Context importance: how far from the point, and at which scale, the model reads.

The patch is a neighbourhood. This permutes part of it – the point's own pixel, a ring of pixels at one distance from it, every ring but the centre, or one window's whole input – and measures the loss of skill, as permutation_importance() does for a variable. The windows are cut around the same point, so a ring is the same ground in every window, and it takes its values from one donor in all of them.

Usage

context_importance(
  by = c("ring", "window"),
  bands = NULL,
  per_variable = FALSE,
  draws = 5L,
  metric = c("ccc", "rmse", "rmse_transform"),
  seed = 42L
)

Arguments

by

"ring" (the default): one row per distance band. "window": one row per window the model reads, that window's whole input permuted – how much the branch carries – and, when the model has a gate between its two branches, the gate read point by point beside it ($gate).

bands

For by = "ring": the bands, a named list of ring numbers – 0 is the point's own pixel, ring d the 8d pixels d steps from it. NULL: the centre, each ring alone, and the context (every ring but the centre); with per_variable, the centre and the context.

per_variable

For by = "ring": one row per variable and band – which variables the model reads through the neighbourhood, and which at the point alone. The variables are those of groups in dsm_importance().

draws

Permutations per band (or variable and band, or window) and per model.

metric

What the table is ranked by, as in permutation_importance().

seed

Seed of the permutations, shared by every model.

Value

An importance_spec, for dsm_importance().

Details

A large importance of the context says the network reads the neighbourhood. It does not say the neighbourhood adds what the centre lacks: at 250 m most covariates are smooth, a pixel three cells away is close to a copy of the centre, and a network can lean on the rim and learn nothing the centre would not have told it. Ring d holds 8d pixels, so the table gives the cost per pixel beside the total: a wider ring costs more for its area alone.

Examples

context_importance(by = "ring")
#> <importance_spec> context by ring, 5 draw(s) | ranked by ccc