font-measurement

even-pretext: Font Measurement Library

Pixel-accurate text measurement for Even Realities G2 smart glasses. Predicts exact layout dimensions matching the LVGL rendering engine in the glasses firmware.

Note: Results may be off for characters not present in the firmware fonts, since the firmware and this library may handle missing glyphs differently in some cases.

Installation

npm install @evenrealities/pretext

Display Constants

• Screen: 576 x 288 pixels
• Line height: 27px (fixed)
• List item height: 40px (fixed)
• List item horizontal padding: 12px per side

API

All functions are exported from @evenrealities/pretext.

getTextWidth(text: string): number

Returns single-line pixel width of a string (with kerning, no wrapping).

import { getTextWidth } from '@evenrealities/pretext';
const width = getTextWidth('Hello, world!'); // => 79

measureTextWrap(text: string, maxWidth: number): MeasureTextResult

Measures multi-line text layout with word wrapping. maxWidth should be the inner content width — subtract padding and border before calling. The returned height is pure text height.

import { measureTextWrap } from '@evenrealities/pretext';

const result = measureTextWrap('The quick brown fox jumps over the lazy dog', 200);
// => { lineCount: 2, height: 54, lineWidths: [192, 96] }

// With padding + border — caller handles the math:
const pad = 8, border = 2;
const m = measureTextWrap('Hello world', 300 - 2 * (pad + border));
const containerHeight = m.height + 2 * (pad + border);

Parameters:

Returns:

pxTruncate(text: string, maxPx: number): string

Truncates a string to fit within a pixel budget, appending '...' if needed. Returns the original string unchanged when it already fits. Uses binary search and handles emoji/surrogate pairs correctly.

import { pxTruncate } from '@evenrealities/pretext';

const label = pxTruncate('Hello, world!', 50); // => 'Hell...'
const fits  = pxTruncate('Hi', 50);            // => 'Hi'

Parameters:

Returns: The original string if it fits, otherwise a truncated string ending with '...'.

Note: This function is single-line only. For multiline text, truncate each line individually.

getAdvW(cp: number): number

Returns raw advance width of a codepoint in 1/16px units (no kerning, no rounding). Useful for debugging or custom measurement logic.

Accounting for Padding and Borders

When a TextContainerProperty has paddingLength or borderWidth, the SDK's LVGL renderer subtracts these from the available text area inside the container. If you measure text against the full container width/height, the content will overflow and a scrollbar appears.

How padding and border affect the text area

Container (width × height)
┌─ border (borderWidth pixels) ─────────────────────┐
│ ┌─ padding (paddingLength pixels) ──────────────┐ │
│ │                                                │ │
│ │   Text renders here                            │ │
│ │   innerWidth  = width  - 2*padding - 2*border  │ │
│ │   innerHeight = height - 2*padding - 2*border  │ │
│ │                                                │ │
│ └────────────────────────────────────────────────┘ │
└────────────────────────────────────────────────────┘

Rules

• paddingLength: N reduces text area by N pixels on all four sides
• borderWidth: N reduces text area by N pixel on all four sides (border is drawn inside the container)
• Both stack: total inset = paddingLength + borderWidth per side
• If neither is set, text renders at the full container width/height

Measuring text for a container with padding/border

import { measureTextWrap } from '@evenrealities/pretext';

const containerW = 560;
const containerH = 258;
const padding = 8;  // paddingLength
const border = 1;   // borderWidth

const inset = padding + border;
const innerW = containerW - 2 * inset;
const innerH = containerH - 2 * inset;
const maxLines = Math.floor(innerH / 27);

// Measure and truncate against the INNER dimensions
const m = measureTextWrap(text, innerW);
if (m.lineCount > maxLines) {
  text = truncateToFitLines(text, innerW, maxLines);
}

Common mistake

// WRONG — measures against container width, ignores padding/border
const m = measureTextWrap(text, 560);

// RIGHT — subtracts padding and border from both sides
const m = measureTextWrap(text, 560 - 2 * (padding + border));

Common Patterns

Size a text container to fit its content

import { measureTextWrap } from '@evenrealities/pretext';

const containerWidth = 300;
const padding = 8;
const innerW = containerWidth - 2 * padding;
const result = measureTextWrap(myText, innerW);
const containerHeight = result.height + 2 * padding;
// Use Math.max(...result.lineWidths) + 2 * padding for tight container width

Truncate text to fit a container

import { pxTruncate } from '@evenrealities/pretext';

// Single-line truncation
const label = pxTruncate(longText, containerWidth);

// Multiline: truncate each line individually
import { measureTextWrap } from '@evenrealities/pretext';
const { lineWidths } = measureTextWrap(longText, containerWidth);
// If the last line is too long after wrapping, truncate it

Instructions

When using this library to size UI containers for Even Realities glasses:

1. Always use this library to predict text dimensions before creating containers. Do not guess pixel sizes.
2. Line height is 27px — multiply by line count for text container height.
3. List items are 40px tall — multiply by item count and add 2 * padding for list container height.
4. Max display is 576 x 288. Ensure containers fit within these bounds.
5. When pairing with the glasses-ui skill, use measureTextWrap results to set container width and height properties precisely.
6. When a container has `paddingLength` or `borderWidth`, subtract these from the container dimensions before measuring text. Failing to do so causes content overflow and a scrollbar. See "Accounting for Padding and Borders" above.

Task

$ARGUMENTS