Explainers

How each GDAL mechanism that wxylib ports works, in GDAL and in wxylib, and how the two are checked against each other. Each has one figure, made by the script beside it.

• How the warper works: chunks, source windows and kernel scale, and why they change the values a warp writes.
• Resampling kernels: bilinear and cubic weights, how they stretch for a coarser target, edges, nodata and the arithmetic order that decides the last digit.
• Curvilinear grids: GDAL’s geolocation-array transformer, the backmap and the search that turns a position back into a cell of a swath.
• Values at points: gdallocationinfo’s cell and InterpolateAtPoint’s kernels, how they differ from a warp’s, and reads planned so each chunk is fetched once.
• Polygons to cells: GDAL’s rasterizer by cell centre and ALL_TOUCHED, what happens to a polygon before the burn, and coverage fractions.
• Overview selection: gdalwarp’s -ovr AUTO rule for which resolution level a warp reads, checked against gdalwarp itself.
• What the tests check: GDAL as the oracle, and the bar of identity.