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libjpeg学习1:简单使用示例
libjpeg这个库主要用于处理jpeg数据,比如将RGB压缩成JPEG,或者将JPEG解压为RGB。其实早在4年前已经接触过,但一直没写过这方面的文章。后来想想还是有必要写出来,至少可以证明自己搞过这东西。
libjpeg使用十分简单,而且源码带有例子程序,下面的代码基本上就是该例子。所以没什么技术含量。
#include <stdio.h> #include <stdlib.h> #include <string.h> #include <setjmp.h> #include <math.h> #include <sys/time.h> #include <time.h> // jpeg库头文件必须放到stdio.h后面 #include "libjpeg/include/jpeglib.h" #include "libjpeg/include/jerror.h" typedef struct my_error_mgr * my_error_ptr; void my_error_exit (j_common_ptr cinfo) { my_error_ptr myerr = (my_error_ptr) cinfo->err; (*cinfo->err->output_message) (cinfo); longjmp(myerr->setjmp_buffer, 1); } // 读取JPG图片数据,并解压到内存中,*rgb_buffer需要自行释放 int read_jpeg_file(const char* jpeg_file, unsigned char** rgb_buffer, int* size, int* width, int* height) { struct jpeg_decompress_struct cinfo; struct my_error_mgr jerr; FILE* fp; JSAMPARRAY buffer; int row_stride = 0; unsigned char* tmp_buffer = NULL; int rgb_size; fp = fopen(jpeg_file, "rb"); if (fp == NULL) { printf("open file %s failed.\n", jpeg_file); return -1; } cinfo.err = jpeg_std_error(&jerr.pub); jerr.pub.error_exit = my_error_exit; if (setjmp(jerr.setjmp_buffer)) { jpeg_destroy_decompress(&cinfo); fclose(fp); return -1; } jpeg_create_decompress(&cinfo); jpeg_stdio_src(&cinfo, fp); jpeg_read_header(&cinfo, TRUE); //cinfo.out_color_space = JCS_RGB; //JCS_YCbCr; // 设置输出格式 jpeg_start_decompress(&cinfo); row_stride = cinfo.output_width * cinfo.output_components; *width = cinfo.output_width; *height = cinfo.output_height; rgb_size = row_stride * cinfo.output_height; // 总大小 *size = rgb_size; buffer = (*cinfo.mem->alloc_sarray)((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1); *rgb_buffer = (unsigned char *)malloc(sizeof(char) * rgb_size); // 分配总内存 printf("debug--:\nrgb_size: %d, size: %d w: %d h: %d row_stride: %d \n", rgb_size, cinfo.image_width*cinfo.image_height*3, cinfo.image_width, cinfo.image_height, row_stride); tmp_buffer = *rgb_buffer; while (cinfo.output_scanline < cinfo.output_height) // 解压每一行 { jpeg_read_scanlines(&cinfo, buffer, 1); // 复制到内存 memcpy(tmp_buffer, buffer[0], row_stride); tmp_buffer += row_stride; } jpeg_finish_decompress(&cinfo); jpeg_destroy_decompress(&cinfo); fclose(fp); return 0; } int write_jpeg_file(const char* jpeg_file, unsigned char* rgb_buffer, int width, int height, int quality) { struct jpeg_compress_struct cinfo; struct jpeg_error_mgr jerr; int row_stride = 0; FILE* fp = NULL; JSAMPROW row_pointer[1]; cinfo.err = jpeg_std_error(&jerr); jpeg_create_compress(&cinfo); fp = fopen(jpeg_file, "wb"); if (fp == NULL) { printf("open file %s failed.\n", jpeg_file); return -1; } jpeg_stdio_dest(&cinfo, fp); cinfo.image_width = width; cinfo.image_height = height; cinfo.input_components = 3; cinfo.in_color_space = JCS_RGB; jpeg_set_defaults(&cinfo); jpeg_set_quality(&cinfo, quality, 1); // todo 1 == true jpeg_start_compress(&cinfo, TRUE); row_stride = width * cinfo.input_components; while (cinfo.next_scanline < cinfo.image_height) { row_pointer[0] = &rgb_buffer[cinfo.next_scanline * row_stride]; jpeg_write_scanlines(&cinfo, row_pointer, 1); } jpeg_finish_compress(&cinfo); jpeg_destroy_compress(&cinfo); fclose(fp); return 0; }
实际上我是使用保存为bmp格式图片的方式进行测试的,bmp图片是RGB分量实际上应该是BGR,所以如果要正确保存bmp,则要进行R、B分量互换,在解压时,libjpeg可以设置输出颜色,使用cinfo.out_color_space,如果不想自己调换R、B,则将其赋值为JCS_EXT_BGR。另外,bmp文件头信息中的高值为负数,否则保存的图片是上下颠倒的。
李迟 2015.6.30 晚饭后