ffmpeg解码JPG和PNG等图片

http://blog.chinaunix.net/uid-25272011-id-3690729.html

 【特别提醒：本文写作时，贴上去的代码，"\n"回车符号的"\"没有了，不知道为啥，所以阅读代码时请注意区分，或者欢迎到我的CSDN网站阅读
        http://blog.csdn.net/jgf_ntu/article/details/8928977】    
    一般我们都是用ffmpeg来解码音视频，如果是JPG和PNG等图片呢，其实跟解码视频是一样的，因为视频也是一幅一幅的图片进行解码的，只不过视频的帧是会前后参考的，而JPG等图片来讲，就是独立的一帧而已。
那么，我们参考之前的一篇文章http://blog.chinaunix.net/uid-25272011-id-3633434.html 【一段ffmpeg视频解码为YUV420P的示例代码】 ，稍作修改即可来演示。
    同时为了能够保存解码后的图片，我们还需要了解一些YUV或者RGB等各种格式的数据的内存存储方式，这些知识可以参照我之前的另一篇文章《YUV420格式解析》 ，这里详细描述了各种格式的空间存储机制。
    一般解码视频时，我们在调用ffmpeg进行解码时，生成的格式一般都是YUV420P的，但解码图皮时可能会有各种形式，如YUVJ422P、YUVJ444P、RGB24等等，本文没有采用ffmpeg的sws_scale函数做统一的转
换，为的是记录如何来存储这些解码后的图片。

     先给出如何从ffmpeg的Frame结构体中保存上述的四种解码后的数据：

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 /**
  * save yuv420p frame [YUV]
  */
 void yuv420p_save(AVFrame *pFrame, AVCodecContext *pCodecCtx)
 {
     int i = 0;

     int width = pCodecCtx->width, height = pCodecCtx->height;
     int height_half = height / 2, width_half = width / 2;
     int y_wrap = pFrame->linesize[0];
     int u_wrap = pFrame->linesize[1];
     int v_wrap = pFrame->linesize[2];

     unsigned char *y_buf = pFrame->data[0];
     unsigned char *u_buf = pFrame->data[1];
     unsigned char *v_buf = pFrame->data[2];

     //save y
     for (i = 0; i < height; i++)
         fwrite(y_buf + i * y_wrap, 1, width, pfout);
     fprintf(stderr, "===>save Y successn");
     //save u
     for (i = 0; i < height_half; i++)
         fwrite(u_buf + i * u_wrap, 1, width_half, pfout);
     fprintf(stderr, "===>save U successn");
     //save v
     for (i = 0; i < height_half; i++)
         fwrite(v_buf + i * v_wrap, 1, width_half, pfout);
     fprintf(stderr, "===>save V successn");

     fflush(pfout);
 }

 /**
  * save yuv422p frame [YUV]
  */
 void yuv422p_save(AVFrame *pFrame, AVCodecContext *pCodecCtx)
 {
     int i = 0;

     int width = pCodecCtx->width, height = pCodecCtx->height;
     int height_half = height / 2, width_half = width / 2;
     int y_wrap = pFrame->linesize[0];
     int u_wrap = pFrame->linesize[1];
     int v_wrap = pFrame->linesize[2];

     unsigned char *y_buf = pFrame->data[0];
     unsigned char *u_buf = pFrame->data[1];
     unsigned char *v_buf = pFrame->data[2];

     //save y
     for (i = 0; i < height; i++)
         fwrite(y_buf + i * y_wrap, 1, width, pfout);
     fprintf(stderr, "===>save Y successn");
     //save u
     for (i = 0; i < height; i++)
         fwrite(u_buf + i * u_wrap, 1, width_half, pfout);
     fprintf(stderr, "===>save U successn");
     //save v
     for (i = 0; i < height; i++)
         fwrite(v_buf + i * v_wrap, 1, width_half, pfout);
     fprintf(stderr, "===>save V successn");

     fflush(pfout);
 }

 /**
  * save rgb24 frame [PPM]
  */
 void rgb24_save(AVFrame *pFrame, AVCodecContext *pCodecCtx)
 {
     int i = 0;
     int width = pCodecCtx->width, height = pCodecCtx->height;

     /* write PPM header */
     fprintf(pfout, "P6n%d %dn255n", width, height);

     /* write pixel data */
     for(i =0; i < height; i++)
         fwrite(pFrame->data[0] + i * pFrame->linesize[0], 1, width * 3, pfout);

     fflush(pfout);
 }

 /**
  * save yuv444p frame [YUV]
  */
 void yuv444p_save(AVFrame *pFrame, AVCodecContext *pCodecCtx)
 {
     int i = 0;

     int width = pCodecCtx->width, height = pCodecCtx->height;
     int y_wrap = pFrame->linesize[0];
     int u_wrap = pFrame->linesize[1];
     int v_wrap = pFrame->linesize[2];

     unsigned char *y_buf = pFrame->data[0];
     unsigned char *u_buf = pFrame->data[1];
     unsigned char *v_buf = pFrame->data[2];

     //save y
     for (i = 0; i < height; i++)
         fwrite(y_buf + i * y_wrap, 1, width, pfout);
     fprintf(stderr, "===>save Y successn");
     //save u
     for (i = 0; i < height; i++)
         fwrite(u_buf + i * u_wrap, 1, width, pfout);
     fprintf(stderr, "===>save U successn");
     //save v
     for (i = 0; i < height; i++)
         fwrite(v_buf + i * v_wrap, 1, width, pfout);
     fprintf(stderr, "===>save V successn");

     fflush(pfout);
 }

     可以对照各种格式看一下代码，应该是很好理解的，下面是其余的main代码，可以编译运行

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 /**
  * decode picture by ffmpeg-1.0 for jpg and png ...
  *
  * 2013-05-14
  *    juguofeng<jgfntu@gmail.com>
  */

 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>

 #include <libavcodec/avcodec.h>
 #include <libavformat/avformat.h>


 FILE *pfout = NULL;
 char ffrvout[128] = { 0 };

 /* how many yuv pic you want to save */
 #define FRAME_NUM 1
 /* enable video demux data save to file */
 //#define ENABLE_DEMUX_SAVE
 /* enable yuv pic save to file */
 #define ENABLE_YUV_SAVE
 /* enable print each video bytes */
 #define ENABLE_PRINT_FRAME_BYTES
 /* how many bytes you want to print */
 #define PRINT_BYTES 30


 /**
  * main thread
  */
 int main(int argc, char *argv[])
 {
     int i;
     char szFileName[128] = {0};
     int decLen = 0;
     int frame = 0;

     AVCodecContext *pCodecCtx = NULL;
     AVFrame *pFrame = NULL;
     AVCodec *pCodec = NULL;
     AVFormatContext *pFormatCtx = NULL;

     if(argc != 3)
     {
         fprintf(stderr, "ERROR:need 3 argument!n");
         exit(-1);
     }

     sprintf(szFileName, "%s", argv[1]);

 #ifdef ENABLE_DEMUX_SAVE
     FILE* frvdemux = fopen("rvdemuxout.rm","wb+");
     if (NULL == frvdemux)
     {
         fprintf(stderr, "create rvdemuxout file failedn");
         exit(1);
     }
 #endif

     /* output yuv file name */
     sprintf(ffrvout, "%s", argv[2]);

     pfout = fopen(ffrvout, "wb+");
     if (NULL == pfout)
     {
         printf("create output file failedn");
         exit(1);
     }
     printf("==========> Begin test ffmpeg call ffmpeg rv decodern");
     av_register_all();

     /* Open input video file */
     //printf("before avformat_open_input [%s]n", szFileName);
     if(avformat_open_input(&pFormatCtx, szFileName, NULL, NULL)!= 0)
     {
         fprintf(stderr, "Couldn't open input filen");
         return -1;
     }
     //printf("after avformat_open_inputn");

     /* Retrieve stream information */
     if(av_find_stream_info(pFormatCtx) < 0)
     {
         printf("av_find_stream_info ERRORn");
         return -1;
     }
     //printf("after av_find_stream_info, n");


     /* Find the first video stream */
     int videoStream = -1;
     printf("==========> pFormatCtx->nb_streams = %dn", pFormatCtx->nb_streams);

     for(i = 0; i < pFormatCtx->nb_streams; i++) {
         if(pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
             videoStream = i;
             printf("the first video stream index: videoStream = %dn",videoStream);
             break;
         }
     }

     if(videoStream == -1)
         return -1;        // Didn't find a video stream

     /* Get a pointer to the codec context for the video stream */
     pCodecCtx = pFormatCtx->streams[videoStream]->codec;
     printf("pCodecCtx->codec_id = %dn", pCodecCtx->codec_id);

     pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
     if(pCodec == NULL) {
         fprintf(stderr, "can not find decoder!n");
         return -1;
     }

     /* Open codec */
     if(avcodec_open(pCodecCtx, pCodec)<0)
     {
         printf("cannot open software codecn");
         return -1; // Could not open codec
     }
     printf("==========> Open software codec successn");

     pFrame = avcodec_alloc_frame();
     if(pFrame == NULL)
     {
         fprintf(stderr, "avcodec_alloc_frame() ERRORn");
         return -1;
     }

     /* flag whether we get a decoded yuv frame */
     int frameFinished;
     int packetno = 0;

     AVPacket packet;
     av_init_packet(&packet);

     while(av_read_frame(pFormatCtx, &packet) >= 0) {
         //printf("[main]avpkt->slice_count=%dn", packet.sliceNum);

         /* Is this a packet from the video stream? */
         if(packet.stream_index == videoStream) {
             packetno++;
 #ifdef ENABLE_PRINT_FRAME_BYTES
         if ( 1 ) {
             int i;
             int size = packet.size < PRINT_BYTES ? packet.size : PRINT_BYTES;
             unsigned char *data = packet.data;
             printf("===>[%5d] [", packet.size);
             for (i = 0; i < size; i++)
                 printf("%02x ", data[i]);
             printf("]n");
         }
 #endif
 #ifdef ENABLE_DEMUX_SAVE
             fwrite(packet.data, 1, packet.size, frvdemux);
 #endif
             //printf("[the %d packet]packet.size = %dn", packetno++, packet.size);

             while (packet.size > 0) {
                 decLen = avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
                 //printf("[video_decode_example]after avcodec_decode_video2,decoded=%dn",decLen);

                 if (decLen < 0)    {
                     fprintf(stderr, "[video_decode_example]Error while decoding frame %dn", frame);
                     //exit(1);
                     /* FIXME if decode one frame err, ignore this frame */
                     decLen = packet.size;
                 }

                 if (frameFinished) {
                     printf("got a yuv framen");
                     //printf(stderr, "[video_decode_example]saving frame %3dn", frame);

                     /* the picture is allocated by the decoder. no need to free it */
                     if (frame == 0) {
                         printf("[video_decode_example]picture->linesize[0]=%d, c->width=%d,c->height=%dn",
                                 pFrame->linesize[0], pCodecCtx->width, pCodecCtx->height);
                         printf("===>YUV format = %dn", pFrame->format);
                     }
 #ifdef ENABLE_YUV_SAVE
                     /* save yuv pic */
                     if (frame < FRAME_NUM) {
                         switch (pFrame->format) {
                             case 0 :    /* YUV420P */
                                 yuv420p_save(pFrame, pCodecCtx);
                                 break;
                             case 2 :    /* RGB24 */
                                 rgb24_save(pFrame, pCodecCtx);
                                 break;
                             case 13 :    /* YUVJ422P */
                                 yuv422p_save(pFrame, pCodecCtx);
                                 break;
                             case 14 :    /* YUVJ444P */
                                 yuv444p_save(pFrame, pCodecCtx);
                                 break;
                             default :
                                 fprintf(stderr, "unsupport YUV format for savingn");
                                 break;
                         }
                         fprintf(stderr, "===>save pic successn");
                     }
 #endif
                     /* frame index grow */
                     frame++;
                 }
                 //printf("===========> %dn", decLen);
                 /* left data in pkt , go on decoding */
                 packet.data += decLen;
                 packet.size -= decLen;
             }
             if (frame == FRAME_NUM) {
                 printf("==========> decoded [%d pkt frames] ---> save [%d YUV frames], enough to stop!n", packetno, FRAME_NUM);
                 break;
             }
         }

         /* FIXME no need free in this file */
         //printf("free packet that was allocated by av_read_framen");
         // Free the packet that was allocated by av_read_frame
         //av_free_packet(&packet);
     }

     printf("decoding job down! begin to freen");
     /* Free the YUV frame */
     av_free(pFrame);

     /* Close the codec */
     avcodec_close(pCodecCtx);

     /* Close the video file */
     av_close_input_file(pFormatCtx);
     fclose(pfout);

     printf("==========> END-OKn");

     return 0;
 }

     最后是Makefile文件

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 # use pkg-config for getting CFLAGS abd LDFLAGS
 FFMPEG_LIBS=libavdevice libavformat libavfilter libavcodec libswscale libavutil
 CFLAGS+=$(shell pkg-config --cflags $(FFMPEG_LIBS))
 LDFLAGS+=$(shell pkg-config --libs $(FFMPEG_LIBS))

 EXAMPLES=pic_dec

 OBJS=$(addsuffix .o,$(EXAMPLES))

 %: %.o
     $(CC) $< $(LDFLAGS) -o $@

 %.o: %.c
     $(CC) $< $(CFLAGS) -c -o $@

 .phony: all clean

 all: $(OBJS) $(EXAMPLES)

 clean:
     rm -rf $(EXAMPLES) $(OBJS)

     注意如果是自己编译的ffmpeg-1.0等版本，安装到例如/usr/local/目录的话，需要在环境变量中设置
     PKG_CONFIG_PATH和LD_LIBRARY_PATH，指定到/usr/local/lib/pkgconfig和/usr/local/lib/目录（如果以后要利用你的PC来交叉编译如VLC等开源代码，最好将这两个变量注释掉，因为交叉编译时的configure脚本会根据
这个配置错误的检查到PC也就是X86结构的lib，这个显然是不对的，会让VLC模块错误的认为你的机子上有了一些第三方的库，但VLC并不知道这是X86结构的）