB_XYZ_Resolution(overview_z) INTO pixel_m;
+    -- grid size in meters
+    grid_m = grid_px * pixel_m;
 
     attributes := _CDB_Aggregable_Attributes_Expression(reloid);
     aggr_attributes := _CDB_Aggregated_Attributes_Expression(reloid);
@@ -690,7 +694,12 @@ AS $$
       aggr_attributes := aggr_attributes || ', ';
     END IF;
 
-    point_geom = Format('ST_SetSRID(ST_MakePoint(gx*%1$s + %2$s, gy*%1$s + %2$s), 3857)', grid_m, grid_m/2);
+    -- points are displaced to it's cell's center
+    offset_m := grid_m/2;
+    -- then corrected to be at the nearest pixel's center (by up to half a pixel)
+    offset_m := offset_m + pixel_m/2 - MOD((grid_m/2)::numeric, pixel_m::numeric)::float8;
+
+    point_geom := Format('ST_SetSRID(ST_MakePoint(gx*%1$s + %2$s, gy*%1$s + %2$s), 3857)', grid_m, offset_m);
 
     -- compute the resulting columns in the same order as in the base table
     WITH cols AS (

From 7b2100b51e07f3ddbd07b95d7cff07e0e51cface Mon Sep 17 00:00:00 2001
From: Javier Goizueta <jgoizueta@gmail.com>
Date: Mon, 25 Apr 2016 13:33:43 +0200
Subject: [PATCH 2/3] Adjust overview coordinates to pixel centers

This makes the adjustment for all grid sizes, not only
for integral number of pixels.
---
 scripts-available/CDB_Overviews.sql | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/scripts-available/CDB_Overviews.sql b/scripts-available/CDB_Overviews.sql
index a12c31b..22703aa 100644
--- a/scripts-available/CDB_Overviews.sql
+++ b/scripts-available/CDB_Overviews.sql
@@ -655,6 +655,10 @@ AS $$
     pixel_m FLOAT8;
     grid_m FLOAT8;
     offset_m FLOAT8;
+    offset_x TEXT;
+    offset_y TEXT;
+    cell_x TEXT;
+    cell_y TEXT;
     aggr_attributes TEXT;
     attributes TEXT;
     columns TEXT;
@@ -694,12 +698,15 @@ AS $$
       aggr_attributes := aggr_attributes || ', ';
     END IF;
 
-    -- points are displaced to it's cell's center
-    offset_m := grid_m/2;
-    -- then corrected to be at the nearest pixel's center (by up to half a pixel)
-    offset_m := offset_m + pixel_m/2 - MOD((grid_m/2)::numeric, pixel_m::numeric)::float8;
+    -- Center of each cell:
+    cell_x := Format('gx*%1$s + %2$s', grid_m, grid_m/2);
+    cell_y := Format('gy*%1$s + %2$s', grid_m, grid_m/2);
 
-    point_geom := Format('ST_SetSRID(ST_MakePoint(gx*%1$s + %2$s, gy*%1$s + %2$s), 3857)', grid_m, offset_m);
+    -- Displacement to the nearest pixel center:
+    offset_x := Format('%2$s/2 - MOD((%1$s)::numeric, (%2$s)::numeric)::float8', cell_x, pixel_m);
+    offset_y := Format('%2$s/2 - MOD((%1$s)::numeric, (%2$s)::numeric)::float8', cell_y, pixel_m);
+
+    point_geom := Format('ST_SetSRID(ST_MakePoint(%1$s + %3$s, %2$s + %4$s), 3857)', cell_x, cell_y, offset_x, offset_y);
 
     -- compute the resulting columns in the same order as in the base table
     WITH cols AS (

From 3c12cf629f75e1814ea558f19bad747520906e3a Mon Sep 17 00:00:00 2001
From: Javier Goizueta <jgoizueta@gmail.com>
Date: Mon, 25 Apr 2016 13:53:59 +0200
Subject: [PATCH 3/3] Optimize overview pixel adjustment for integer-pixel
 cells

---
 scripts-available/CDB_Overviews.sql | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/scripts-available/CDB_Overviews.sql b/scripts-available/CDB_Overviews.sql
index 22703aa..ab0b606 100644
--- a/scripts-available/CDB_Overviews.sql
+++ b/scripts-available/CDB_Overviews.sql
@@ -703,8 +703,14 @@ AS $$
     cell_y := Format('gy*%1$s + %2$s', grid_m, grid_m/2);
 
     -- Displacement to the nearest pixel center:
-    offset_x := Format('%2$s/2 - MOD((%1$s)::numeric, (%2$s)::numeric)::float8', cell_x, pixel_m);
-    offset_y := Format('%2$s/2 - MOD((%1$s)::numeric, (%2$s)::numeric)::float8', cell_y, pixel_m);
+    IF MOD(grid_px::numeric, 1.0::numeric) = 0 THEN
+      offset_m := pixel_m/2 - MOD((grid_m/2)::numeric, pixel_m::numeric)::float8;
+      offset_x := Format('%s', offset_m);
+      offset_y := Format('%s', offset_m);
+    ELSE
+      offset_x := Format('%2$s/2 - MOD((%1$s)::numeric, (%2$s)::numeric)::float8', cell_x, pixel_m);
+      offset_y := Format('%2$s/2 - MOD((%1$s)::numeric, (%2$s)::numeric)::float8', cell_y, pixel_m);
+    END IF;
 
     point_geom := Format('ST_SetSRID(ST_MakePoint(%1$s + %3$s, %2$s + %4$s), 3857)', cell_x, cell_y, offset_x, offset_y);