Semantics for AffineTransform composition is reversed

[adampowerdeciphex]

When composing multiple AffineTransforms, one would expect that the composed transform is the equivalent of applying the individual transforms from left to right. Instead it is the same as applying right to left.

The solution would be to reverse the matrix multiplication of the compose() function. Alternatively this could be made clear in the docs as there may be people that depend on this right to left behaviour.

Sample code

use geo::{AffineOps, AffineTransform, Point};

fn main() {
    let point = Point::new(1.,0.);

    let translate = AffineTransform::translate(1., 0.);
    let scale = AffineTransform::scale(4., 1., [0., 0.]);
    let composed = translate.compose(&scale);

    assert_eq!(point.affine_transform(&translate), Point::new(2., 0.));
    assert_eq!(point.affine_transform(&scale), Point::new(4., 0.));
    assert_eq!(point.affine_transform(&translate).affine_transform(&scale), Point::new(8., 0.));

    // This assert fails, as the composed transform instead produces Point::new(5., 0.)
    assert_eq!(point.affine_transform(&composed), Point::new(8., 0.));
}

[lnicola]

For prior art, see https://gdal.org/api/raster_c_api.html#_CPPv424GDALComposeGeoTransformsPKdPKdPd.

[lnicola]

But I don't understand, why do you say it's right-to-left? It looks left-to-right to me (composed is equivalent to translate, then scale).

[adampowerdeciphex]

Sorry I should have mentioned that the final assert here fails. The expected result would be Point::new(8., 0.), but instead produces Point::new(5., 0.)

[michaelkirk]

Fixed in #1196 - please take a look. Thank you for catching this and providing a test!

[adampowerdeciphex]

Fantastic, thanks for the quick fix! 😃