static int poll_frame(AVFilterLink *link)

{

    AVFilterContext   *s = link->src;

    OverlayContext *over = s->priv;

    int ret = avfilter_poll_frame(s->inputs[OVERLAY]);



    if (ret == AVERROR_EOF)

        ret = !!over->overpicref;



    return ret && avfilter_poll_frame(s->inputs[MAIN]);

}
