static int vcr1_decode_frame(AVCodecContext *avctx, void *data,
                             int *got_frame, AVPacket *avpkt)
{
    const uint8_t *buf        = avpkt->data;
    int buf_size              = avpkt->size;
    VCR1Context *const a      = avctx->priv_data;
    AVFrame *const p          = data;
    const uint8_t *bytestream = buf;
    int i, x, y, ret;
    if ((ret = ff_get_buffer(avctx, p, 0)) < 0) {
        av_log(avctx, AV_LOG_ERROR, "get_buffer() failed\n");
        return ret;
    }
    p->pict_type = AV_PICTURE_TYPE_I;
    p->key_frame = 1;
    if (buf_size < 32)
    for (i = 0; i < 16; i++) {
        a->delta[i] = *bytestream++;
        bytestream++;
        buf_size--;
    }
    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]];
            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;
                buf_size   -= 4;
            }
        } else {
            if (buf_size < 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;
                buf_size   -= 4;
            }
        }
    }
    *got_frame = 1;
    return buf_size;
packet_small:
    av_log(avctx, AV_LOG_ERROR, "Input packet too small.\n");
    return AVERROR_INVALIDDATA;
}