offset= a->offset[y&3];
offset= a->offset[y&3] - a->delta[ bytestream[2]&0xF ];
static int decode_frame(AVCodecContext *avctx,                         void *data, int *data_size,                        uint8_t *buf, int buf_size){    VCR1Context * const a = avctx->priv_data;    AVFrame *picture = data;    AVFrame * const p= (AVFrame*)&a->picture;    uint8_t *bytestream= buf;    int i, x, y;    *data_size = 0;    /* special case for last picture */    if (buf_size == 0) {        return 0;    }    if(p->data[0])        avctx->release_buffer(avctx, p);    p->reference= 0;    if(avctx->get_buffer(avctx, p) < 0){        fprintf(stderr, "get_buffer() failed\n");        return -1;    }    p->pict_type= I_TYPE;    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= &a->picture.data[0][ y*a->picture.linesize[0] ];        if((y&3) == 0){            uint8_t *cb= &a->picture.data[1][ (y>>2)*a->picture.linesize[1] ];            uint8_t *cr= &a->picture.data[2][ (y>>2)*a->picture.linesize[2] ];            for(i=0; i<4; i++)                a->offset[i]= *(bytestream++);            offset= a->offset[0];            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[1];                *(cr++) = bytestream[3];                                bytestream+= 4;            }        }else{            offset= a->offset[y&3];            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;            }        }    }    *picture= *(AVFrame*)&a->picture;    *data_size = sizeof(AVPicture);    emms_c();        return buf_size;}