if (s->first_slice_line && ((n < 2) || (n > 3))) c = 1024;
if(s->first_slice_line && n!=3){ if(n!=2) c= 1024; if(n!=1 && s->mb_x == s->resync_mb_x) a= 1024; }
static int h263_pred_dc(MpegEncContext * s, int n, uint16_t **dc_val_ptr){    int x, y, wrap, a, c, pred_dc, scale;    int16_t *dc_val, *ac_val;    /* find prediction */    if (n < 4) {        x = 2 * s->mb_x + 1 + (n & 1);        y = 2 * s->mb_y + 1 + ((n & 2) >> 1);        wrap = s->mb_width * 2 + 2;        dc_val = s->dc_val[0];        ac_val = s->ac_val[0][0];        scale = s->y_dc_scale;    } else {        x = s->mb_x + 1;        y = s->mb_y + 1;        wrap = s->mb_width + 2;        dc_val = s->dc_val[n - 4 + 1];        ac_val = s->ac_val[n - 4 + 1][0];        scale = s->c_dc_scale;    }    /* B C     * A X      */    a = dc_val[(x - 1) + (y) * wrap];    c = dc_val[(x) + (y - 1) * wrap];        /* No prediction outside GOB boundary */    if (s->first_slice_line && ((n < 2) || (n > 3)))        c = 1024;    pred_dc = 1024;    /* just DC prediction */    if (a != 1024 && c != 1024)        pred_dc = (a + c) >> 1;    else if (a != 1024)        pred_dc = a;    else        pred_dc = c;        /* we assume pred is positive */    //pred_dc = (pred_dc + (scale >> 1)) / scale;    *dc_val_ptr = &dc_val[x + y * wrap];    return pred_dc;}