Example: Thresholding

Thresholding changes voxel values but not the image frame. RITK exposes a retaining threshold and a binary indicator:

• ThresholdImageFilter::outside(lower, upper, outside) keeps values in the closed interval and replaces values outside it.
• BinaryThresholdImageFilter::new(lower, upper, foreground, background) writes the foreground value inside the interval and background elsewhere.

The complete pipeline figure shows both behavior classes. The threshold panel is the suppression stage; the binary morphology panels consume a binary indicator made from the same sigmoid output.

let retained = ThresholdImageFilter::outside(0.0, 0.58, 0.0)
    .apply_native(&sigmoid, &backend)?;
let mask = BinaryThresholdImageFilter::new(0.62, 1.0, 1.0, 0.0)
    .apply_native(&sigmoid, &backend)?;
assert_eq!(retained.shape(), sigmoid.shape());
assert_eq!(mask.shape(), sigmoid.shape());

The interval is inclusive for the binary filter. That boundary matters for label maps and should be tested with values exactly at both endpoints. Values outside the interval are not clipped into the interval; they become the configured background.

Source and verification

Source: crates/ritk-filter/examples/book_processing_pipeline.rs

cargo run -p ritk-filter --example book_processing_pipeline -- \
  docs/book/figures/processing_pipeline.svg

The native filter tests cover endpoint inclusion, outside suppression, shape preservation, and native/generic parity. The figure verifies that the foreground topology is visible and that thresholding is not an independently contrast-stretched display artifact.