Changes between Version 1 and Version 2 of UserWikiVariableBuffer

Timestamp:

11/25/19 10:43:50 (5 years ago)

Author:

mdavis

Comment:

--

UserWikiVariableBuffer

@@ -3 +3 @@
 Source: [https://gis.stackexchange.com/questions/340968/varying-size-buffer-along-a-line-with-postgis GIS StackExchange: Varying size buffer along a line with PostGIS] 
 
-Using the function given below, variable-distance buffers can be generated.
+The function below generates variable-distance buffers.
+It operates by creating the union of a series of segment buffers .
+Each segment buffer is the convex hull of the buffers of the segment start and end points, 
+with the buffer distance computed proportionally to the fractional distance along the line.
+
+**Note: currently only the end distance can be given.  The start distance is approximately 0.**
 
 === Example ===
 {{{
-SELECT gid, ST_VariableBufferFromLine(geom,10.0) AS geom FROM mylines
+SELECT ST_VariableBufferFromLine( 'LINESTRING( 0 0, 100 0)' , 10 );
 }}}
 
@@ -14 +19 @@
 CREATE OR REPLACE FUNCTION ST_VariableBufferFromLine(
     geom GEOMETRY, 
-    Length_BufferSize_Ratio NUMERIC
+    end_dist NUMERIC
 )
-
 RETURNS GEOMETRY AS
-
 $BODY$
 
 WITH 
     step1 AS 
-        (SELECT ST_DumpPoints(geom) AS dump, 
-        ST_Length(geom) AS len, 
+        (SELECT ST_DumpPoints(geom) AS dump,  
         geom),
     step2 AS
         (SELECT (dump).path[1], 
-        ST_Buffer((dump).geom, GREATEST(ST_LineLocatePoint(geom, (dump).geom)*len/Length_BufferSize_Ratio,0.001)) AS geom 
-         FROM step1),
+            ST_Buffer( (dump).geom, 
+                GREATEST(end_dist * ST_LineLocatePoint(geom, (dump).geom), 0.001)) AS geom 
+        FROM step1),
     step3 AS
         (SELECT 
-        ST_ConvexHull(ST_Union(geom, LEAD(geom) OVER(ORDER BY path))) AS geom 
+            ST_ConvexHull(ST_Union(geom, LEAD(geom) OVER(ORDER BY path))) AS geom 
         FROM step2)
 SELECT ST_Union(geom) AS geom FROM step3
 
 $BODY$
-
 LANGUAGE SQL;
 }}}