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const INF = 1e20;
// lookup table for gamma-corrected, signed squared alpha distance values
const alphaTable = new Float64Array(256);
for (let i = 0; i < 256; i++) {
const d = 0.5 - Math.pow(i / 255, 1 / 2.2);
alphaTable[i] = d * Math.abs(d);
}
alphaTable[255] = -INF;
export default class TinySDF {
constructor({
fontSize = 24,
buffer = 3,
radius = 8,
cutoff = 0.25,
fontFamily = 'sans-serif',
fontWeight = 'normal',
fontStyle = 'normal',
lang = null
} = {}) {
this.buffer = buffer; // padding around a glyph's bounding box
this.radius = radius; // how many pixels around the glyph edge are encoded as signed distances
this.cutoff = cutoff; // how much of the SDF byte range represents inside vs outside the edge
this.lang = lang; // language of the Canvas drawing context
// make the canvas size big enough to both have the specified buffer around the glyph
// for "halo", and account for some glyphs possibly being larger than their font size
const size = this.size = fontSize + buffer * 4;
const canvas = this._createCanvas(size);
const ctx = this.ctx = canvas.getContext('2d', {willReadFrequently: true});
ctx.font = `${fontStyle} ${fontWeight} ${fontSize}px ${fontFamily}`;
ctx.textBaseline = 'alphabetic';
ctx.textAlign = 'left'; // Necessary so that RTL text doesn't have different alignment
ctx.fillStyle = 'black';
// two grids of squared distances: one for the outside of the glyph shape, one for the inside;
// the signed distance is derived as sqrt(outer) - sqrt(inner)
this.gridOuter = new Float64Array(size * size);
this.gridInner = new Float64Array(size * size);
this.f = new Float64Array(size);
this.z = new Float64Array(size + 1);
this.v = new Uint16Array(size);
}
_createCanvas(size) {
if (typeof OffscreenCanvas !== 'undefined') {
return new OffscreenCanvas(size, size);
}
const canvas = document.createElement('canvas');
canvas.width = canvas.height = size;
return canvas;
}
draw(char) {
const {
width: glyphAdvance,
actualBoundingBoxAscent,
actualBoundingBoxDescent,
actualBoundingBoxLeft,
actualBoundingBoxRight
} = this.ctx.measureText(char);
// The integer/pixel part of the alignment is encoded in metrics.glyphTop/glyphLeft
// The remainder is implicitly encoded in the rasterization
const glyphTop = Math.ceil(actualBoundingBoxAscent);
// actualBoundingBoxLeft is positive when ink extends LEFT of the origin (per spec),
// so negate to get the ink's left edge in canvas x-coords (positive = right of origin)
const glyphLeft = Math.floor(-actualBoundingBoxLeft);
// If the glyph overflows the canvas size, it will be clipped at the bottom/right
const glyphWidth = Math.max(0, Math.min(this.size - this.buffer, Math.ceil(actualBoundingBoxRight) - glyphLeft));
const glyphHeight = Math.max(0, Math.min(this.size - this.buffer, glyphTop + Math.ceil(actualBoundingBoxDescent)));
const width = glyphWidth + 2 * this.buffer;
const height = glyphHeight + 2 * this.buffer;
const len = Math.max(width * height, 0);
const data = new Uint8ClampedArray(len);
const glyph = {data, width, height, glyphWidth, glyphHeight, glyphTop, glyphLeft, glyphAdvance};
if (glyphWidth === 0 || glyphHeight === 0) return glyph;
const {ctx, buffer, gridInner, gridOuter} = this;
if (this.lang) ctx.lang = this.lang;
ctx.clearRect(buffer, buffer, glyphWidth, glyphHeight);
ctx.fillText(char, buffer - glyphLeft, buffer + glyphTop);
const imgData = ctx.getImageData(buffer, buffer, glyphWidth, glyphHeight);
// default: outside the glyph (INF distance) for outer, inside (0 distance) for inner
gridOuter.fill(INF, 0, len);
gridInner.fill(0, 0, len);
// for anti-aliased pixels, treat partial coverage as a distance approximation:
// a fully covered pixel gets 0 outer / INF inner; a partial pixel gets a small
// non-zero outer or inner distance based on how far its coverage deviates from 0.5
let imgIdx = 3; // start at the alpha channel of the first pixel
for (let y = 0; y < glyphHeight; y++) {
let j = (y + buffer) * width + buffer;
for (let x = 0; x < glyphWidth; x++, imgIdx += 4, j++) {
const a = imgData.data[imgIdx]; // alpha value
if (a === 0) continue; // empty pixels
const t = alphaTable[a];
gridOuter[j] = Math.max(0, t);
gridInner[j] = Math.max(0, -t);
}
}
edt(gridOuter, 0, 0, width, height, width, this.f, this.v, this.z);
// Pad the inner EDT region by 1 px so ink pixels touching the bbox edge can see the
// outside-ink seeds in the buffer region; clamp to buffer so we don't underflow when buffer=0
const pad = Math.min(buffer, 1);
edt(gridInner, buffer - pad, buffer - pad, glyphWidth + 2 * pad, glyphHeight + 2 * pad, width, this.f, this.v, this.z);
// encode signed distance as a byte: inside the glyph maps to high values, outside to low,
// with the edge gradient spanning [-radius * cutoff, radius * (1 - cutoff)] pixels around the edge;
// Uint8ClampedArray clamps beyond that
const scale = 255 / this.radius;
const base = 255 * (1 - this.cutoff);
for (let i = 0; i < len; i++) {
const d = Math.sqrt(gridOuter[i]) - Math.sqrt(gridInner[i]);
data[i] = Math.round(base - scale * d);
}
return glyph;
}
}
// 2D Euclidean squared distance transform by Felzenszwalb & Huttenlocher https://cs.brown.edu/~pff/papers/dt-final.pdf
function edt(data, x0, y0, width, height, gridSize, f, v, z) {
for (let x = x0; x < x0 + width; x++) edt1d(data, y0 * gridSize + x, gridSize, height, f, v, z);
for (let y = y0; y < y0 + height; y++) edt1d(data, y * gridSize + x0, 1, width, f, v, z);
}
// 1D squared distance transform
function edt1d(grid, offset, stride, length, f, v, z) {
v[0] = 0;
z[0] = -INF;
z[1] = INF;
f[0] = grid[offset];
for (let q = 1, k = 0, s = 0; q < length; q++) {
f[q] = grid[offset + q * stride];
const q2 = q * q;
do {
const r = v[k];
s = (f[q] - f[r] + q2 - r * r) / (q - r) / 2;
} while (s <= z[k] && --k > -1);
k++;
v[k] = q;
z[k] = s;
z[k + 1] = INF;
}
for (let q = 0, k = 0; q < length; q++) {
while (z[k + 1] < q) k++;
const r = v[k];
const qr = q - r;
grid[offset + q * stride] = f[r] + qr * qr;
}
}