Jpeglib读取jpg文件

整理自：

http://hi.baidu.com/lewutian/item/e8eed42664ee61122a0f1c89

http://blog.csdn.net/mcgrady_tracy/article/details/7439066

1.下载编译库

下载库：

http://www.ijg.org/ 网址下面的windows版本的。本文下载的是jpegsr9a.zip。

编译库：

libjpeg，以vs2008为例, 首先要把jconfig.vc复制为jconfig.h。

打开VS2008的command line prompt命令行提示符，切换到解压的libjpeg目录下，输入"nmake -f makefile.vc"

完成后取出libjpeg.lib就行了

2.项目中引入库

将编译出来的lib目录和lib名称配置到VC的linker中，头文件引入就可以了。

3.实例代码（Jpeglib读取图像函数说明见代码注释）

//// JEPGFile.cpp : Defines the entry point for the console application.
////
//
#include "stdafx.h"
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "jpeglib.h"

#define PUT_2B(array,offset,value)
         (array[offset] = (char) ((value) & 0xFF),
          array[offset+1] = (char) (((value) >> 8) & 0xFF))
 #define PUT_4B(array,offset,value)
         (array[offset] = (char) ((value) & 0xFF),
          array[offset+1] = (char) (((value) >> 8) & 0xFF),
          array[offset+2] = (char) (((value) >> 16) & 0xFF),
          array[offset+3] = (char) (((value) >> 24) & 0xFF))

 void write_bmp_header(j_decompress_ptr cinfo, FILE *output_file)
 {
         //cinfo已经转换为小端模式了
         char bmpfileheader[14];
         char bmpinfoheader[40];
         long headersize, bfSize;
         int bits_per_pixel, cmap_entries;


         int step;

         /* Compute colormap size and total file size */
         if (cinfo->out_color_space == JCS_RGB) {
                 if (cinfo->quantize_colors) {
                         /* Colormapped RGB */
                         bits_per_pixel = 8;
                         cmap_entries = 256;
                 } else {
                         /* Unquantized, full color RGB */
                         bits_per_pixel = 24;
                         cmap_entries = 0;
                 }
         } else {
                 /* Grayscale output.  We need to fake a 256-entry colormap. */
                 bits_per_pixel = 8;
                 cmap_entries = 256;
         }

         step = cinfo->output_width * cinfo->output_components;

         while ((step & 3) != 0) step++;

         /* File size */
         headersize = 14 + 40 + cmap_entries * 4; /* Header and colormap */

        bfSize = headersize + (long) step * (long) cinfo->output_height;

         /* Set unused fields of header to 0 */
         memset(bmpfileheader, 0, sizeof(bmpfileheader));
         memset(bmpinfoheader, 0 ,sizeof(bmpinfoheader));

         /* Fill the file header */
         bmpfileheader[0] = 0x42;/* first 2 bytes are ASCII 'B', 'M' */
         bmpfileheader[1] = 0x4D;
         PUT_4B(bmpfileheader, 2, bfSize); /* bfSize */
         /* we leave bfReserved1 & bfReserved2 = 0 */
         PUT_4B(bmpfileheader, 10, headersize); /* bfOffBits */

         /* Fill the info header (Microsoft calls this a BITMAPINFOHEADER) */
         PUT_2B(bmpinfoheader, 0, 40);   /* biSize */
         PUT_4B(bmpinfoheader, 4, cinfo->output_width); /* biWidth */
         PUT_4B(bmpinfoheader, 8, cinfo->output_height); /* biHeight */
         PUT_2B(bmpinfoheader, 12, 1);   /* biPlanes - must be 1 */
         PUT_2B(bmpinfoheader, 14, bits_per_pixel); /* biBitCount */
         /* we leave biCompression = 0, for none */
         /* we leave biSizeImage = 0; this is correct for uncompressed data */
         if (cinfo->density_unit == 2) { /* if have density in dots/cm, then */
                 PUT_4B(bmpinfoheader, 24, (INT32) (cinfo->X_density*100)); /* XPels/M */
                 PUT_4B(bmpinfoheader, 28, (INT32) (cinfo->Y_density*100)); /* XPels/M */
         }
         PUT_2B(bmpinfoheader, 32, cmap_entries); /* biClrUsed */
         /* we leave biClrImportant = 0 */

         if (fwrite(bmpfileheader, 1, 14, output_file) != (size_t) 14) {
                 printf("write bmpfileheader error
");
         }
         if (fwrite(bmpinfoheader, 1, 40, output_file) != (size_t) 40) {
                 printf("write bmpinfoheader error
");
         }

         if (cmap_entries > 0) {
         }
 }

 void write_pixel_data(j_decompress_ptr cinfo, unsigned char *output_buffer, FILE *output_file)
 {
         int rows, cols;
         int row_width;
         int step;
         unsigned char *tmp = NULL;

        unsigned char *pdata;

         row_width = cinfo->output_width * cinfo->output_components;
         step = row_width;
         while ((step & 3) != 0) step++;

         pdata = (unsigned char *)malloc(step);
         memset(pdata, 0, step);

         // JPEG左上角的开始一行行写的数据，转换为BMP的从左下角开始一行行写的数据
         // 也就是JPEG最末的行放置到BMP最开始行，JPEG最开始行放置到BMP最末行就对了。
         tmp = output_buffer + row_width * (cinfo->output_height - 1);
         for (rows = 0; rows < cinfo->output_height; rows++) {
                 for (cols = 0; cols < row_width; cols += 3) {
                        // 大端模式转换为小端模式
                         pdata[cols + 2] = tmp[cols + 0];
                         pdata[cols + 1] = tmp[cols + 1];
                         pdata[cols + 0] = tmp[cols + 2];
                 }
                 tmp -= row_width;
                 fwrite(pdata, 1, step, output_file);
         }

         free(pdata);
 }


 /*读JPEG文件相当于解压文件*/

 int read_jpeg_file(const char *input_filename, const char *output_filename)
 {
         struct jpeg_decompress_struct cinfo;
         struct jpeg_error_mgr jerr;
         FILE *input_file;
         FILE *output_file;
         JSAMPARRAY buffer;
         int row_width;

         unsigned char *output_buffer;
         unsigned char *tmp = NULL;

         cinfo.err = jpeg_std_error(&jerr);

         if ((input_file = fopen(input_filename, "rb")) == NULL) {
                 fprintf(stderr, "can't open %s
", input_filename);
                 return -1;
         }

         if ((output_file = fopen(output_filename, "wb")) == NULL) {

                fprintf(stderr, "can't open %s
", output_filename);
                 return -1;
         }

         // Initialization of JPEG compression objects
         jpeg_create_decompress(&cinfo);

         /* Specify data source for decompression */
         jpeg_stdio_src(&cinfo, input_file);

         /* 1.设置读取jpg文件头部，Read file header, set default decompression parameters */
         (void) jpeg_read_header(&cinfo, TRUE);

         /* 2.开始解码数据 Start decompressor */
         (void) jpeg_start_decompress(&cinfo);

         row_width = cinfo.output_width * cinfo.output_components;

         /* 3.跳过读取的头文件字节Make a one-row-high sample array that will go away when done with image */
         buffer = (*cinfo.mem->alloc_sarray)
                 ((j_common_ptr) &cinfo, JPOOL_IMAGE, row_width, 1);

         write_bmp_header(&cinfo, output_file);

         output_buffer = (unsigned char *)malloc(row_width * cinfo.output_height);
         memset(output_buffer, 0, row_width * cinfo.output_height);
         tmp = output_buffer;

         /* 4.Process data由左上角从上到下行行扫描 */
         while (cinfo.output_scanline < cinfo.output_height) {
                 (void) jpeg_read_scanlines(&cinfo, buffer, 1);

                 memcpy(tmp, *buffer, row_width);
                 tmp += row_width;
         }

        // 写入数据，注意大小端转换和图像数据坐标表示差异在内存中的顺序
         write_pixel_data(&cinfo, output_buffer, output_file);

         free(output_buffer);

         (void) jpeg_finish_decompress(&cinfo);

         jpeg_destroy_decompress(&cinfo);

         /* Close files, if we opened them */
         fclose(input_file);
         fclose(output_file);

        return 0;
 }


int write_jpeg_file(char * filename, int quality)
{

  struct jpeg_compress_struct cinfo;
  unsigned char  * image_buffer;
  int i = 0;
  struct jpeg_error_mgr jerr;
  /* More stuff */
  FILE * outfile;  /* target file */
  JSAMPROW row_pointer[1]; /* pointer to JSAMPLE row[s] */
  int row_stride;  /* physical row width in image buffer */

  /* Step 1: allocate and initialize JPEG compression object */

  /* We have to set up the error handler first, in case the initialization
   * step fails.  (Unlikely, but it could happen if you are out of memory.)
   * This routine fills in the contents of struct jerr, and returns jerr's
   * address which we place into the link field in cinfo.
   */
   /*1.第一步创建jpeg compress 对象*/
  cinfo.err = jpeg_std_error(&jerr);
  /* Now we can initialize the JPEG compression object. */
  jpeg_create_compress(&cinfo);

  /* Step 2: specify data destination (eg, a file) */
  /* Note: steps 2 and 3 can be done in either order. */

  /* Here we use the library-supplied code to send compressed data to a
   * stdio stream. You can also write your own code to do something else.
   * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
   * requires it in order to write binary files.
   */
   /*写的方式打开文件*/
  if ((outfile = fopen(filename, "wb")) == NULL) {
 fprintf(stderr, "can't open %s
", filename);
 exit(1);
  }
  jpeg_stdio_dest(&cinfo, outfile);

  /* Step 3: set parameters for compression */

  /* First we supply a description of the input image.
   * Four fields of the cinfo struct must be filled in:
   */
   /*2.设置 压缩参数 libjpeg中的宽度和高度是两个全局的
  我这默认设置成640 480。根据demo中的说明color_space必须
  得设置*/
  cinfo.image_width = 640; /* image width and height, in pixels */
  cinfo.image_height = 480;
  cinfo.input_components = 3;  /* # of color components per pixel */
  cinfo.in_color_space = JCS_RGB; /* colorspace of input image */
  /* Now use the library's routine to set default compression parameters.
   * (You must set at least cinfo.in_color_space before calling this,
   * since the defaults depend on the source color space.)
   */
  jpeg_set_defaults(&cinfo);
  /* Now you can set any non-default parameters you wish to.
   * Here we just illustrate the use of quality (quantization table) scaling:
   */
   /*设置quality为2*/
  jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);

  /* Step 4: Start compressor */

  /* TRUE ensures that we will write a complete interchange-JPEG file.
   * Pass TRUE unless you are very sure of what you're doing.
   */
   /*3. 开始压缩*/
  jpeg_start_compress(&cinfo, TRUE);

  /* Step 5: while (scan lines remain to be written) */
  /*     jpeg_write_scanlines(...); */

  /* Here we use the library's state variable cinfo.next_scanline as the
   * loop counter, so that we don't have to keep track ourselves.
   * To keep things simple, we pass one scanline per call; you can pass
   * more if you wish, though.
   */
  row_stride = 640 * 3; /* JSAMPLEs per row in image_buffer */
 image_buffer = (unsigned char*)malloc(640*480*3);

 if (NULL == image_buffer)
 {
  return -1;
 }
 for(i=0; i< 640*480; i++)
   {
     // 4.构造的数据,数据是RGB形式Pixel
     image_buffer[i*3] = 255/*i*255*/;
     image_buffer[i*3+1] = 255/*128-(i*255)&0x7f*/;
     image_buffer[i*3+2] = 0/*255-(i*255)&0xff*/;
   }

  while (cinfo.next_scanline < cinfo.image_height) {
 /* jpeg_write_scanlines expects an array of pointers to scanlines.
  * Here the array is only one element long, but you could pass
  * more than one scanline at a time if that's more convenient.
  */
 // 5.将数据扫描输入,image_buffer数据是RGB形式Pixel
 row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
 (void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
  }

  /* Step 6: Finish compression */
  // 6.完成压缩
  jpeg_finish_compress(&cinfo);
  /* After finish_compress, we can close the output file. */
  fclose(outfile);

  /* Step 7: release JPEG compression object */

  /* This is an important step since it will release a good deal of memory. */
  jpeg_destroy_compress(&cinfo);

  /* And we're done! */
}


 int main(int argc, char *argv[])
 {
     read_jpeg_file("f:\data\animal.jpg", "f:\data\animal.bmp");
     write_jpeg_file("f:\data\createJpg.jpg", 2);

     return 0;
 }