x-Height Ratio From Font File

Extract head.unitsPerEm and OS/2.sxHeight from caller hex standalone SFNT font bytes; absent/zero/old OS/2 x-height is explicitly unavailable, not guessed from cap height or a full font validity certification.

Description

A font’s x-height ratio relates a native metric to units per em.

A font’s x-height ratio relates a native metric to units per em. Missing native metrics should be represented separately from measured outline height.^{[1, 2]}

Paste even-length hexadecimal standalone TrueType/OpenType SFNT bytes. Parse a bounded table directory, validate nonoverlap/in-bounds tables, head magic and unitsPerEm 16–16384; read OS/2 v2+ sxHeight at offset 86. Return units, x-height, ratio and availability.

When to use x-Height Ratio From Font File

Use x-Height Ratio From Font File when you need to evaluate the supplied records under the calculation and evidence rules below. Missing OS/2, versions below 2 or zero x-height return unavailable with zero placeholders. WOFF/collections, truncated/overlapping tables and negative x-height fail. No checksum or complete font validity is verified.

Prepare measured font or typeset-layout values and the explicit design policy requested. For text or font bytes, use the supported syntax explained below.

How to use x-Height Ratio From Font File and understand its results

Prepare measured font or typeset-layout values and the explicit design policy requested. For text or font bytes, use the supported syntax explained below.
Enter fontFileHex using the field table and calculation rules, then run the tool.
Read unitsPerEm, nativeXHeightUnits, xHeightRatio, nativeXHeightAvailable. The explanation below defines the units, signs, ordering and any flags or placeholder values.
If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.

Input field	Meaning	Value type; see rules below	Requirement
fontFileHex	Font file hex	string	Required

ratio=sxHeight/unitsPerEm when available, otherwise zero

Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.

Output field	Meaning	Value type
unitsPerEm	Units per em	number
nativeXHeightUnits	Native x height units	number
xHeightRatio	X height ratio	number
nativeXHeightAvailable	Native x height available	boolean

json

{
  "input": {
    "fontFileHex": "00010000000200200001000068656164000000000000002c000000364f532f320000000000000064000000600000000000000000000000005f0f3cf5000003e8000000000000000000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001f40000000000000000"
  },
  "output": {
    "unitsPerEm": 1000,
    "nativeXHeightUnits": 500,
    "xHeightRatio": 0.5,
    "nativeXHeightAvailable": true
  }
}

Limitations and assumptions

Missing OS/2, versions below 2 or zero x-height return unavailable with zero placeholders. WOFF/collections, truncated/overlapping tables and negative x-height fail. No checksum or complete font validity is verified.

Numeric inputs, where present, must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Extremely large text inputs are rejected. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors. A successfully returned false flag is an evaluated outcome; its meaning is explained above.

Alternative or Complementary methods

Use Inline Code Monospace x-Height Match: Matching inline code to body x-height requires independent metrics from both fonts. Equal x-height does not imply equal weight or baselines. For a complementary task, use Fallback Metrics Ascent/Descent Override: Font metrics overrides and size-adjust scaling interact. The requested percentages must account for scaling to reach the target layout metrics.

Further background: OpenType OS/2: sxHeight.

References

x-Height Ratio From Font File — tool.io
OpenType OS/2: sxHeight — Microsoft

Similar or alternative tools

Inline Code Monospace x-Height Match
Matching inline code to body x-height requires independent metrics from both fonts. Equal x-height does not imply equal weight or baselines.
Fallback Metrics Ascent/Descent Override
Font metrics overrides and size-adjust scaling interact. The requested percentages must account for scaling to reach the target layout metrics.