A local adjustment ran as a second chain after every global operation,
then blended by the mask. So global contrast -30 with -20 on a face was
contrast -30, the rest of the chain, and contrast -20 again on the
result, rather than -50 where contrast runs. The two edits compounded in
ways neither slider showed; a flattening applied to an already
flattened picture is how the shadows of a night shot went magenta.
A layer's setting is now an offset from its default, added to the
global setting (clamped to the parameter's range; a moved switch or
choice replaces it) and run at that operation's own place in the chain.
At each operation the global fragment and each touching layer's
combined fragment read the same input colour, and the pixel moves by
each layer's weighted difference: c_g + sum w_i (c_i - c_g). At full
weight that is the combined setting exactly, at zero the global result
exactly, and no setting is applied twice. An offset that brings an
operation back to neutral emits an empty version, which undoes the
global setting inside the mask.
Blending the colours rather than the uniforms is deliberate: the tone
curve and colour mixer emit code only for the channels and bands that
are touched, so the global and combined versions of one operation need
not share a uniform set.
A photograph with no masks compiles to the same shader byte for byte.
Test: global -30 with a whole-frame layer at -20 renders within one
count of global -50.
dtourolle
fc54523093