= Variable-Distance Buffer =

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] 

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.

=== Example ===
{{{
SELECT ST_VariableBufferFromLine( 'LINESTRING( 0 0, 100 0)' , 1, 10 );
}}}

=== Code ===
{{{
CREATE OR REPLACE FUNCTION ST_VariableBufferFromLine(
    geom GEOMETRY, 
    start_dist NUMERIC,
    end_dist NUMERIC
)
RETURNS GEOMETRY AS
$BODY$

WITH 
    step1 AS 
        (SELECT ST_DumpPoints(geom) AS dump,  
        geom),
    step2 AS
        (SELECT (dump).path[1], 
            ST_Buffer( (dump).geom, 
                start_dist + (end_dist - start_dist) * ST_LineLocatePoint(geom, (dump).geom) 
                ) AS geom 
        FROM step1),
    step3 AS
        (SELECT 
            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;
}}}