ffmpeg解码JPG和PNG等图片
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《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：
/**
* 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 success\n");
//save u
for (i = 0; i < height_half; i++)
fwrite(u_buf + i * u_wrap, 1, width_half, pfout);
fprintf(stderr, "===>save U success\n");
//save v
for (i = 0; i < height_half; i++)
fwrite(v_buf + i * v_wrap, 1, width_half, pfout);
fprintf(stderr, "===>save V success\n");
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 success\n");
//save u
for (i = 0; i < height; i++)
fwrite(u_buf + i * u_wrap, 1, width_half, pfout);
fprintf(stderr, "===>save U success\n");
//save v
for (i = 0; i < height; i++)
fwrite(v_buf + i * v_wrap, 1, width_half, pfout);
fprintf(stderr, "===>save V success\n");
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, "P6\n%d %d\n255\n", 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 success\n");
//save u
for (i = 0; i < height; i++)
fwrite(u_buf + i * u_wrap, 1, width, pfout);
fprintf(stderr, "===>save U success\n");
//save v
for (i = 0; i < height; i++)
fwrite(v_buf + i * v_wrap, 1, width, pfout);
fprintf(stderr, "===>save V success\n");
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/**
/**
* decode picture by ffmpeg-1.0 for jpg and png ...
*
* 2013-05-14
* juguofeng
*/
#include
#include
#include
#include
#include
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 failed\n");
exit(1);
}
#endif
/* output yuv file name */
sprintf(ffrvout, "%s", argv[2]);
pfout = fopen(ffrvout, "wb+");
if (NULL == pfout)
{
printf("create output file failed\n");
exit(1);
}
printf("==========> Begin test ffmpeg call ffmpeg rv decoder\n");
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 file\n");
return -1;
}
//printf("after avformat_open_input\n");
/* Retrieve stream information */
if(av_find_stream_info(pFormatCtx) < 0)
{
printf("av_find_stream_info ERROR\n");
return -1;
}
//printf("after av_find_stream_info, \n");
/* Find the first video stream */
int videoStream = -1;
printf("==========> pFormatCtx->nb_streams = %d\n", 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 = %d\n",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 = %d\n", 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 codec\n");
return -1; // Could not open codec
}
printf("==========> Open software codec success\n");
pFrame = avcodec_alloc_frame();
if(pFrame == NULL)
{
fprintf(stderr, "avcodec_alloc_frame() ERROR\n");
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=%d\n", 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 = %d\n", packetno++, packet.size);
while (packet.size > 0) {
decLen = avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
//printf("[video_decode_example]after avcodec_decode_video2,decoded=%d\n",decLen);
if (decLen < 0) {
fprintf(stderr, "[video_decode_example]Error while decoding frame %d\n", frame);
//exit(1);
/* FIXME if decode one frame err, ignore this frame */
decLen = packet.size;
}
if (frameFinished) {
printf("got a yuv frame\n");
//printf(stderr, "[video_decode_example]saving frame %3d\n", 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=%d\n",
pFrame->linesize[0], pCodecCtx->width, pCodecCtx->height);
printf("===>YUV format = %d\n", 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 saving\n");
break;
}
fprintf(stderr, "===>save pic success\n");
}
#endif
/* frame index grow */
frame++;
}
//printf("===========> %d\n", 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_frame\n");
// Free the packet that was allocated by av_read_frame
//av_free_packet(&packet);
}
printf("decoding job down! begin to free\n");
/* 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-OK\n");
return 0;
}
最后是Makefile文件
# 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）