to_geotiff: round-trip descending x and ascending y (#1716)

[brendancol]

Summary

to_geotiff lost the sign of pixel_width and pixel_height when writing DataArrays with descending x or ascending y coords. The writer stored abs(...) in ModelPixelScaleTag (33550) and the reader hard-coded a north-up reconstruction, so the round trip silently changed the georeference.

Repro

import numpy as np
import xarray as xr
from xrspatial.geotiff import to_geotiff, open_geotiff

x = np.array([200.0, 190.0, 180.0, 170.0, 160.0])  # descending
y = np.array([50.0, 40.0, 30.0, 20.0])
arr = np.arange(20, dtype=np.float32).reshape(4, 5)
da = xr.DataArray(arr, dims=('y', 'x'), coords={'y': y, 'x': x})
to_geotiff(da, '/tmp/out.tif', crs=4326)
print(open_geotiff('/tmp/out.tif').coords['x'].values)
# Before: [-200. -190. -180. -170. -160.]  (sign flipped)
# After:  [ 200.  190.  180.  170.  160.]

Fix

• Writer (build_geo_tags): when orientation is non-standard (pixel_width < 0 or pixel_height > 0), emit ModelTransformationTag (34264) instead of ModelPixelScaleTag + ModelTiepointTag. ModelPixelScale requires positive scales per the GeoTIFF spec, so the transformation tag's signed 4x4 affine is the right route here.
• North-up branch is unchanged: ModelPixelScaleTag + ModelTiepointTag are still emitted bit-for-bit identically.
• Reader was already correct: it prefers ModelTransformationTag when present and falls back to scale + tiepoint otherwise.
• write_vrt: also fixed dst_y_off / dst_x_off to use the top-left corner of each source's geographic extent rather than origin_y / origin_x directly. Without this, mosaics built from sources with ascending y placed tiles outside the VRT extent (which the new writer behavior now exposes for the first time).

Tests

New xrspatial/geotiff/tests/test_descending_coords_1716.py:

• test_descending_x_roundtrip -- descending x coords survive
• test_ascending_y_roundtrip -- ascending y coords survive
• test_descending_x_and_ascending_y_roundtrip -- both axes flipped
• test_north_up_still_uses_pixel_scale_and_tiepoint -- standard orientation keeps the existing tag layout
• test_descending_x_uses_transformation_tag -- non-standard orientation emits 34264 and omits 33550 / 33922
• test_ascending_y_uses_transformation_tag -- same check on the y axis

Full geotiff test suite passes apart from two pre-existing failures (test_window_clamps_to_raster_bounds, test_window_clamps_negative_offsets) that fail on main and are unrelated.

Closes #1716

[Copilot]

Pull request overview

Fixes GeoTIFF round-tripping for DataArrays with non-standard axis orientation (descending x and/or ascending y) by encoding the full signed affine transform in GeoTIFF tags, and updates VRT mosaic placement to correctly compute offsets for these orientations.

Changes:

• Write ModelTransformationTag (34264) instead of ModelPixelScaleTag + ModelTiepointTag when the transform has descending x or ascending y.
• Teach the writer to serialize ModelTransformationTag (16 doubles) into IFDs for both regular and streaming writes.
• Fix write_vrt destination offsets to use each source’s true top/left edges (works for ascending-y sources) and add regression tests for #1716.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
xrspatial/geotiff/_geotags.py	Emits ModelTransformationTag for non-north-up transforms to preserve pixel scale sign.
xrspatial/geotiff/_writer.py	Serializes ModelTransformationTag values into TIFF tags (16 doubles).
xrspatial/geotiff/_vrt.py	Computes VRT tile offsets from source top/left extent rather than assuming north-up origin.
xrspatial/geotiff/tests/test_descending_coords_1716.py	Adds regression coverage for descending/ascending coordinate round-trips and tag layout expectations.

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