lib jpeg举例

/* * example.c * * This file illustrates how to use the IJG code as a subroutine library * to read or write JPEG image files.   * This code will not do anything useful as-is, but it may be helpful as a * skeleton for constructing routines that call the JPEG library.   */#include <stdio.h>/* * JPEG库文件. * 包括在包含jpeglib.h库之前所需要的系统头文件. * (stdio.h支持在 ANSI-conforming 系统中.) * 同时还要包含 "jerror.h". */#include "jpeglib.h"/* * <setjmp.h> 用于可选的错误恢复机制，该机制在例子的第二部分有. */#include <setjmp.h>/******************** JPEG COMPRESSION SAMPLE INTERFACE *******************//* 该部分指出如何向JPEG压缩器中填充数据. * We present a minimal version that does not worry about refinements such * as error recovery (the JPEG code will just exit() if it gets an error). *//* * IMAGE DATA FORMATS: * * 标准输入图像是矩形阵列的像素，每个像素有相同的元素组成（颜色通道）. * 每行像素是一个数组 SAMPLEs (类型为 unsigned chars). * 对于一行中的每个像素，各颜色数据值是相邻的，例如，24bit的RGB色彩： R,G,B,R,G,B,R,G,B,...  * * 例如，我们假定这些数据结构与我们在内存中存储的图像相匹配，因此可传递一个指针，指向图像缓存区.  * 特别的，我们说图像是RGB色彩，通过一下方式描述： */extern JSAMPLE * image_buffer;/* Points to large array of R,G,B-order data */extern int image_height;/* Number of rows in image */extern int image_width;/* Number of columns in image *//* * JPEG 压缩器，目标文件名和压缩质量因子 */GLOBAL(void)write_JPEG_file (char * filename, int quality){  /* 该结构体包含JPEG压缩参数和指针.结构体、多个压缩/解压缩过程，指定任何一个结构体为一个“JPEG对象”*/

  struct jpeg_compress_struct cinfo;

  /* 该结构描述一个JPEG错误句柄。如果压缩失败会打印 stderr 信息并调用 exit() <span style="font-family: Arial, Helvetica, sans-serif;">  */</span>

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  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 */  /* 第1步：分配和初始化JPEG压缩对象*/  /* 先设置错误句柄，以防初始化失败。该程序会填充jerr内容并返回jerr's的地址，它存放则会cinfo的链接地址。 */  cinfo.err = jpeg_std_error(&jerr);  /* Now we can initialize the JPEG compression object. */  jpeg_create_compress(&cinfo);  /* 第2步：知道数据的目标文件 */  /* 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*/.     if ((outfile = fopen(filename, "wb")) == NULL) {    fprintf(stderr, "can't open %s\n", filename);    exit(1);  }  jpeg_stdio_dest(&cinfo, outfile);  /* 第3步：对压缩器设置参数 */  /* 首先应支持输入图像的描述，应包含4个cinfo结构体的区域*/     cinfo.image_width = image_width; /* image width and height, in pixels */  cinfo.image_height = image_height;  cinfo.input_components = 3;/* # of color components per pixel */  cinfo.in_color_space = JCS_RGB; /* colorspace of input image */  /* 设置默认的压缩参数，调用前至少包括cinfo.in_color_space，因为默认的包括源颜色空间*/     jpeg_set_defaults(&cinfo);<pre name="code" class="html">  

   /* 设置你期望的菲默认参数。*/

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.   */  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 = image_width * 3;/* JSAMPLEs per row in image_buffer */  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.     */    row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];    (void) jpeg_write_scanlines(&cinfo, row_pointer, 1);  }  /* Step 6: Finish compression */  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! */}/* * SOME FINE POINTS: * * In the above loop, we ignored the return value of jpeg_write_scanlines, * which is the number of scanlines actually written.  We could get away * with this because we were only relying on the value of cinfo.next_scanline, * which will be incremented correctly.  If you maintain additional loop * variables then you should be careful to increment them properly. * Actually, for output to a stdio stream you needn't worry, because * then jpeg_write_scanlines will write all the lines passed (or else exit * with a fatal error).  Partial writes can only occur if you use a data * destination module that can demand suspension of the compressor. * (If you don't know what that's for, you don't need it.) * * If the compressor requires full-image buffers (for entropy-coding * optimization or a multi-scan JPEG file), it will create temporary * files for anything that doesn't fit within the maximum-memory setting. * (Note that temp files are NOT needed if you use the default parameters.) * On some systems you may need to set up a signal handler to ensure that * temporary files are deleted if the program is interrupted.  See libjpeg.txt. * * Scanlines MUST be supplied in top-to-bottom order if you want your JPEG * files to be compatible with everyone else's.  If you cannot readily read * your data in that order, you'll need an intermediate array to hold the * image.  See rdtarga.c or rdbmp.c for examples of handling bottom-to-top * source data using the JPEG code's internal virtual-array mechanisms. *//******************** JPEG DECOMPRESSION SAMPLE INTERFACE *******************//* This half of the example shows how to read data from the JPEG decompressor. * It's a bit more refined than the above, in that we show: *   (a) how to modify the JPEG library's standard error-reporting behavior; *   (b) how to allocate workspace using the library's memory manager. * * Just to make this example a little different from the first one, we'll * assume that we do not intend to put the whole image into an in-memory * buffer, but to send it line-by-line someplace else.  We need a one- * scanline-high JSAMPLE array as a work buffer, and we will let the JPEG * memory manager allocate it for us.  This approach is actually quite useful * because we don't need to remember to deallocate the buffer separately: it * will go away automatically when the JPEG object is cleaned up. *//* * ERROR HANDLING: * * The JPEG library's standard error handler (jerror.c) is divided into * several "methods" which you can override individually.  This lets you * adjust the behavior without duplicating a lot of code, which you might * have to update with each future release. * * Our example here shows how to override the "error_exit" method so that * control is returned to the library's caller when a fatal error occurs, * rather than calling exit() as the standard error_exit method does. * * We use C's setjmp/longjmp facility to return control.  This means that the * routine which calls the JPEG library must first execute a setjmp() call to * establish the return point.  We want the replacement error_exit to do a * longjmp().  But we need to make the setjmp buffer accessible to the * error_exit routine.  To do this, we make a private extension of the * standard JPEG error handler object.  (If we were using C++, we'd say we * were making a subclass of the regular error handler.) * * Here's the extended error handler struct: */struct my_error_mgr {  struct jpeg_error_mgr pub;/* "public" fields */  jmp_buf setjmp_buffer;/* for return to caller */};typedef struct my_error_mgr * my_error_ptr;/* * Here's the routine that will replace the standard error_exit method: */METHODDEF(void)my_error_exit (j_common_ptr cinfo){  /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */  my_error_ptr myerr = (my_error_ptr) cinfo->err;  /* Always display the message. */  /* We could postpone this until after returning, if we chose. */  (*cinfo->err->output_message) (cinfo);  /* Return control to the setjmp point */  longjmp(myerr->setjmp_buffer, 1);}/* * Sample routine for JPEG decompression.  We assume that the source file name * is passed in.  We want to return 1 on success, 0 on error. */GLOBAL(int)read_JPEG_file (char * filename){  /* 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 my_error_mgr jerr;  /* More stuff */  FILE * infile;/* source file */  JSAMPARRAY 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.pub);  jerr.pub.error_exit = my_error_exit;  /* Establish the setjmp return context for my_error_exit to use. */  if (setjmp(jerr.setjmp_buffer)) {    /* If we get here, the JPEG code has signaled an error.     * We need to clean up the JPEG object, close the input file, and return.     */    jpeg_destroy_decompress(&cinfo);    fclose(infile);    return 0;  }  /* 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;  /* 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_stride, 1);  /* 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.   */  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);    /* Assume put_scanline_someplace wants a pointer and sample count. */    put_scanline_someplace(buffer[0], row_stride);  }  /* 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;}/* * SOME FINE POINTS: * * In the above code, we ignored the return value of jpeg_read_scanlines, * which is the number of scanlines actually read.  We could get away with * this because we asked for only one line at a time and we weren't using * a suspending data source.  See libjpeg.txt for more info. * * We cheated a bit by calling alloc_sarray() after jpeg_start_decompress(); * we should have done it beforehand to ensure that the space would be * counted against the JPEG max_memory setting.  In some systems the above * code would risk an out-of-memory error.  However, in general we don't * know the output image dimensions before jpeg_start_decompress(), unless we * call jpeg_calc_output_dimensions().  See libjpeg.txt for more about this. * * Scanlines are returned in the same order as they appear in the JPEG file, * which is standardly top-to-bottom.  If you must emit data bottom-to-top, * you can use one of the virtual arrays provided by the JPEG memory manager * to invert the data.  See wrbmp.c for an example. * * As with compression, some operating modes may require temporary files. * On some systems you may need to set up a signal handler to ensure that * temporary files are deleted if the program is interrupted.  See libjpeg.txt. */