Build a base contrast mask

contrast_mask() is the extension point for new contrast types: it returns a base contrast matrix with one row per base condition (no basis expansion) and one column per contrast column. The unified driver contrast_from_mask() handles basis expansion, basis filtering and packaging into a contrast object compatible with contrast_weights.event_model().

Usage

contrast_mask(x, term, ...)

Arguments

x

A contrast_spec object.

term

An event_term against which the mask is computed.

...

Additional arguments passed to methods.

Value

A list with elements:

• weights : numeric matrix, n_base_conditions x n_contrast_cols, row names = base condition names.

• condnames : character vector of base condition names.

Details

To register a custom contrast type:

1. Define a constructor that produces a list with class c("my_spec", "contrast_spec", "list").

2. Implement contrast_mask.my_spec() returning the base weights.

3. Implement contrast_weights.my_spec() as a one-liner:

   contrast_weights.my_spec <- function(x, term, ...) {
     contrast_from_mask(contrast_mask(x, term, ...), x, term)
   }

The built-in spec classes (pair_contrast_spec, oneway_contrast_spec, poly_contrast_spec, ...) implement contrast_weights() directly, so they do not require contrast_mask() methods.