static int vcr1_decode_frame(AVCodecContext *avctx, void *data,
                             int *got_frame, AVPacket *avpkt)
{
    VCR1Context *const a      = avctx->priv_data;
    AVFrame *const p          = data;
    const uint8_t *bytestream = avpkt->data;
    const uint8_t *bytestream_end = bytestream + avpkt->size;
    int i, x, y, ret;

    if(avpkt->size < 16 + avctx->height + avctx->width*avctx->height*5/8){
        av_log(avctx, AV_LOG_ERROR, "Insufficient input data.\n");
        return AVERROR(EINVAL);
    }

    if ((ret = ff_get_buffer(avctx, p, 0)) < 0)
        return ret;
    p->pict_type = AV_PICTURE_TYPE_I;
    p->key_frame = 1;

    for (i = 0; i < 16; i++) {
        a->delta[i] = *bytestream++;
        bytestream++;
    }

    for (y = 0; y < avctx->height; y++) {
        int offset;
        uint8_t *luma = &p->data[0][y * p->linesize[0]];

        if ((y & 3) == 0) {
            uint8_t *cb = &p->data[1][(y >> 2) * p->linesize[1]];
            uint8_t *cr = &p->data[2][(y >> 2) * p->linesize[2]];

            av_assert0 (bytestream_end - bytestream >= 4 + avctx->width);

            for (i = 0; i < 4; i++)
                a->offset[i] = *bytestream++;

            offset = a->offset[0] - a->delta[bytestream[2] & 0xF];
            for (x = 0; x < avctx->width; x += 4) {
                luma[0]     = offset += a->delta[bytestream[2] & 0xF];
                luma[1]     = offset += a->delta[bytestream[2] >>  4];
                luma[2]     = offset += a->delta[bytestream[0] & 0xF];
                luma[3]     = offset += a->delta[bytestream[0] >>  4];
                luma       += 4;

                *cb++       = bytestream[3];
                *cr++       = bytestream[1];

                bytestream += 4;
            }
        } else {
            av_assert0 (bytestream_end - bytestream >= avctx->width / 2);

            offset = a->offset[y & 3] - a->delta[bytestream[2] & 0xF];

            for (x = 0; x < avctx->width; x += 8) {
                luma[0]     = offset += a->delta[bytestream[2] & 0xF];
                luma[1]     = offset += a->delta[bytestream[2] >>  4];
                luma[2]     = offset += a->delta[bytestream[3] & 0xF];
                luma[3]     = offset += a->delta[bytestream[3] >>  4];
                luma[4]     = offset += a->delta[bytestream[0] & 0xF];
                luma[5]     = offset += a->delta[bytestream[0] >>  4];
                luma[6]     = offset += a->delta[bytestream[1] & 0xF];
                luma[7]     = offset += a->delta[bytestream[1] >>  4];
                luma       += 8;
                bytestream += 4;
            }
        }
    }

    *got_frame = 1;

    return bytestream - avpkt->data;
}