terrain_layer_weights

Evaluates a material layer stack's rules into two RGBA weight textures — layers 0-3 in one, 4-7 in the other, with the eight shares at a texel summing to one.

One thread per destination weight texel. The dispatch is per tile: a tile writes the texels whose centres fall inside its interior, reading the field through its own apron-inclusive buffer. Because every read is placed from the field minimum and the texel size in world metres — never from the tile's own origin — the texels one tile writes agree exactly with its neighbour's along the seam.

The six inputs

input	unit	where it comes from
height	metres, world Y	the height channel, bilinear
slope	degrees, 0 flat / 90 vertical	central difference of height over gradientStep metres
curvature	1/metres, + gully / − ridge	Laplacian of height, magnitude ≈ 1/radius
mask	0..1	a bound mask.<name> channel, bilinear
noise	0..1	terrain_fbm's gradient noise over world position
channel	the channel's own stored values	any bound field channel, bilinear

slope and curvature are derived here rather than stored, so they cost no channel and can never fall behind the ground they describe.

mask and channel both read a named channel through the same four slots and the same bilinear tap; they differ in what the band is written against. A mask channel holds 0..1, so its band is in that range. A channel band is in the values the channel stores — cubic metres of deposited sediment, metres of water depth, whatever the op that wrote it produces — with no rescaling on the way in. That is what lets a layer put sand where the water actually dropped it and wet shore where water actually stands.

A rule is a band, whatever it reads

Every rule carries min, max, softness and jitter in the unit of its own input. A layer covers the ground by its strength times the product of its rules; a layer with no rules covers unconditionally. invert takes the complement of the band, which is how "anywhere but here" is written.

softness is a feather half-width and jitter is how far noise is allowed to move the value before it is banded — metres on a height rule, degrees on a slope rule. Each rule draws its jitter from its own noise offset, so two rules on one layer do not wander in step.

The layers are a stack

Working from the top down, each layer takes its coverage share of whatever is still showing. A cliff rule that fully passes therefore leaves nothing of the grass under it, a layer at half coverage lets half the layer below through, and whatever no layer claimed goes to layer 0 — the ground the stack stands on. The eight shares add to one by construction, with nothing divided by a sum, and an unclaimed texel is a surface rather than a hole.

Rules are numbered bottom-up across the whole stack and each layer reads its own range of them, so covering downward moves no rule's noise offset.

Bindings

params carries the whole stack: four head lanes, eight layer lanes, then three lanes per rule. height is the field's height tile. chan0..chan3 are the field channels the stack's rules name, in sorted order — a slot no rule reads is bound to the height tile and never sampled. Four is what the kernel declares; a compute stage carries eight storage buffers and params and height take two, so six is the ceiling a stack could be given.

weightsLo and weightsHi are rgba8 storage targets, which is the widest format a material slot can sample: a material reads every slot through a filtering sampler, and the 32-bit float formats are not filterable.