Apply an arbitrary affine transform

Transforms every coordinate by the six coefficients of an affine matrix. The geometry type of the output matches the input.

Usage

ga_affine_transform(geometry, a, b, xoff, d, e, yoff)

Arguments

geometry

a GeoArrow point, linestring, multilinestring, polygon, or multipolygon array

a, b, d, e

the linear part of the matrix; each length 1 or the same length as geometry

xoff, yoff

the translation part; each length 1 or the same length as geometry

Value

a GeoArrow array of the same geometry type as geometry

Details

The coefficients map a coordinate to x' = a * x + b * y + xoff and y' = d * x + e * y + yoff, the same ordering PostGIS ST_Affine and Shapely use. The identity transform is a = 1, b = 0, xoff = 0, d = 0, e = 1, yoff = 0.

This is the general form behind ga_translate(), ga_scale_xy(), ga_rotate_around_centroid(), and ga_skew(). Reach for those when they fit, since they are clearer at the call site. Use this one to apply a matrix you already have, or to compose several steps into a single pass over the coordinates.

All six coefficients are recycled against geometry, so a different transform can be applied to every row.

References

AffineOps

See also

Other affine: ga_rotate_around_center(), ga_rotate_around_centroid(), ga_scale_xy(), ga_skew(), ga_skew_xy(), ga_translate()

Examples

ring <- rbind(c(0, 0), c(1, 0), c(1, 1), c(0, 1), c(0, 0))
square <- sf::st_polygon(list(ring))
g <- geoarrow::as_geoarrow_array(sf::st_sfc(square))

# double the width and shift right by 10
res <- ga_affine_transform(g, a = 2, b = 0, xoff = 10, d = 0, e = 1, yoff = 0)
sf::st_as_sfc(geoarrow::as_geoarrow_vctr(res))
#> Geometry set for 1 feature 
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: 10 ymin: 0 xmax: 12 ymax: 1
#> CRS:           NA
#> POLYGON ((10 0, 12 0, 12 1, 10 1, 10 0))