bmp转jpg（使用libjpeg）

　　jpg压缩原理可以参考这篇文章http://hi.baidu.com/tiandsp/item/f5a2dcde6ef1405bd73aae41，我很早以前转的一篇文章。

　　没有使用libjpeg的压缩代码可以看看这篇文章http://hi.baidu.com/tiandsp/item/9b5843c58a3b4474cfd4f841，也是我很早以前转的。

　　这次使用libjpeg库压缩和上一篇的解压正好对应起来，有好多函数名称我都是对称的起的，所以结合起来看效果更好。

　　和上一篇一样，只能处理24位和8位的图像。

　　代码如下：

#include <iostream>#include <stdio.h>extern "C"{#include "jpeglib.h"};#pragma comment(lib,"jpeg.lib")using namespace std;#pragma pack(2)        //两字节对齐，否则bmp_fileheader会占16Bytestruct bmp_fileheader{    unsigned short    bfType;        //若不对齐，这个会占4Byte    unsigned long    bfSize;    unsigned short    bfReverved1;    unsigned short    bfReverved2;    unsigned long    bfOffBits;};struct bmp_infoheader{    unsigned long    biSize;    unsigned long    biWidth;    unsigned long    biHeight;    unsigned short    biPlanes;    unsigned short    biBitCount;    unsigned long    biCompression;    unsigned long    biSizeImage;    unsigned long    biXPelsPerMeter;    unsigned long    biYpelsPerMeter;    unsigned long    biClrUsed;    unsigned long    biClrImportant;};FILE *input_file;FILE *output_file;struct bmp_fileheader bfh;struct bmp_infoheader bih;unsigned char *src_buffer;unsigned char *dst_buffer;void read_bmp_header(){        fread(&bfh,sizeof(struct bmp_fileheader),1,input_file);    fread(&bih,sizeof(struct bmp_infoheader),1,input_file);}void read_bmp_data(){    fseek(input_file,bfh.bfOffBits,SEEK_SET);    src_buffer=new unsigned char[bih.biWidth*bih.biHeight*bih.biBitCount/8];    fread(src_buffer,sizeof(unsigned char)*bih.biWidth*bih.biHeight*bih.biBitCount/8,1,input_file);    unsigned long width=bih.biWidth;    unsigned long height=bih.biHeight;    unsigned short depth=unsigned short(bih.biBitCount/8);    unsigned char *src_point;    unsigned char *dst_point;    dst_buffer=new unsigned char[width*height*depth];        src_point=src_buffer+width*depth*(height-1);    dst_point=dst_buffer+width*depth*(height-1);    for (unsigned long i=0;i<height;i++)    {        for (unsigned long j=0;j<width*depth;j+=depth)        {            if (depth==1)        //处理灰度图            {                dst_point[j]=src_point[j];            }            if (depth==3)        //处理彩色图            {                dst_point[j+2]=src_point[j+0];                dst_point[j+1]=src_point[j+1];                dst_point[j+0]=src_point[j+2];            }        }        dst_point-=width*depth;        src_point-=width*depth;    }}void synthese_jpeg(){    struct jpeg_compress_struct cinfo;    struct jpeg_error_mgr jerr;    JSAMPARRAY buffer;    unsigned long width=bih.biWidth;    unsigned long height=bih.biHeight;    unsigned short depth=unsigned short(bih.biBitCount/8);    unsigned char *point;    cinfo.err=jpeg_std_error(&jerr);        //libjpeg各种配置    jpeg_create_compress(&cinfo);    jpeg_stdio_dest(&cinfo,output_file);    cinfo.image_width=width;    cinfo.image_height=height;    cinfo.input_components=depth;    if (depth==1)        cinfo.in_color_space=JCS_GRAYSCALE;    else        cinfo.in_color_space=JCS_RGB;    jpeg_set_defaults(&cinfo);    jpeg_set_quality(&cinfo,20,TRUE);    //中间的值为压缩质量，越大质量越好    jpeg_start_compress(&cinfo,TRUE);    buffer=(*cinfo.mem->alloc_sarray)            ((j_common_ptr)&cinfo,JPOOL_IMAGE,width*depth,1);    point=dst_buffer+width*depth*(height-1);    while (cinfo.next_scanline<height)    {        memcpy(*buffer,point,width*depth);        jpeg_write_scanlines(&cinfo,buffer,1);        point-=width*depth;    }    jpeg_finish_compress(&cinfo);    jpeg_destroy_compress(&cinfo);}int main(){    input_file=fopen("lena_gray.bmp","rb");    output_file=fopen("lena.jpg","wb");    read_bmp_header();    read_bmp_data();    synthese_jpeg();    fclose(input_file);    fclose(output_file);    delete[] src_buffer;    delete[] dst_buffer;    cout<<"good job."<<endl;    cin.get();    return 0;}