/**
 * How much two screenshots differ, and where.
 *
 * A ruling used to know only whether the pixels moved: `sha256(before) !==
 * sha256(after)`, a boolean in which a one-pixel nudge and a whole re-layout
 * are the same answer. That is enough to hold the guard that says nothing may
 * pass on unchanged pixels, and not enough to say anything to the person
 * reading the verdict about what the fix actually did.
 *
 * The comparison is exact: every channel, no tolerance, alpha ignored. lookout
 * captures with animations disabled at a fixed device scale, so two runs of an
 * unchanged page are byte-identical; a tolerance would only let a real change
 * hide under it. That is also why no perceptual metric is used here: SSIM and
 * its relatives score a one-pixel misalignment at ~1.0, which is exactly the
 * defect class this repository exists to file.
 *
 * The algorithm was `tools/ui-check`'s, where it found three real bugs that
 * every other gate passed over. It is promoted here rather than copied, and
 * that tool now imports it.
 */
export interface PixelBox {
    x: number;
    y: number;
    w: number;
    h: number;
}
export interface PixelDiff {
    /** Pixels differing, including every pixel one image has and the other does not. */
    changed: number;
    /** Pixels in the union of both images: max width x max height. */
    total: number;
    /** changed / total, 0 to 1. */
    fraction: number;
    /** The smallest rectangle containing every difference; null when there are none. */
    box: PixelBox | null;
    /**
     * changed / box area. Near 1 means one solid region moved; small means the
     * change is scattered, which is what a re-layout looks like.
     */
    density: number;
    before: {
        width: number;
        height: number;
    };
    after: {
        width: number;
        height: number;
    };
    /** The images are not the same size, which is itself a change. */
    sizeChanged: boolean;
}
/**
 * Compare two PNGs. Never throws: an image that will not decode returns null,
 * which callers read as "changed, and by how much was not measured" rather
 * than as "unchanged".
 *
 * Different dimensions are the common re-layout case, not an error. The
 * overlapping region is compared pixel by pixel and everything outside it
 * counts as changed, so a page that grew reports the growth rather than
 * refusing to answer.
 */
export declare function diffPng(before: Buffer | string, after: Buffer | string): Promise<PixelDiff | null>;
/**
 * What changed, as a sentence for the verdict. Says the shape of the change,
 * not just its size: one region moving and the same count scattered across the
 * frame mean different things to whoever is reading the ruling.
 */
export declare function changeSaid(d: PixelDiff): string;
/**
 * The changed region of the after image, cropped and enlarged. Best effort:
 * returns null rather than costing a ruling its verdict, because a picture of
 * the change is a convenience and the measurement is the evidence.
 */
export declare function writeDiffCrop(afterPng: string, box: PixelBox, outPath: string): Promise<string | null>;