分析encode_one_frame函数.
先来看下结构体Sourceframe的定义:
typedef struct { // Size info int x_size, y_framesize, y_fieldsize; char *yf, *uf, *vf; //!< frame representation char *yt, *ut, *vt; //!< top field char *yb, *ub, *vb; //!< bottom field } Sourceframe;
可见,其中将来要存得就是原始的YUV帧的尺寸和YUV信息, 接着看srcframe指针变量的定义:
static Sourceframe *srcframe;
接着看对srcframe的赋值操作:
srcframe = AllocSourceframe (img->width, img->height);
进入AllocSourceframe函数看内部:
static Sourceframe *AllocSourceframe (int xs, int ys) { Sourceframe *sf = NULL; const unsigned int bytes_y = xs*ys; const unsigned int bytes_uv = (xs*ys)/4; if ((sf = calloc (1, sizeof (Sourceframe))) == NULL) no_mem_exit ("ReadOneFrame: sf"); if (sf->yf == NULL) if ((sf->yf = calloc (1, bytes_y)) == NULL) no_mem_exit ("ReadOneFrame: sf->yf"); if (sf->yt == NULL) if ((sf->yt = calloc (1, bytes_y/2)) == NULL) no_mem_exit ("ReadOneFrame: sf->yt"); if (sf->yb == NULL) if ((sf->yb = calloc (1, bytes_y/2)) == NULL) no_mem_exit ("ReadOneFrame: sf->yb"); if (sf->uf == NULL) if ((sf->uf = calloc (1, bytes_uv)) == NULL) no_mem_exit ("ReadOneFrame: sf->uf"); if (sf->ut == NULL) if ((sf->ut = calloc (1, bytes_uv/2)) == NULL) no_mem_exit ("ReadOneFrame: sf->ut"); if (sf->ub == NULL) if ((sf->ub = calloc (1, bytes_uv/2)) == NULL) no_mem_exit ("ReadOneFrame: sf->ub"); if (sf->vf == NULL) if ((sf->vf = calloc (1, bytes_uv)) == NULL) no_mem_exit ("ReadOneFrame: sf->vf"); if (sf->vt == NULL) if ((sf->vt = calloc (1, bytes_uv/2)) == NULL) no_mem_exit ("ReadOneFrame: sf->vt"); if (sf->vb == NULL) if ((sf->vb = calloc (1, bytes_uv/2)) == NULL) no_mem_exit ("ReadOneFrame: sf->vb"); sf->x_size = xs; sf->y_framesize = ys; sf->y_fieldsize = ys/2; return sf; }
可以看到,实际上是就是在对内存上分配存储空间,并让栈指针sf指向分配的堆内存,返回sf后,srcframe指针就指向了该块对内存.
接着执行下面的语句:
ReadOneFrame (FrameNumberInFile, input->infile_header, img->width, img->height, srcframe);
执行后,就对srcframe指针指向的堆内存赋值了,进入ReadOneFrame函数大致看一下:
static void ReadOneFrame (int FrameNoInFile, int HeaderSize, int xs, int ys, Sourceframe *sf) { int i; const unsigned int bytes_y = xs*ys; const unsigned int bytes_uv = (xs*ys)/4; const int framesize_in_bytes = bytes_y + 2*bytes_uv; assert (xs % MB_BLOCK_SIZE == 0); assert (ys % MB_BLOCK_SIZE == 0); assert (p_in != NULL); assert (sf != NULL); assert (sf->yf != NULL); assert (FrameNumberInFile == FrameNoInFile); // printf ("ReadOneFrame: frame_no %d xs %d ys %d\n", FrameNoInFile, xs, ys); if (fseek (p_in, HeaderSize, SEEK_SET) != 0) error ("ReadOneFrame: cannot fseek to (Header size) in p_in", -1); // the reason for the following loop is to support source files bigger than // MAXINT. In most operating systems, including Windows, it is possible to // fseek to file positions bigger than MAXINT by using this relative seeking // technique. StW, 12/30/02 // Skip starting frames for (i=0; i<input->start_frame; i++) if (fseek (p_in, framesize_in_bytes, SEEK_CUR) != 0) { printf ("ReadOneFrame: cannot advance file pointer in p_in beyond frame %d, looping to picture zero\n", i); if (fseek (p_in, HeaderSize, SEEK_SET) != 0) report_stats_on_error(); exit (-1); } for (i=0; i<FrameNoInFile; i++) if (fseek (p_in, framesize_in_bytes, SEEK_CUR) != 0) { printf ("ReadOneFrame: cannot advance file pointer in p_in beyond frame %d, looping to picture zero\n", i); if (fseek (p_in, HeaderSize, SEEK_SET) != 0) error ("ReadOneFrame: cannot fseek to (Header size) in p_in", -1); } // Here we are at the correct position for the source frame in the file. Now // read it. if (fread (sf->yf, 1, bytes_y, p_in) != bytes_y) { printf ("ReadOneFrame: cannot read %d bytes from input file, unexpected EOF?, exiting", bytes_y); report_stats_on_error(); exit (-1); } if (fread (sf->uf, 1, bytes_uv, p_in) != bytes_uv) { printf ("ReadOneFrame: cannot read %d bytes from input file, unexpected EOF?, exiting", bytes_uv); report_stats_on_error(); exit (-1); } if (fread (sf->vf, 1, bytes_uv, p_in) != bytes_uv) { printf ("ReadOneFrame: cannot read %d bytes from input file, unexpected EOF?, exiting", bytes_uv); report_stats_on_error(); exit (-1); } // Complete frame is read into sf->?f, now setup // top and bottom field (sf->?t and sf->?b) GenerateFieldComponent (sf->yf, sf->yt, sf->yb, xs, ys); GenerateFieldComponent (sf->uf, sf->ut, sf->ub, xs/2, ys/2); GenerateFieldComponent (sf->vf, sf->vt, sf->vb, xs/2, ys/2); }
其中的p_in就是一个文件指针,指向了原始的YUV文件,p_in是一个全局变量,对p_in的赋值在Configure函数中,摘录如下:
// Open Files if ((p_in=fopen(input->infile,"rb"))==NULL) { snprintf(errortext, ET_SIZE, "Input file %s does not exist",input->infile); error (errortext, 500); }
其中的input->infile是从配置文件(encoder_baseline.cfg)中得到的,input->infile指向了“foreman_part_qcif.yuv”
回头再看语句:
ReadOneFrame (FrameNumberInFile, input->infile_header, img->width, img->height, srcframe);
易知,把"foreman_part_qcif.yuv"中的像素值塞进了srcframe指针指向的堆内存, 通过上面语句塞进对内存之后怎么办呢?看下面的语句:
CopyFrameToOldImgOrgVariables (srcframe);
进入CopyFrameToOldImgOrgVariables函数看一下:
static void CopyFrameToOldImgOrgVariables (Sourceframe *sf) { int x, y; for (y=0; y<sf->y_framesize; y++) for (x=0; x<sf->x_size; x++) { imgY_org_frm [y][x] = sf->yf[y*sf->x_size+x]; } for (y=0; y<sf->y_framesize/2; y++) for (x=0; x<sf->x_size/2; x++) { imgUV_org_frm[0][y][x] = sf->uf[y*sf->x_size/2+x]; imgUV_org_frm[1][y][x] = sf->vf[y*sf->x_size/2+x]; } }
可见,这个函数实现的功能是把srcframe指向的对内存中的数据往imgY_org_frm和imgUV_org_frm中倾倒, 而后又有:
imgY_org = imgY_org_frm;
imgUV_org = imgUV_org_frm;
也就是说,指向的地方相同,所以全局的 imgY_org和imgUV_org 中也就有了"foreman_part_qcif.yuv"中的YUV信息,而且都根据将来的需要进行了一定的组织(数据排列).
以后用imgY_org和imgUV_org就相当于用到了"foreman_part_qcif.yuv"中的像素值. 以上就是YUV数据流向程序的简要过程.