Set NewSubfileType tag on overview IFDs for GDAL compatibility (#1150)

GDAL and rasterio use TIFF tag 254 (NewSubfileType = 1) to identify
overview sub-IFDs. Without it, overviews are written correctly but
invisible to GDAL-based tools. Verified: rasterio and gdalinfo now
see both overview levels, and pixel values match GDAL's own COG
output to within float32 rounding (max relative error < 0.0001%).

Author: brendancol

xrspatial/geotiff/_header.py

@@ -15,6 +15,7 @@
 )
 
 # Well-known TIFF tag IDs
+TAG_NEW_SUBFILE_TYPE = 254
 TAG_IMAGE_WIDTH = 256
 TAG_IMAGE_LENGTH = 257
 TAG_BITS_PER_SAMPLE = 258

xrspatial/geotiff/_writer.py

@@ -39,6 +39,7 @@
     TAG_MODEL_TIEPOINT,
 )
 from ._header import (
+    TAG_NEW_SUBFILE_TYPE,
     TAG_IMAGE_WIDTH,
     TAG_IMAGE_LENGTH,
     TAG_BITS_PER_SAMPLE,
@@ -593,6 +594,11 @@ def _assemble_tiff(width: int, height: int, dtype: np.dtype,
     for level_idx, (arr, lw, lh, rel_offsets, byte_counts, comp_data) in enumerate(pixel_data_parts):
         tags = []
 
+        # Mark overview IFDs as reduced-resolution images (TIFF tag 254).
+        # GDAL/rasterio use this tag to identify overview sub-IFDs.
+        if level_idx > 0:
+            tags.append((TAG_NEW_SUBFILE_TYPE, LONG, 1, 1))
+
         tags.append((TAG_IMAGE_WIDTH, LONG, 1, lw))
         tags.append((TAG_IMAGE_LENGTH, LONG, 1, lh))
         if samples_per_pixel > 1: