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16Byte
struct bmp_fileheader
{unsigned short    bfType;        //若不对齐，这个会占4Byteunsigned 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;elsecinfo.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;
}