/**
 * @license
 * Copyright (C) Gnucoop soc. coop.
 *
 * This file is part of the Advanced JSON forms (ajf).
 *
 * Advanced JSON forms (ajf) is free software: you can redistribute it and/or
 * modify it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the License,
 * or (at your option) any later version.
 *
 * Advanced JSON forms (ajf) is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero
 * General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with Advanced JSON forms (ajf).
 * If not, see http://www.gnu.org/licenses/.
 *
 */
/**
 * Encodes an image made only of black pixels over a transparent background
 * (e.g. a signature drawn on a canvas) as a palette-indexed PNG.
 *
 * Since every pixel is either transparent or some shade of black
 * (anti-aliased strokes only vary the alpha channel), the palette never
 * needs more than 256 entries, so a color type 3 (indexed) PNG can be built
 * regardless of the image content. This is dramatically smaller than the
 * plain RGBA PNG produced by `canvas.toDataURL('image/png')`, which the
 * browser always encodes at 32 bits per pixel with no palette reduction.
 */
export declare function canvasToIndexedPngDataUrl(canvas: HTMLCanvasElement): string;