It sounds like you'd like to apply an affine transformation (ST_Affine) to your
geometry.
Two simple wrappers may help you:
ST_Scale will expand/scale a polygon by a specified value in x and y.
ST_Translate will translate the geometry if you wanted to scale at the
midpoint of the geometry.
SELECT
ST_AsText(
ST_Translate(
ST_Scale(geom, 3, 2),
-3 * (ST_Xmin(geom)+ST_XMax(geom))/2 + ((ST_Xmin(geom)+ST_XMax(geom))/2),
-2 * (ST_Ymin(geom)+ST_YMax(geom))/2 + ((ST_Ymin(geom)+ST_YMax(geom))/2))
)
FROM
(SELECT 'POLYGON (( 2 5, 2 6, 3 6, 3 5, 2 5 ))'::geometry AS geom) a;
st_astext
------------------------------------------
POLYGON((1 4.5,1 6.5,4 6.5,4 4.5,1 4.5))
(1 row)
-- Or use ST_Affine directly:
SELECT
ST_AsText(
ST_Affine(
geom, 3, 0, 0, 2,
-3 * (ST_Xmin(geom)+ST_XMax(geom))/2 + ((ST_Xmin(geom)+ST_XMax(geom))/2),
-2 * (ST_Ymin(geom)+ST_YMax(geom))/2 + ((ST_Ymin(geom)+ST_YMax(geom))/2))
)
FROM
(SELECT 'POLYGON (( 2 5, 2 6, 3 6, 3 5, 2 5 ))'::geometry AS geom) a;
st_astext
------------------------------------------
POLYGON((1 4.5,1 6.5,4 6.5,4 4.5,1 4.5))
(1 row)
Ideally, it looks like there should an optional parameter to ST_Scale that would allow a user to scale about the
midpoint or lower left directly. I'll add this as a wish list item.
Cheers,
Kevin
Yurka wrote:
I'm wondering if such thing exists.
I have a polygon that i need to expand by x for each coordinate
equivalent would be a function ST_Expand(), but ST_Expand expands the
bounding box not the polygon itself.
maybe someone has a workaround?
thanks in advance.
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