static int filter_frame(AVFilterLink *inlink, AVFilterBufferRef *picref)
{
    AVFilterContext *ctx  = inlink->dst;
    TileContext *tile    = ctx->priv;
    AVFilterLink *outlink = ctx->outputs[0];
    unsigned x0, y0;

    if (!tile->current) {
        outlink->out_buf = ff_get_video_buffer(outlink, AV_PERM_WRITE,
                                               outlink->w, outlink->h);
        if (!outlink->out_buf)
            return AVERROR(ENOMEM);
        avfilter_copy_buffer_ref_props(outlink->out_buf, picref);
        outlink->out_buf->video->w = outlink->w;
        outlink->out_buf->video->h = outlink->h;

        /* fill surface once for margin/padding */
        if (tile->margin || tile->padding)
            ff_fill_rectangle(&tile->draw, &tile->blank,
                              outlink->out_buf->data,
                              outlink->out_buf->linesize,
                              0, 0, outlink->w, outlink->h);
    }

    get_current_tile_pos(ctx, &x0, &y0);
    ff_copy_rectangle2(&tile->draw,
                       outlink->out_buf->data, outlink->out_buf->linesize,
                       inlink ->cur_buf->data, inlink ->cur_buf->linesize,
                       x0, y0, 0, 0, inlink->w, inlink->h);

    avfilter_unref_bufferp(&inlink->cur_buf);
    if (++tile->current == tile->nb_frames)
        end_last_frame(ctx);

    return 0;
}