static int decode_slice(AVCodecContext *c, void *arg)
{
    FFV1Context *fs   = *(void **)arg;
    FFV1Context *f    = fs->avctx->priv_data;
    int width, height, x, y, ret;
    const int ps      = av_pix_fmt_desc_get(c->pix_fmt)->comp[0].step_minus1 + 1;
    AVFrame * const p = f->cur;
    int i, si;

    for( si=0; fs != f->slice_context[si]; si ++)
        ;

    if(f->fsrc && !p->key_frame)
        ff_thread_await_progress(&f->last_picture, si, 0);

    if(f->fsrc && !p->key_frame) {
        FFV1Context *fssrc = f->fsrc->slice_context[si];
        FFV1Context *fsdst = f->slice_context[si];
        av_assert1(fsdst->plane_count == fssrc->plane_count);
        av_assert1(fsdst == fs);

        if (!p->key_frame)
            fsdst->slice_damaged |= fssrc->slice_damaged;

        for (i = 0; i < f->plane_count; i++) {
            PlaneContext *psrc = &fssrc->plane[i];
            PlaneContext *pdst = &fsdst->plane[i];

            av_free(pdst->state);
            av_free(pdst->vlc_state);
            memcpy(pdst, psrc, sizeof(*pdst));
            pdst->state = NULL;
            pdst->vlc_state = NULL;

            if (fssrc->ac) {
                pdst->state = av_malloc(CONTEXT_SIZE * psrc->context_count);
                memcpy(pdst->state, psrc->state, CONTEXT_SIZE * psrc->context_count);
            } else {
                pdst->vlc_state = av_malloc(sizeof(*pdst->vlc_state) * psrc->context_count);
                memcpy(pdst->vlc_state, psrc->vlc_state, sizeof(*pdst->vlc_state) * psrc->context_count);
            }
        }
    }

    if (f->version > 2) {
        if (ffv1_init_slice_state(f, fs) < 0)
            return AVERROR(ENOMEM);
        if (decode_slice_header(f, fs) < 0) {
            fs->slice_damaged = 1;
            return AVERROR_INVALIDDATA;
        }
    }
    if ((ret = ffv1_init_slice_state(f, fs)) < 0)
        return ret;
    if (f->cur->key_frame)
        ffv1_clear_slice_state(f, fs);

    width  = fs->slice_width;
    height = fs->slice_height;
    x      = fs->slice_x;
    y      = fs->slice_y;

    if (!fs->ac) {
        if (f->version == 3 && f->micro_version > 1 || f->version > 3)
            get_rac(&fs->c, (uint8_t[]) { 129 });
        fs->ac_byte_count = f->version > 2 || (!x && !y) ? fs->c.bytestream - fs->c.bytestream_start - 1 : 0;
        init_get_bits(&fs->gb,
                      fs->c.bytestream_start + fs->ac_byte_count,
                      (fs->c.bytestream_end - fs->c.bytestream_start - fs->ac_byte_count) * 8);
    }

    av_assert1(width && height);
    if (f->colorspace == 0) {
        const int chroma_width  = FF_CEIL_RSHIFT(width,  f->chroma_h_shift);
        const int chroma_height = FF_CEIL_RSHIFT(height, f->chroma_v_shift);
        const int cx            = x >> f->chroma_h_shift;
        const int cy            = y >> f->chroma_v_shift;
        decode_plane(fs, p->data[0] + ps*x + y*p->linesize[0], width, height, p->linesize[0], 0);

        if (f->chroma_planes) {
            decode_plane(fs, p->data[1] + ps*cx+cy*p->linesize[1], chroma_width, chroma_height, p->linesize[1], 1);
            decode_plane(fs, p->data[2] + ps*cx+cy*p->linesize[2], chroma_width, chroma_height, p->linesize[2], 1);
        }
        if (fs->transparency)
            decode_plane(fs, p->data[3] + ps*x + y*p->linesize[3], width, height, p->linesize[3], 2);
    } else {
        uint8_t *planes[3] = { p->data[0] + ps * x + y * p->linesize[0],
                               p->data[1] + ps * x + y * p->linesize[1],
                               p->data[2] + ps * x + y * p->linesize[2] };
        decode_rgb_frame(fs, planes, width, height, p->linesize);
    }
    if (fs->ac && f->version > 2) {
        int v;
        get_rac(&fs->c, (uint8_t[]) { 129 });
        v = fs->c.bytestream_end - fs->c.bytestream - 2 - 5*f->ec;
        if (v) {
            av_log(f->avctx, AV_LOG_ERROR, "bytestream end mismatching by %d\n", v);
            fs->slice_damaged = 1;
        }
    }

    emms_c();

    ff_thread_report_progress(&f->picture, si, 0);

    return 0;
}