static int mpeg4_decode_sprite_trajectory(Mpeg4DecContext *ctx, GetBitContext *gb)
{
    MpegEncContext *s = &ctx->m;
    int a     = 2 << s->sprite_warping_accuracy;
    int rho   = 3  - s->sprite_warping_accuracy;
    int r     = 16 / a;
    int alpha = 0;
    int beta  = 0;
    int w     = s->width;
    int h     = s->height;
    int min_ab, i, w2, h2, w3, h3;
    int sprite_ref[4][2];
    int virtual_ref[2][2];

    // only true for rectangle shapes
    const int vop_ref[4][2] = { { 0, 0 },         { s->width, 0 },
                                { 0, s->height }, { s->width, s->height } };
    int d[4][2]             = { { 0, 0 }, { 0, 0 }, { 0, 0 }, { 0, 0 } };

    if (w <= 0 || h <= 0)
        return AVERROR_INVALIDDATA;

    for (i = 0; i < ctx->num_sprite_warping_points; i++) {
        int length;
        int x = 0, y = 0;

        length = get_vlc2(gb, sprite_trajectory.table, SPRITE_TRAJ_VLC_BITS, 3);
        if (length)
            x = get_xbits(gb, length);

        if (!(ctx->divx_version == 500 && ctx->divx_build == 413))
            skip_bits1(gb);     /* marker bit */

        length = get_vlc2(gb, sprite_trajectory.table, SPRITE_TRAJ_VLC_BITS, 3);
        if (length)
            y = get_xbits(gb, length);

        skip_bits1(gb);         /* marker bit */
        ctx->sprite_traj[i][0] = d[i][0] = x;
        ctx->sprite_traj[i][1] = d[i][1] = y;
    }
    for (; i < 4; i++)
        ctx->sprite_traj[i][0] = ctx->sprite_traj[i][1] = 0;

    while ((1 << alpha) < w)
        alpha++;
    while ((1 << beta) < h)
        beta++;  /* typo in the mpeg4 std for the definition of w' and h' */
    w2 = 1 << alpha;
    h2 = 1 << beta;

    // Note, the 4th point isn't used for GMC
    if (ctx->divx_version == 500 && ctx->divx_build == 413) {
        sprite_ref[0][0] = a * vop_ref[0][0] + d[0][0];
        sprite_ref[0][1] = a * vop_ref[0][1] + d[0][1];
        sprite_ref[1][0] = a * vop_ref[1][0] + d[0][0] + d[1][0];
        sprite_ref[1][1] = a * vop_ref[1][1] + d[0][1] + d[1][1];
        sprite_ref[2][0] = a * vop_ref[2][0] + d[0][0] + d[2][0];
        sprite_ref[2][1] = a * vop_ref[2][1] + d[0][1] + d[2][1];
    } else {
        sprite_ref[0][0] = (a >> 1) * (2 * vop_ref[0][0] + d[0][0]);
        sprite_ref[0][1] = (a >> 1) * (2 * vop_ref[0][1] + d[0][1]);
        sprite_ref[1][0] = (a >> 1) * (2 * vop_ref[1][0] + d[0][0] + d[1][0]);
        sprite_ref[1][1] = (a >> 1) * (2 * vop_ref[1][1] + d[0][1] + d[1][1]);
        sprite_ref[2][0] = (a >> 1) * (2 * vop_ref[2][0] + d[0][0] + d[2][0]);
        sprite_ref[2][1] = (a >> 1) * (2 * vop_ref[2][1] + d[0][1] + d[2][1]);
    }
    /* sprite_ref[3][0] = (a >> 1) * (2 * vop_ref[3][0] + d[0][0] + d[1][0] + d[2][0] + d[3][0]);
     * sprite_ref[3][1] = (a >> 1) * (2 * vop_ref[3][1] + d[0][1] + d[1][1] + d[2][1] + d[3][1]); */

    /* this is mostly identical to the mpeg4 std (and is totally unreadable
     * because of that...). Perhaps it should be reordered to be more readable.
     * The idea behind this virtual_ref mess is to be able to use shifts later
     * per pixel instead of divides so the distance between points is converted
     * from w&h based to w2&h2 based which are of the 2^x form. */
    virtual_ref[0][0] = 16 * (vop_ref[0][0] + w2) +
                         ROUNDED_DIV(((w - w2) *
                                      (r * sprite_ref[0][0] - 16 * vop_ref[0][0]) +
                                      w2 * (r * sprite_ref[1][0] - 16 * vop_ref[1][0])), w);
    virtual_ref[0][1] = 16 * vop_ref[0][1] +
                        ROUNDED_DIV(((w - w2) *
                                     (r * sprite_ref[0][1] - 16 * vop_ref[0][1]) +
                                     w2 * (r * sprite_ref[1][1] - 16 * vop_ref[1][1])), w);
    virtual_ref[1][0] = 16 * vop_ref[0][0] +
                        ROUNDED_DIV(((h - h2) * (r * sprite_ref[0][0] - 16 * vop_ref[0][0]) +
                                     h2 * (r * sprite_ref[2][0] - 16 * vop_ref[2][0])), h);
    virtual_ref[1][1] = 16 * (vop_ref[0][1] + h2) +
                        ROUNDED_DIV(((h - h2) * (r * sprite_ref[0][1] - 16 * vop_ref[0][1]) +
                                     h2 * (r * sprite_ref[2][1] - 16 * vop_ref[2][1])), h);

    switch (ctx->num_sprite_warping_points) {
    case 0:
        s->sprite_offset[0][0] =
        s->sprite_offset[0][1] =
        s->sprite_offset[1][0] =
        s->sprite_offset[1][1] = 0;
        s->sprite_delta[0][0]  = a;
        s->sprite_delta[0][1]  =
        s->sprite_delta[1][0]  = 0;
        s->sprite_delta[1][1]  = a;
        ctx->sprite_shift[0]   =
        ctx->sprite_shift[1]   = 0;
        break;
    case 1:     // GMC only
        s->sprite_offset[0][0] = sprite_ref[0][0] - a * vop_ref[0][0];
        s->sprite_offset[0][1] = sprite_ref[0][1] - a * vop_ref[0][1];
        s->sprite_offset[1][0] = ((sprite_ref[0][0] >> 1) | (sprite_ref[0][0] & 1)) -
                                 a * (vop_ref[0][0] / 2);
        s->sprite_offset[1][1] = ((sprite_ref[0][1] >> 1) | (sprite_ref[0][1] & 1)) -
                                 a * (vop_ref[0][1] / 2);
        s->sprite_delta[0][0]  = a;
        s->sprite_delta[0][1]  =
        s->sprite_delta[1][0]  = 0;
        s->sprite_delta[1][1]  = a;
        ctx->sprite_shift[0]   =
        ctx->sprite_shift[1]   = 0;
        break;
    case 2:
        s->sprite_offset[0][0] = (sprite_ref[0][0] << (alpha + rho)) +
                                 (-r * sprite_ref[0][0] + virtual_ref[0][0]) *
                                 (-vop_ref[0][0]) +
                                 (r * sprite_ref[0][1] - virtual_ref[0][1]) *
                                 (-vop_ref[0][1]) + (1 << (alpha + rho - 1));
        s->sprite_offset[0][1] = (sprite_ref[0][1] << (alpha + rho)) +
                                 (-r * sprite_ref[0][1] + virtual_ref[0][1]) *
                                 (-vop_ref[0][0]) +
                                 (-r * sprite_ref[0][0] + virtual_ref[0][0]) *
                                 (-vop_ref[0][1]) + (1 << (alpha + rho - 1));
        s->sprite_offset[1][0] = ((-r * sprite_ref[0][0] + virtual_ref[0][0]) *
                                  (-2 * vop_ref[0][0] + 1) +
                                  (r * sprite_ref[0][1] - virtual_ref[0][1]) *
                                  (-2 * vop_ref[0][1] + 1) + 2 * w2 * r *
                                  sprite_ref[0][0] - 16 * w2 + (1 << (alpha + rho + 1)));
        s->sprite_offset[1][1] = ((-r * sprite_ref[0][1] + virtual_ref[0][1]) *
                                  (-2 * vop_ref[0][0] + 1) +
                                  (-r * sprite_ref[0][0] + virtual_ref[0][0]) *
                                  (-2 * vop_ref[0][1] + 1) + 2 * w2 * r *
                                  sprite_ref[0][1] - 16 * w2 + (1 << (alpha + rho + 1)));
        s->sprite_delta[0][0] = (-r * sprite_ref[0][0] + virtual_ref[0][0]);
        s->sprite_delta[0][1] = (+r * sprite_ref[0][1] - virtual_ref[0][1]);
        s->sprite_delta[1][0] = (-r * sprite_ref[0][1] + virtual_ref[0][1]);
        s->sprite_delta[1][1] = (-r * sprite_ref[0][0] + virtual_ref[0][0]);

        ctx->sprite_shift[0]  = alpha + rho;
        ctx->sprite_shift[1]  = alpha + rho + 2;
        break;
    case 3:
        min_ab = FFMIN(alpha, beta);
        w3     = w2 >> min_ab;
        h3     = h2 >> min_ab;
        s->sprite_offset[0][0] = (sprite_ref[0][0] << (alpha + beta + rho - min_ab)) +
                                 (-r * sprite_ref[0][0] + virtual_ref[0][0]) *
                                 h3 * (-vop_ref[0][0]) +
                                 (-r * sprite_ref[0][0] + virtual_ref[1][0]) *
                                 w3 * (-vop_ref[0][1]) +
                                 (1 << (alpha + beta + rho - min_ab - 1));
        s->sprite_offset[0][1] = (sprite_ref[0][1] << (alpha + beta + rho - min_ab)) +
                                 (-r * sprite_ref[0][1] + virtual_ref[0][1]) *
                                 h3 * (-vop_ref[0][0]) +
                                 (-r * sprite_ref[0][1] + virtual_ref[1][1]) *
                                 w3 * (-vop_ref[0][1]) +
                                 (1 << (alpha + beta + rho - min_ab - 1));
        s->sprite_offset[1][0] = (-r * sprite_ref[0][0] + virtual_ref[0][0]) *
                                 h3 * (-2 * vop_ref[0][0] + 1) +
                                 (-r * sprite_ref[0][0] + virtual_ref[1][0]) *
                                 w3 * (-2 * vop_ref[0][1] + 1) + 2 * w2 * h3 *
                                 r * sprite_ref[0][0] - 16 * w2 * h3 +
                                 (1 << (alpha + beta + rho - min_ab + 1));
        s->sprite_offset[1][1] = (-r * sprite_ref[0][1] + virtual_ref[0][1]) *
                                 h3 * (-2 * vop_ref[0][0] + 1) +
                                 (-r * sprite_ref[0][1] + virtual_ref[1][1]) *
                                 w3 * (-2 * vop_ref[0][1] + 1) + 2 * w2 * h3 *
                                 r * sprite_ref[0][1] - 16 * w2 * h3 +
                                 (1 << (alpha + beta + rho - min_ab + 1));
        s->sprite_delta[0][0] = (-r * sprite_ref[0][0] + virtual_ref[0][0]) * h3;
        s->sprite_delta[0][1] = (-r * sprite_ref[0][0] + virtual_ref[1][0]) * w3;
        s->sprite_delta[1][0] = (-r * sprite_ref[0][1] + virtual_ref[0][1]) * h3;
        s->sprite_delta[1][1] = (-r * sprite_ref[0][1] + virtual_ref[1][1]) * w3;

        ctx->sprite_shift[0]  = alpha + beta + rho - min_ab;
        ctx->sprite_shift[1]  = alpha + beta + rho - min_ab + 2;
        break;
    }
    /* try to simplify the situation */
    if (s->sprite_delta[0][0] == a << ctx->sprite_shift[0] &&
        s->sprite_delta[0][1] == 0 &&
        s->sprite_delta[1][0] == 0 &&
        s->sprite_delta[1][1] == a << ctx->sprite_shift[0]) {
        s->sprite_offset[0][0] >>= ctx->sprite_shift[0];
        s->sprite_offset[0][1] >>= ctx->sprite_shift[0];
        s->sprite_offset[1][0] >>= ctx->sprite_shift[1];
        s->sprite_offset[1][1] >>= ctx->sprite_shift[1];
        s->sprite_delta[0][0] = a;
        s->sprite_delta[0][1] = 0;
        s->sprite_delta[1][0] = 0;
        s->sprite_delta[1][1] = a;
        ctx->sprite_shift[0] = 0;
        ctx->sprite_shift[1] = 0;
        s->real_sprite_warping_points = 1;
    } else {
        int shift_y = 16 - ctx->sprite_shift[0];
        int shift_c = 16 - ctx->sprite_shift[1];
        for (i = 0; i < 2; i++) {
            s->sprite_offset[0][i] <<= shift_y;
            s->sprite_offset[1][i] <<= shift_c;
            s->sprite_delta[0][i]  <<= shift_y;
            s->sprite_delta[1][i]  <<= shift_y;
            ctx->sprite_shift[i]     = 16;
        }
        s->real_sprite_warping_points = ctx->num_sprite_warping_points;
    }

    return 0;
}