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ADR 0037 - Rejected: dynamic image insertion

  • Status: rejected
  • Date: 2026-05-17
  • Spec target: XTL 0.1 (rejection; no spec change)
  • Affects: ADR-0034 (Corollary 2 / 3 application)

Context​

JXLS ships a jx:image command that lets a template author embed an image whose binary source is supplied at render time:

<!-- JXLS-style, conceptual -->
jx:image(href="${item.logoPath}", x="1", y="1")

The same shape appears in xltpl and several JXLS forks. The typical use case is per-customer logos, signature images, or QR codes that vary row-to-row.

The natural XTL spelling would be something like:

{{ IMAGE([LogoPath]) }}
{{ @image [QRBase64] }}

This ADR records the decision not to ship that feature in XTL 0.1 (or 1.0). The rejection is deliberate, not an oversight, so that future proposers see the rationale before reopening the question.

Per ADR-0034 Corollary 3, this is the first feature-level rejection ADR following that informational principle.

Considered Options​

A. Ship dynamic image insertion in 0.1. Adds a new directive or function with binary-data semantics.

B. Ship a static image-anchor preservation only (ADR-0036 item 1) and reject runtime image insertion. Templates ship images already in the file; the engine preserves them; no runtime binary pipeline.

C. Defer (no decision yet). Leaves the question open and recurring.

Decision​

Adopt B. Dynamic image insertion is out of scope for XTL 0.1 and not on the roadmap to 1.0.

Why rejected​

Five distinct objections, each sufficient on its own:

1. The use case is narrow in practice​

Audit of real Korean vendor templates (거래명세서, 정산서, 발주서, 인보이스) over the conversation that produced ADR-0033 surfaced: every observed image was a static company logo or stamp, authored once into the template, not a per-row dynamic asset. The "per-customer logo" scenario JXLS targets is real but vanishingly rare in the operations workflows XTL is designed for. Authoring effort for a feature with thin demand is wasted authoring effort that delays features with broad demand.

2. Binary-data pipeline breaks the browser story​

XTL's browser flow (xl3.io) operates on two .xlsx files: a template and a source data workbook. Both are self-contained. A dynamic image feature requires additional binary inputs at render time — file paths the browser cannot resolve, base64 blobs that bloat the source workbook, or external URLs the engine would need to fetch. Each shape breaks one of XTL's design axes:

  • File paths → host-specific I/O leak; browser cannot resolve.
  • Base64 in cells → source-workbook size explosion; not how operators author data.
  • Fetched URLs → introduces a network surface; security implications; cannot run offline.

The static-image preservation in ADR-0036 sidesteps all three: images live in the template (where author already put them), not in the source data.

3. Cross-impl portability cost is high​

Image placement in OOXML uses xdr:twoCellAnchor / oneCellAnchor with EMU coordinates and per-image relationship parts. Each port (xl3-py, future Rust/Go) would need to:

  • Parse the row/column the directive targets
  • Compute EMU offsets for one-cell or two-cell anchoring
  • Generate the xl/media/imageN.{png,jpg} parts
  • Wire the worksheet's _rels/sheet*.xml.rels correctly
  • Handle MIME type detection / re-encoding for the input source

This is ~500 lines per port for a feature few authors will use. ADR-0034 Corollary 1 says absorb experience cheaply; this feature is the inverse — expensive for thin payoff.

4. Excel-native function alternative exists for the common case​

Excel has IMAGE() (Microsoft 365 / 2024+), HYPERLINK() for clickable logos, and conditional formatting "Icon Sets" for status indicators. Authors who really need data-driven visuals have author-side options that don't require an XTL directive.

A future ADR could add IMAGE() to the XTL function table as a pass-through to Excel's native IMAGE() evaluation, but that is a much smaller surface than jx:image and is not part of this ADR's rejection — it would be a separate proposal if demand surfaces.

5. Encourages templates to grow non-portable behavior​

The moment a template depends on per-render binary assets, the template stops being a self-contained artifact. README §41-52 states the thesis: "The template is the handover artifact. It can be reviewed, versioned, archived, and passed to the next operator without asking them to read the automation code." A template that says {{ @image [LogoPath] }} is only meaningful in the context of some host system that knows where logo paths resolve. The template is no longer the contract.

What is not rejected​

  • Static image preservation (the template's own embedded images survive to output). Preserved per ADR-0036 item 1.
  • Excel's native =IMAGE("url") formula evaluating inside the workbook. Out of scope of this ADR — orthogonal question.
  • Future reconsideration if production users report dynamic image insertion as a blocker. ADRs are revisable per GOVERNANCE.md § "Disagreements". A reopening would need to address all five objections above with concrete evidence.

Consequences​

  • Future proposers of dynamic image features can cite this ADR to know the bar (overcome all five objections) before drafting.
  • No new directive, no new function in 0.1.
  • No conformance fixture is added — rejected ADRs do not need fixtures; the rejection IS the contract.
  • docs/internal/jxls-absorption-plan.md Category C item C6 links to this ADR.
  • Listed in INFORMATIONAL_ADRS of src/__tests__/spec-coverage.test.ts so the ADR↔fixture coverage check passes.

References​

  • ADR-0034 — Relationship to prior-art template engines (Corollary 3: some prior-art choices are explicitly out of scope)
  • ADR-0036 — Template feature preservation matrix (item 1: static image preservation, the kept side of this rejection)
  • README § "Why xl3 exists" / § "What xl3 emphasizes" (the thesis the rejection protects)
  • GOVERNANCE.md § "Disagreements" (revisability)