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T1 fan-out · creators / media: nika:glob lists the folder, for_each fans one nika:image_fx task per photo, and a fixed ops chain (grayscale → dither → pixelate) renders each one. No model in the loop — the pipeline is pure pixels, so the same input produces the same bytes on every run, on every machine.

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The job

Batch-stylizing a shoot means opening an editor, replaying the same five clicks per photo, and hoping you replayed them identically. This workflow IS the preset: the ops chain is written once in YAML, the batch fans out in parallel, and re-running it a year from now yields byte-identical output — the style is version-controlled with your repo.

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The shape

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The file

t1-image-fx-batch.nika.yaml
nika: v1
workflow: image-fx-batch
description: "Folder of PNG files → deterministic art (grayscale · dither · pixelate) · offline batch"

vars:
  photos: "./photos" # the input folder · every .png in it gets the treatment

permits:
  exec: false
  fs:
    # a RELATIVE glob walks from `.` (the engine gates the walk root), so
    # the read boundary is the cwd subtree — and `vars.photos` is caller-
    # overridable, so no narrower literal glob can be proven to cover it.
    # Fixing the folder? Tighten this to `photos/**`. The write stays tight
    read: ["**"]
    write: ["out/**"]
  tools: ["nika:glob", "nika:image_fx"]

tasks:
  # The folder is the work list — no hand-maintained array to drift.
  - id: shots
    invoke:
      tool: "nika:glob"
      args: { pattern: "${{ vars.photos }}/*.png" }

  # One deterministic darkroom pass per photo · the ops run IN ORDER.
  - id: stylize
    depends_on: [shots]
    for_each: ${{ tasks.shots.output }}
    max_parallel: 4
    invoke:
      tool: "nika:image_fx"
      args:
        input: "${{ item }}"
        ops:
          - grayscale: {}
          - dither: { mode: bayer4 }
          - pixelate: { block: 4 }
        out: "out/${{ item }}"

outputs:
  rendered:
    # an empty ./photos skips the fan-out (its output is null) — the guard
    # keeps the typed contract honest on the very first exploratory run
    value: "${{ tasks.stylize.output != null ? tasks.stylize.output : [] }}"
    type: array
    description: "The stylized artifact receipts · one per input photo"

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Two things this file teaches

  1. The ops vocabulary is a closed set — dither: { mode: bayer4 } is one of the engine’s named modes; misspell it (bayer) and nika check answers with the whole legal set instead of guessing. The checker teaches the vocabulary before the run spends a cycle.
  2. A fan-out that can produce nothing still binds — the collect step guards with ${{ tasks.stylize.output != null ? … : [] }} (quoted, because a bare CEL ternary contains : and YAML would claim it). An empty folder degrades to an empty manifest, not a crash.

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Run it

git clone https://github.com/supernovae-st/nika-spec && cd nika-spec
nika run examples/showcase/t1-image-fx-batch.nika.yaml
ls out/
This workflow is newer than the currently shipped engine pack: nika examples run showcase/t1-image-fx-batch says unknown example until the next engine tag re-vendors the pack. Until then, run it from the spec checkout as above — the file is the same one the pack will embed, sha-pinned in the spec manifest.
Point vars.photos at any directory of PNG files. Because every op is deterministic, the diff of fx-out/ across two runs is empty — which makes the output safe to commit, and the workflow safe to put in CI.