greensky/pan/geom — mesh building for the pan

An accumulator you push faces into, and the shapes the salt flat is made of. Everything here is pure geometry: it produces flat position / index / normal / uv arrays, and Geom.upload hands them to renderer.mesh.create.

Used only by greensky/pan.

Winding

renderer.mesh.create reads a triangle's front face as the one where cross(v1 - v0, v2 - v0) points out of it. Geom.quad takes its four corners in that order and emits (0,1,2) + (0,2,3), so every builder below states its corners front-face-first and no face needs cull = "none".

The surface

local Geom = require("greensky.pan.geom")

local a = Geom.acc()                                  -- an empty accumulator
Geom.vert(a, x,y,z, nx,ny,nz, u,v)                    -- one vertex, returns its index
Geom.tri(a, i0, i1, i2)
Geom.quad(a, p0, p1, p2, p3, nx, ny, nz, uvScale)     -- a flat quad, planar UVs
Geom.box(a, x0,y0,z0, x1,y1,z1, uvScale, skipBottom?) -- an axis-aligned box
Geom.disc(a, cx, y, cz, radii, segs, uvScale)         -- a horizontal disc, ring radii inner to outer
Geom.plate(a, cx, cz, r, y, sides, rot, yBase?, uvScale)  -- a flat polygon with an optional skirt
Geom.field(a, cx, cz, sizeX, sizeZ, nx, nz, fn, uvScale)  -- a height field, fn(x, z) in world space
Geom.upload(a)                                        -- -> MeshHandle
Geom.stats(a)                                         -- { verts, tris }
Geom.rng(seed)                                        -- :next() 0..1, :range(lo, hi)

uvScale is metres per UV unit. Geom.field takes its normal from four neighbouring samples of fn, so a wave shades correctly without the caller differentiating it.

Geom.rng is a plain LCG. The film has to be byte-identical run to run, so nothing in the pan may reach for math.random.

Note on runtime meshes

A mesh made by renderer.mesh.create from raw geometry is not CPU-resident, so renderer.mesh.setVertices refuses it — renderer.mesh.update with whole geometry is the way to change one after the fact, and it costs about 5 ms for 9,400 vertices. The pan builds static geometry once and animates it by moving and scaling the entity instead.