How to properly rotate a raster

Tue Mar 19 06:58:16 PDT 2024

An easier way to compare what is happening is to look at the ST_Polygon output of the two.  https://postgis.net/docs/en/RT_ST_Polygon.html

I’m thinking the confusion (which I had thought myself) is that the width and height is measured against the X y axis, but now I think about it, I think it’s measured along the skew so width and height would remain the same in both cases.  In the case of your polygon you rotate and then save as raster, the new raster is axis aligned and so there you do see the width and height change.

So when I run this query:

SELECT ST_Polygon(r1.rast) AS orig_geom_rast, ST_Polygon(r2.rast) AS rotated_geom_rast, ST_Width(r1.rast) AS orig_width, ST_Width(r2.rast) AS rotated_width

                FROM (SELECT rast FROM rast_not_rotated) AS r1,

                                (SELECT ST_SetRotation(rast, -0.5 ) AS rast FROM rast_not_rotated) AS r2;

The dashed polygon is the original polygon and the filled in one is after you rotate the raster.  In both cases the ST_Width and ST_Height do not change so that’s not a good measure of what is going on.

From: Eloi Ribeiro <mail at eloiribeiro.eu> 
Sent: Tuesday, March 19, 2024 6:56 AM
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Subject: RE: How to properly rotate a raster

Hi all,

Here an example with the code.

If you compare rast_rotated_from_raster with rast_rotated_from_vector, the latest is rotated as expected and the first is not.

I guess, I must be doing something wrong when creating rast_rotated_from_raster.

Any guess?

-- non rotated raster

DROP TABLE IF EXISTS public.rast_not_rotated;

CREATE TABLE public.rast_not_rotated AS

    SELECT ST_AsRaster(t.geom,1000,1000,'8BUI',1,0) AS rast 

    FROM (SELECT ST_GeomFromText('POLYGON((166989.36462303242 505764.77048559673,

                                 166996.25788353998 505576.476318614,

                                 167265.54134611748 505585.94966528506,

                                 167258.67088940175 505774.2438322678,

                                 166989.36462303242 505764.77048559673))',28992) geom) t;

ALTER TABLE public.rast_not_rotated ADD COLUMN rid SERIAL PRIMARY KEY;

CREATE INDEX rast_not_rotated_rast_gist ON public.rast_not_rotated USING gist (st_convexhull(rast));

SELECT AddRasterConstraints('public'::name, 'rast_not_rotated'::name,'rast'::name);

-- rotate previous raster

DROP TABLE IF EXISTS public.rast_rotated_from_raster;

CREATE TABLE public.rast_rotated_from_raster AS SELECT ST_SetRotation(rast, -0.035162353342492736) rast FROM public.rast_not_rotated;

ALTER TABLE public.rast_rotated_from_raster ADD COLUMN rid SERIAL PRIMARY KEY;

CREATE INDEX rast_rotated_from_raster_rast_gist ON public.rast_rotated_from_raster USING gist (st_convexhull(rast));

SELECT AddRasterConstraints('public'::name, 'rast_rotated_from_raster'::name,'rast'::name);

-- rotate polygon and then save as raster

DROP TABLE IF EXISTS public.rast_rotated_from_vector;

CREATE TABLE public.rast_rotated_from_vector AS

    SELECT ST_AsRaster(t.geom,1000,1000,'8BUI',1,0) AS rast 

    FROM (SELECT ST_Rotate(

                    ST_GeomFromText('POLYGON((166989.36462303242 505764.77048559673,

                                    166996.25788353998 505576.476318614,

                                    167265.54134611748 505585.94966528506,

                                    167258.67088940175 505774.2438322678,

                                    166989.36462303242 505764.77048559673))',28992)

                                    ,-0.035162353342492736, 166989.36462303242, 505764.77048559673) geom) t;

ALTER TABLE public.rast_rotated_from_vector ADD COLUMN rid SERIAL PRIMARY KEY;

CREATE INDEX rast_rotated_from_vector_rast_gist ON public.rast_rotated_from_vector USING gist (st_convexhull(rast));

SELECT AddRasterConstraints('public'::name, 'rast_rotated_from_vector'::name,'rast'::name);

Eloi

On Monday, March 18th, 2024 at 19:11, Eloi Ribeiro <mail at eloiribeiro.eu <mailto:mail at eloiribeiro.eu> > wrote:

Hi Regina and all,

Elaborating a bit more. I have a vector layer consisting of a rectangle (farm plot), and this rectangle is subdivided into several (polygons) stripes parallel to the outer edge of the rectangle. I want to produce a raster where the rectangle and stripes long edges are parallel to the latitude line.

Using ST_Rotate(geom, -0.035, x, y) in all vector layers together with ST_AsRaster(), I managed to get what I want. Good.

Nevertheless, if I first produce the raster and then try to rotated it using ST_SetRotation(rast, -0.035), I barely see any rotation. In this step, I must be doing something wrong.

I would like to first rasterize vector and then rotate the raster. Instead of rasterize non rotated vector and then, again rasterize rotated vector.

The change in size (width and height), I mentioned in my first message, was while testing with 0.5 rad, not with -0.035 rad.

-- input raster

SELECT ST_Width(rast), ST_Height(rast), ST_SkewX(rast) FROM plot_3;

--  st_width | st_height | st_skewx 

-- ----------+-----------+----------

--      1000 |      1000 |        0

-- output raster of ST_SetRotation(rast, -0.035162353342492736)

SELECT ST_Width(rast), ST_Height(rast), ST_SkewX(rast) FROM plot_3_rotated_from_raster;

--  st_width | st_height |       st_skewx       

-- ----------+-----------+----------------------

--      1000 |      1000 | 0.006952538312005269

-- output raster of ST_SetRotation(rast, -0.035162353342492736)

SELECT ST_Width(rast), ST_Height(rast), ST_SkewX(rast) FROM plot_3_rotated_from_vect;

--  st_width | st_height | st_skewx 

-- ----------+-----------+----------

--       977 |       964 |        0

-- output raster of ST_SetRotation(rast, -0.5)

SELECT ST_Width(rast), ST_Height(rast), ST_SkewX(rast) FROM plot_3_rotated_from_raster_05;

--  st_width | st_height |      st_skewx       

-- ----------+-----------+---------------------

--      1000 |      1000 | 0.09481479675190897

Cheers,

Eloi

On Monday, March 18th, 2024 at 14:01, Regina Obe <lr at pcorp.us <mailto:lr at pcorp.us> > wrote:

By size, what do you mean exactly? It would change the width and height.

What are you expecting rotation to do. Perhaps you can give example out of the below and some sample

ST_Width, ST_Height, ST_SkewX before and after and what you were expecting.

https://postgis.net/docs/en/RT_ST_SetRotation.html

From: Eloi Ribeiro <mail at eloiribeiro.eu <mailto:mail at eloiribeiro.eu> > 
Sent: Monday, March 18, 2024 4:57 AM
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org <mailto:postgis-users at lists.osgeo.org> >
Subject: How to properly rotate a raster

Hi all,

I need to rotate a raster, for that I'm using the function ST_SetRotation, like so: 

CREATE TABLE plot_3_rotated AS 
SELECT ST_SetRotation(rast, 0.03516235334249185) rast 
FROM plot_3;

But I see that is not producing the expected results and is changing size of the raster. What am I doing wrong?

Cheers,

Eloi

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