static int filter_frame(AVFilterLink *inlink, AVFilterBufferRef *cur_buf)
{
    AlphaExtractContext *extract = inlink->dst->priv;
    AVFilterLink *outlink = inlink->dst->outputs[0];
    AVFilterBufferRef *out_buf =
        ff_get_video_buffer(outlink, AV_PERM_WRITE, outlink->w, outlink->h);
    int ret;

    if (!out_buf) {
        ret = AVERROR(ENOMEM);
        goto end;
    }
    avfilter_copy_buffer_ref_props(out_buf, cur_buf);

    if (extract->is_packed_rgb) {
        int x, y;
        uint8_t *pin, *pout;
        for (y = 0; y < out_buf->video->h; y++) {
            pin = cur_buf->data[0] + y * cur_buf->linesize[0] + extract->rgba_map[A];
            pout = out_buf->data[0] + y * out_buf->linesize[0];
            for (x = 0; x < out_buf->video->w; x++) {
                *pout = *pin;
                pout += 1;
                pin += 4;
            }
        }
    } else {
        const int linesize = abs(FFMIN(out_buf->linesize[Y], cur_buf->linesize[A]));
        int y;
        for (y = 0; y < out_buf->video->h; y++) {
            memcpy(out_buf->data[Y] + y * out_buf->linesize[Y],
                   cur_buf->data[A] + y * cur_buf->linesize[A],
                   linesize);
        }
    }

    ret = ff_filter_frame(outlink, out_buf);

end:
    avfilter_unref_buffer(cur_buf);
    return ret;
}