Polaris MCFG — A License-Safe Metric-Compatible Font Generator, Plus the LLM Eval Rubric Thread Next to It

Wed, 06 May 2026 00:00:00 +0900

Overview

PolarisOffice/polaris_mcfg appeared on 2026-04-26 — a tool that looks like it came out of the Polaris Office product team. It extracts only the layout metrics from restricted fonts (think Hancom fonts, internal commercial fonts) and grafts them onto freely-licensed fonts like Noto Sans and Pretendard to produce a new font. The result: original line breaks and page boundaries preserved, license now safe. What makes the chatroom timing interesting is that the conversation immediately around this share was about LLM evaluation rubrics — two topics that look unrelated but both belong to production-grade engineering practice.

graph TD
 Source["Source font.ttf <br/> (commercial/restricted)"] --> Extract["mcfg extract"]
 Extract --> Metrics["metrics.json <br/> advance/ascender/descender"]
 Free["Free font.ttf <br/> (Noto Sans/Pretendard)"] --> Generate["mcfg generate"]
 Metrics --> Generate
 Generate --> Output["Polaris font.ttf <br/> OFL-safe"]
 Output --> Validate["mcfg validate <br/> HarfBuzz render regression"]
 Validate --> Pass["PASS <br/> advance widths match <br/> render within ±0.5 percent"]

The Problem It Solves

Open a Hancom-authored .hwp or .docx in another environment and line breaks and page splits drift. The visible glyph shapes aren’t the issue — the numeric metrics are: advance width, ascender, descender, line gap. polaris_mcfg solves this with one clean cut: never touch the outline, only graft the numbers from one font onto another’s design.

The Clean Separation — License-Safe Boundary

The data the tool handles is numbers only. Glyph outlines are never extracted, never copied. The visible design of the output font is 100% from the free font, and so is its license. The standard there is the SIL Open Font License (OFL) 1.1 — finalized in 2007 by Victor Gaultney and Nicolas Spalinger at SIL International, untouched for nearly 20 years, the de facto free-license standard for the font industry. Both Noto Sans and Pretendard ship under OFL.

CLI

Subcommand	Purpose
`mcfg extract <font.ttf>`	Metrics → JSON
`mcfg compare a b`	Diff two fonts (or two JSONs); text/json/html output
`mcfg generate --metrics … --design …`	Produce the synthesized font
`mcfg validate <font> --against …`	Verify the metrics actually match

mcfg extract NotoSansKR-Bold.ttf -o bold.json

mcfg generate \
 --metrics bold.json \
 --design NotoSansKR-Regular.ttf \
 --output PolarisBoldMetrics-Regular.ttf \
 --apply global,advance \
 --license-text "SIL Open Font License 1.1"

mcfg validate PolarisBoldMetrics-Regular.ttf \
 --against NotoSansKR-Bold.ttf \
 --render-default \
 --render-tolerance-pct 0.5
# → result: PASS (advance widths match, rendering within ±0.5%)

Validation runs through HarfBuzz, the de facto OpenType shaping engine — the only way to confirm the metric graft really worked is to render real text and compare pixels.

Milestones and License Responsibility

M1 (metric extractor + JSON schema) through M7 (packaging and docs) are all complete; 84 tests pass. Tool code is MIT; output fonts inherit the design font’s license (OFL or similar). One important caveat: whether the source font’s EULA permits metric extraction is the user’s responsibility (Requirements.md §6). The tool is not an automated license-laundering machine — it’s an honest separation tool, and the README is explicit about that.

The LLM Eval Rubric Thread Next to It

Around the same time, an unexpectedly pointed take on LLM evaluation surfaced:

“Vector similarity and RAGAS metrics aren’t really suitable for grading. Free-form grading inevitably has to go through an LLM, and the standard practice is to write the evaluation rubric first and base everything on that.”

This single line compresses the production wisdom of LLM-as-Judge into three points. (1) Vector similarity and RAGAS score semantic match but don’t constitute a grading standard. (2) Free-form grading must call an LLM — rule-based scoring won’t reach. (3) Write the rubric first. “Tell me if this answer is good” doesn’t work as a prompt; you need an explicit grading scheme before you’ll get consistency.

This matches exactly where every modern LLM eval framework — DeepEval, Evidently, OpenAI Evals — is heading. Rubric-driven judging is now the standard.

Insights

That a font metric extractor and an LLM evaluation rubric thread emerge at the same moment signals something about the audience: these are people who are actually shipping product. The two topics look unrelated but the underlying move is identical — both are about reducing intuition-dependent territory to explicit, verifiable rules. The font tool reduces “are these metrics compatible” to a HarfBuzz rendering regression. LLM-as-Judge reduces “is this answer good” to a rubric. Both topics demand an automated verification step before they’re production-ready, and that verification step ends up defining the tool’s identity. The fact that polaris_mcfg has a validate subcommand at all, and that LLM eval frameworks treat rubrics as first-class objects, are expressions of the same engineering instinct. In production “it just works” is not a finishing line — explicit criteria + automated verification + regression tracking is the new bar, and these two topics point to the same place from very different starting points.

References

Tool repo and demo

PolarisOffice/polaris_mcfg — Metric-Compatible Font Generator (MIT, Python, 4 stars)
Demo / docs site

Font ecosystem

HarfBuzz — OpenType shaping engine
SIL Open Font License — de facto free-license standard (OFL 1.1, 2007)
SIL International — OFL stewards
Noto Sans and Pretendard — OFL-licensed Hangul fonts

LLM evaluation methodology

RAGAS — RAG evaluation framework
DeepEval — LLM-as-Judge + rubric-based eval
Evidently — ML/LLM monitoring and eval
OpenAI Evals — OpenAI’s official eval framework

Harfbuzz on ICE-ICE-BEAR-BLOG