autokerning/dist/calibrate.js

autokerning computes suggested kerning values for glyph pairs from TrueType/OpenType fonts by rendering glyph bitmaps, applying a small Gaussian blur, and measuring pixel overlap across horizontal offsets. It can be used programmatically (as an imported E

import { renderGlyph } from "./glyph.js";
import { gaussianBlur } from "./blur.js";
import { kernPair } from "./kernPair.js";
const TUNING_CHARS = "lno";
let KERNEL_WIDTH = 20; // 0.2 * FONT_SIZE (100)
/** Calibrate the kernel width and find overlap thresholds */
export function findS(font) {
    let kernelWidth = KERNEL_WIDTH;
    while (true) {
        const ss = [];
        for (const char of TUNING_CHARS) {
            const glyph = renderGlyph(font, char);
            const blurred = { ...glyph, bitmap: gaussianBlur(glyph.bitmap) };
            // Self-kerning: how much does a character overlap with itself?
            const kern = kernPair(blurred, blurred, 0, 1e10);
            const s = overlap(blurred, blurred, kern);
            ss.push(s);
        }
        const minS = Math.min(...ss);
        const maxS = Math.max(...ss);
        // If the self-overlap is balanced (min > max/2), we're good
        if (minS > maxS / 2) {
            return [minS, maxS];
        }
        // Otherwise, increase kernel width and try again
        kernelWidth += 2;
        if (kernelWidth > 200) {
            throw new Error("Failed to find reasonable kernel size.");
        }
    }
}
/** Compute pixel overlap (sum of squared products) */
function overlap(left, right, kern) {
    const height = Math.max(left.height, right.height);
    const rightX = left.advance - kern;
    let sum = 0;
    for (let y = 0; y < height; y++) {
        for (let x = 0; x < left.width + right.width; x++) {
            const lx = x;
            const rx = x - rightX;
            const lVal = lx >= 0 && lx < left.width && y < left.height
                ? left.bitmap.get(y, lx)
                : 0;
            const rVal = rx >= 0 && rx < right.width && y < right.height
                ? right.bitmap.get(y, rx)
                : 0;
            sum += lVal * rVal;
        }
    }
    return sum;
}