The camera preview, the library preview and the exported file each carried
their own copy of the grain shader, and each sized it off the rect it happened
to be drawing on. Two things were wrong with that. The field was a single value
hash, so it looked like digital static rather than film: measured on the noise
plane, neighbour correlation was flat and the lattice sat on the picture's own
axes; and it was the same field in every session, so two photos from two
launches printed identical grain. The size rule was wrong the other way round:
canvas units in Skia are dp, so on a 3x phone the floor of one dp pinned a
390dp-wide preview to a THREE device-pixel cell while the file printed a
1080th of its width — 2.77x coarser than the picture it was previewing, which
is the preview-vs-file mismatch all over again, just the other direction.
Grain now lives in one module, src/utils/grainShader.ts, imported by the
preview (both surfaces), the phone's export and the web engine, so the three
cannot drift. The noise is three octaves of value noise over a density that is
the mean of three hashes — a bell, the way an emulsion's density swings —
turned 20 degrees off the axes, seeded once per app launch, so a session prints
one roll and the next launch prints another. Only coarser octaves: a finer one
lands under the pixel and the clumping is lost outright. The cell is one
1/1080th of the PICTURE's width, floored at one pixel OF THE TARGET — an
export's own output pixel, a preview's device pixel (1 / PixelRatio.get())
instead of a whole dp.
Measured on the plane itself with the real module (grainSize harness): the file
at 4000px and a 390dp camera preview at 3x agree on the same fractional lag,
rho 0.017 against 0.011, while the floor this replaces reads 0.329 — visibly
coarser, as claimed; both keep the spread the AMOUNT knob was tuned on
(sigma 57.8 / 57.9) and still clump (rho(1) 0.91 / 0.39); R, G and B never move
apart, split 0. GRAIN_SKSL compiles on the Skia runtime, and the compile is now
swallowed into null at module scope like the other three effects, so a shader
this app cannot compile costs the grain rather than a launch.
Not ported: the Kotlin grainOverlay probe (per-pixel, path is off by default).
3dtours
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