2026/08/13 JPEG图像

一、JPEG图像原理与应用

基本概念

JPEG（Joint Photographic Experts Group）指的是联合图像专家组，是国际标准化组织ISO制订并于1992年发布的一种面向连续色调静止图像的压缩编码标准，所以也被称为JPEG标准。

同样，JPEG也是一种常用的图像存储格式，JPEG的文件格式有两种文件扩展名：.jpg和.jpeg，这两种扩展名是相同的，我们可以把.jpg的文件改名为.jpeg，而对文件本身不会有任何影响。

和其他相同图像质量的文件格式相比，JPEG格式的压缩比是最高的，也就是说在图像质量相差不大的情况下，JPEG文件所占的内存更小。所以JPEG图像一般适合进行网络传输。

编解码库

由于JPEG格式的文件是经过压缩的，所以用户无法向访问BMP位图一样直接读取JPEG文件中的内容，而是需要使用对应的libjpeg解码库对JPEG文件进行解码，从而得到解码之后的RGB颜色分量，然后把解码之后的RGB颜色分量写入到LCD屏即可。

libjpeg是一款开源的JPEG图像库，可以用于编码JPEG文件格式或者用于解码JPEG文件格式，这套库是由IJG（Independent JPEG Group）独立小组进行维护与发布，libjpeg库完全利用C语言设计并且内部集成各种用于编解码算法的函数接口，广泛用于各种图像处理，比如OpenCV（开源的跨平台计算机视觉库）读取图像的底层实现逻辑就是基于libjpeg库的。

库的使用



为了可以把一张jpg图片显示在LCD上，所以需要把jpg图片进行解压，解压之后就可以得到图片内部的像素点的颜色分量，就可以把像素点的颜色分量向LCD的像素点写入。就需要完全掌握jpg图片的解压流程。

解码流程
(1)创建解码对象，并且对解码对象进行初始化，另外需要创建错误处理对象，并和解码对象进行关联。

(2)打开待解码的jpg图片，使用fopen的时候需要添加选项”b”，以二进制方式打开文件！

(3)读取待解码图片的文件头，并把图像信息和解码对象进行关联，通过解码对象对jpg图片进行解码

(4)可以选择设置解码参数，如果打算以默认参数对jpg图片进行解码，则可以省略该步骤！

(5)开始对jpg图片进行解码，调用函数之后开始解码，可以得到图像宽、图像高等信息！

(6)开始设计一个循环，在循环中每次读取1行的图像数据，并写入到LCD中，注意：转换算法需要用户自己设计。

(7)在所有的图像数据都已经解码完成后，则调用函数完成解码即可，然后释放相关资源！

程序设计
1.完成libjpeg库的移植，并设计程序实现在LCD上的任意位置显示一张任意大小的jpg图片，注意不要越界。

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
/*
 * Include file for users of JPEG library.
 * You will need to have included system headers that define at least
 * the typedefs FILE and size_t before you can include jpeglib.h.
 * (stdio.h is sufficient on ANSI-conforming systems.)
 * You may also wish to include "jerror.h".
 */

#include "jpeglib.h"


int * lcd_mp;


//成功返回1 失败返回0
int read_JPEG_file (char * filename,int start_x,int start_y)
{
  /* This struct contains the JPEG decompression parameters and pointers to
   * working space (which is allocated as needed by the JPEG library).
   */
  struct jpeg_decompress_struct cinfo;
  /* We use our private extension JPEG error handler.
   * Note that this struct must live as long as the main JPEG parameter
   * struct, to avoid dangling-pointer problems.
   */
  struct jpeg_error_mgr jerr;
  /* More stuff */
  FILE * infile;			/* source file */
  unsigned char * buffer;		/* Output row buffer */
  int row_stride;			/* physical row width in output buffer */

  /* In this example we want to open the input file before doing anything else,
   * so that the setjmp() error recovery below can assume the file is open.
   * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
   * requires it in order to read binary files.
   */

  if ((infile = fopen(filename, "rb")) == NULL) {
    fprintf(stderr, "can't open %s\n", filename);
    return 0;
  }

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

  /* We set up the normal JPEG error routines, then override error_exit. */
  cinfo.err = jpeg_std_error(&jerr);

  /* Now we can initialize the JPEG decompression object. */
  jpeg_create_decompress(&cinfo);

  /* Step 2: specify data source (eg, a file) */

  jpeg_stdio_src(&cinfo, infile);

  /* Step 3: read file parameters with jpeg_read_header() */

  (void) jpeg_read_header(&cinfo, TRUE);
  /* We can ignore the return value from jpeg_read_header since
   *   (a) suspension is not possible with the stdio data source, and
   *   (b) we passed TRUE to reject a tables-only JPEG file as an error.
   * See libjpeg.txt for more info.
   */

  /* Step 4: set parameters for decompression */

  /* In this example, we don't need to change any of the defaults set by
   * jpeg_read_header(), so we do nothing here.
   */

  /* Step 5: Start decompressor */

  (void) jpeg_start_decompress(&cinfo);
  /* We can ignore the return value since suspension is not possible
   * with the stdio data source.
   */

  /* We may need to do some setup of our own at this point before reading
   * the data.  After jpeg_start_decompress() we have the correct scaled
   * output image dimensions available, as well as the output colormap
   * if we asked for color quantization.
   * In this example, we need to make an output work buffer of the right size.
   */ 
  /* JSAMPLEs per row in output buffer */
  row_stride = cinfo.output_width * cinfo.output_components;  //计算一行的大小

  /* Make a one-row-high sample array that will go away when done with image */
  buffer = calloc(1,row_stride);

  /* Step 6: while (scan lines remain to be read) */
  /*           jpeg_read_scanlines(...); */

  /* Here we use the library's state variable cinfo.output_scanline as the
   * loop counter, so that we don't have to keep track ourselves.
   */
  int data = 0;

  while (cinfo.output_scanline < cinfo.output_height) 
  {
    /* jpeg_read_scanlines expects an array of pointers to scanlines.
     * Here the array is only one element long, but you could ask for
     * more than one scanline at a time if that's more convenient.
     */
    (void) jpeg_read_scanlines(&cinfo, &buffer, 1); //从上到下，从左到右  RGB RGB RGB RGB 
  	
  	for (int i = 0; i < cinfo.output_width; ++i)  //012 345
  	{
  		data |= buffer[3*i]<<16;	//R
		data |= buffer[3*i+1]<<8;	//G
		data |= buffer[3*i+2];  	//B 

		//把像素点写入到LCD的指定位置
		lcd_mp[800*start_y + start_x + 800*(cinfo.output_scanline-1) + i] = data;

		data = 0;
  	}
  
  }

  /* Step 7: Finish decompression */

  (void) jpeg_finish_decompress(&cinfo);
  /* We can ignore the return value since suspension is not possible
   * with the stdio data source.
   */

  /* Step 8: Release JPEG decompression object */

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

  /* After finish_decompress, we can close the input file.
   * Here we postpone it until after no more JPEG errors are possible,
   * so as to simplify the setjmp error logic above.  (Actually, I don't
   * think that jpeg_destroy can do an error exit, but why assume anything...)
   */
  fclose(infile);

  /* At this point you may want to check to see whether any corrupt-data
   * warnings occurred (test whether jerr.pub.num_warnings is nonzero).
   */

  /* And we're done! */
  return 1;
}


int main(int argc, char const *argv[])
{
	//1.打开LCD   open  
	int lcd_fd = open("/dev/fb0",O_RDWR);

	//2.对LCD进行内存映射  mmap
	lcd_mp = (int *)mmap(NULL,800*480*4,PROT_READ|PROT_WRITE,MAP_SHARED,lcd_fd,0);

	//3.显示一张jpg
	read_JPEG_file ("demo.jpg",0,0);

	return 0;
}

2.完成libjpeg库的移植，并设计程序实现把bmp图片转换为jpg图片，学***g图片的压缩算法。