Resampling

warp resamples a source window into a target, for any sample type (u8, i8, u16, i16, u32, i32, f32, f64) and any number of pixel-interleaved bands, in one pass:

let out = warp(&transformer, &SourceBuffer { .. }, dst_ncol, dst_nrow, &opts);

SourceBuffer is the window you read, laid out (row, col, band), with its offset in the full source image. WarpOptions holds everything else; WarpOptions::default() is nearest neighbour with GDAL's defaults. Build it with WarpOptions::new(alg) and the with_ methods, one per option:

let opts = WarpOptions::new(ResampleAlg::Average)
    .with_footprint(Footprint::Exact { densify: 1 })
    .with_src_nodata(Nodata::Value(-9999.0));

WarpOptions and the option enums are #[non_exhaustive], so new options can arrive in minor releases. Outside rwarp this means no struct literals for WarpOptions, GeolocOptions or AverageThresholds, and a _ arm when matching on the enums.

Kernel widening when downsampling

When the output is coarser than the source, GDAL widens the bilinear, cubic and lanczos kernels to cover each output pixel's source area. rwarp does the same by default:

• WidenWhenDownsampling estimates the target/source scale over the target window, as GDAL does per warp chunk, then widens the kernel support to ceil(radius / scale) on any axis where the scale is below 1. Upsampling never narrows the kernel.
• Fixed { x, y } gives the scale, like GDAL's XSCALE and YSCALE. Use it to make results independent of how the target is tiled.
• Point keeps the fixed 2 x 2, 4 x 4 and 6 x 6 support at every ratio, which is gdalwarp -wo XSCALE=1 -wo YSCALE=1.

As in GDAL, bilinear and cubic widen only once the scale reaches 0.5 or less; lanczos widens at any scale below 1.

One deliberate difference: rwarp estimates the scale over the window actually being warped. GDAL takes its estimate at the top-left of the whole output for every chunk, so a chunked gdalwarp can widen by the wrong amount away from the origin (see GDAL parity).

Average and mode

Both weigh the source pixels a target pixel's footprint covers, measuring area in source pixel space:

• Footprint::GdalBox is GDAL's footprint: the axis-aligned box between the transformed top-left and bottom-right corners of the target pixel, with fractional weights at its edges. It matches gdalwarp, quirks included; under rotation or strong curvature the box can be far from the true footprint.
• Footprint::Exact { densify } transforms the target pixel's outline and intersects it exactly with the source pixels. With renormalisation over valid pixels, Average is then first-order conservative remapping. It costs about 25 times the box. The same overlaps are available as a weight table; see Conservative weights.

Mode counts covered area per value and breaks ties with ModeTies. Counts are re-compared every time a value's count grows, in scan order, so First is the tied value that reached the winning count first. GDAL does this for 8- and 16-bit data; rwarp does it for every type.

Output values

• Integer outputs are clamped to the type and rounded half up, floor(v + 0.5), as GDAL does with an integer working type. Nearest and mode copy source values and never round.
• Nodata avoidance: a resampled value equal to the nodata value is moved off it by one step, so a valid pixel never reads back as missing.
• Source bounds: sample points outside [0, n) of the source are off the source for every method except average and mode, which use footprints.