# A hand-written node. `rust:` names the type in `crate::ops` that implements
# `Operation`; everything else about it — its descriptor, its parameters, its
# WGSL — comes from that type rather than from this file.
#
# It appears here anyway so that `ops/` lists the whole pipeline in order.
# A chain half-declared here and half-ordered in Rust would be worse than
# either alone: the order is the one thing a reader comes to this directory
# to learn.
id: tone_curve
order: 60

# Both, and this is the case the plural exists for: the RGB curve is
# tonal and the per-channel curves are chromatic. Filing it under one
# would hide it from half the people looking for it.
rust: ToneCurve

why_rust: |
  Four curves — master, red, green, blue — of five control points each,
  presented as one widget, with an interpolator and a monotonicity guarantee
  behind them. Its neutral is a *relationship* between parameters rather than
  a set of values — the identity diagonal, on every channel — which is not
  something the declarative `active:` rule can express; its `presentation()`
  spans forty parameters rather than describing one; and its fragment is
  *assembled* rather than written, because each curve reaches the shader only
  when it has been moved off the diagonal. A declared node's `wgsl:` is one
  fixed block of text, which is the right shape for nearly everything here and
  the wrong one for a node whose cost has to follow what the photographer
  actually touched.

placement: |
  After the fixed-weight region controls, so the curve is the final word on
  tone: a photographer reaches for it to fix what those controls could not
  place exactly.

  Within the node, the master curve runs before the per-channel ones. Both
  orders are visibly different images and the reasons for this one are written
  out in `src/ops/curve.rs`: the master is tonal and hue-preserving, the
  per-channel curves are the chromatic grade over the tones it produced, and a
  point placed on a channel curve should act on the tone the photographer can
  see rather than on the one the master is about to move.