autokerning/dist/overlap.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 { logger } from "./log.js";
let debugCount = 0;
/**
 * Compute pixel overlap using composition logic from Python version
 *
 * Intersection rectangle layout:
 * - Right glyph at x ∈ [0, right.width)
 * - Left glyph at x ∈ [lOffset, lOffset + left.width)
 * - Only pixels in both ranges contribute to overlap
 */
export function overlap(left, right, kern) {
    const height = left.height;
    // Calculate offset where left glyph is positioned relative to right
    const lOffset = -(left.advance + left.bboxOffsetX) + right.bboxOffsetX - kern;
    // Find intersection range in x-dimension
    const xStart = Math.max(0, lOffset);
    const xEnd = Math.min(right.width, lOffset + left.width);
    const width = Math.max(0, xEnd - xStart);
    if (debugCount < 3 && width > 0) {
        logger.debug(`[OVERLAP] pair=${left.char}${right.char}, kern=${kern
            .toString()
            .padStart(4)}, lOffset=${lOffset
            .toFixed(2)
            .padStart(8)}, [${xStart.toFixed(0)}, ${xEnd.toFixed(0)}) width=${width
            .toFixed(0)
            .padStart(4)}`);
    }
    if (width <= 0) {
        return 0;
    }
    // Compute intersection by multiplying pixel values
    let sum = 0;
    // Use direct typed-array access to avoid ndarray.get overhead inside tight loops
    const rData = right.bitmap.data;
    const lData = left.bitmap.data;
    const rW = right.width;
    const lW = left.width;
    const rH = right.height;
    const lH = left.height;
    for (let y = 0; y < height; y++) {
        const rRowOff = y * rW;
        const lRowOff = y * lW;
        for (let x = xStart; x < xEnd; x++) {
            // Right glyph pixel at (x, y)
            let rVal = 0;
            if (x >= 0 && x < rW && y < rH) {
                rVal = rData[rRowOff + x] ?? 0;
            }
            // Left glyph pixel at (x - lOffset, y)
            let lVal = 0;
            const lx = x - lOffset;
            const li = Math.floor(lx);
            if (li >= 0 && li < lW && y < lH) {
                lVal = lData[lRowOff + li] ?? 0;
            }
            // Sum product of squared values
            sum += lVal * lVal * rVal * rVal;
        }
    }
    if (debugCount < 3 && width > 0) {
        logger.error(`         -> overlap=${sum.toFixed(2).padStart(8)}`);
        debugCount++;
    }
    return sum;
}
// Export function to set bbox offset (needed during glyph creation)
export function setGlyphBboxOffset(glyph, offsetX) {
    glyph.bboxOffsetX = offsetX;
}