libJPEG-turbo库使用示例代码

libJPEG库是用于编码数据为JPEG格式或者解码JPEG格式图片的常用库，OpenCV读取图像底层实现就是利用libJPEG库，而libJPEG-turbo则效率更高。

具体怎么编译编译libJPEG库源码得到lib库的方法很容易搜到，不多做介绍。

下面的代码包含了该库中常用的API用法，包括读取一幅JPEG图片并解码到内存、编码内存中的数据为JPEG图片写入本地等。

 #include <iostream>
 #include <stdio.h>
 #include <setjmp.h>
 #include <string.h>
 #include <stdlib.h>
 #include <jpeglib.h>
 #include <opencv2/opencv.hpp>

 using namespace std;

 int read_JPEG_file(string strImageName)
 {
     /* 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 */
     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(strImageName.c_str(), "rb")) == NULL) {
         fprintf(stderr, "can't open %s\n", strImageName);
         return -;
     }

     /* 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);
     /* 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 -1;
     //}  

     /* 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() */
     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.
     */
     printf("image_width = %d\n", cinfo.image_width);
     printf("image_height = %d\n", cinfo.image_height);
     printf("num_components = %d\n", cinfo.num_components);

     /* 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.
     */
     printf("enter scale M/N:\n");
     //scanf("%d/%d", &cinfo.scale_num, &cinfo.scale_denom);
     cinfo.scale_num = ;
     cinfo.scale_denom = ;
     printf("scale to : %d/%d\n", cinfo.scale_num, cinfo.scale_denom);

     /* Step 5: Start decompressor */
     jpeg_start_decompress(&cinfo);
     /* We can ignore the return value since suspension is not possible
     * with the stdio data source.
     */

     //输出的图象的信息
     printf("output_width = %d\n", cinfo.output_width);
     printf("output_height = %d\n", cinfo.output_height);
     printf("output_components = %d\n", cinfo.output_components);

     /* 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, );

     /* 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.
         */
         jpeg_read_scanlines(&cinfo, buffer, );
         /* Assume put_scanline_someplace wants a pointer and sample count. */
         //put_scanline_someplace(buffer[0], row_stride);
     }

     /* Step 7: Finish decompression */
     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).
     */

     return ;
 }

 int write_JPEG_file(string strImageName, int quality)
 {
     unsigned char* image_buffer;    /* Points to large array of R,G,B-order data */
     int image_height = ; /* Number of rows in image */
     int image_width = ;  /* Number of columns in image */

                             /* This struct contains the JPEG compression parameters and pointers to
                             * working space (which is allocated as needed by the JPEG library).
                             * It is possible to have several such structures, representing multiple
                             * compression/decompression processes, in existence at once.  We refer
                             * to any one struct (and its associated working data) as a "JPEG object".
                             */
     struct jpeg_compress_struct cinfo;

     /* This struct represents a JPEG error handler.  It is declared separately
     * because applications often want to supply a specialized error handler
     * (see the second half of this file for an example).  But here we just
     * take the easy way out and use the standard error handler, which will
     * print a message on stderr and call exit() if compression fails.
     * 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 * outfile;     /* target file */
     JSAMPROW row_pointer[];    /* 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.
                         */
     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.
     */
     if ((outfile = fopen(strImageName.c_str(), "wb")) == NULL) {
         fprintf(stderr, "can't open %s\n", strImageName);
         //exit(1);
         return -;
     }
     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:
     */
     cinfo.image_width = image_width;    /* image width and height, in pixels */
     cinfo.image_height = image_height;
     cinfo.input_components = ;     /* # 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:
     */
     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 * ;   /* JSAMPLEs per row in image_buffer */

     image_buffer = new unsigned char[row_stride * cinfo.image_height];
     memset(image_buffer, 0xff, row_stride * cinfo.image_height);

     int line = ;
     //while (cinfo.next_scanline < cinfo.image_height) {
     while (line < 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];
         row_pointer[] = &image_buffer[line * row_stride];
         jpeg_write_scanlines(&cinfo, row_pointer, );

         line++;
     }

     delete image_buffer;

     /* 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);

     return ;
 }

 struct Image
 {
     int bpp;
     int width;
     int height;
     unsigned char* data;
 };

 struct jerror_mgr
 {
     jpeg_error_mgr base;
     jmp_buf        jmp;
 };

 METHODDEF(void) jerror_exit(j_common_ptr jinfo)
 {
     jerror_mgr* err = (jerror_mgr*)jinfo->err;
     longjmp(err->jmp, );
 }

 METHODDEF(void) joutput_message(j_common_ptr)
 {
 }

 bool Image_LoadJpeg(Image* image, unsigned char* img_data, unsigned int img_size)
 {
     jpeg_decompress_struct jinfo;
     jerror_mgr jerr;

     jinfo.err = jpeg_std_error(&jerr.base);
     jerr.base.error_exit = jerror_exit;
     jerr.base.output_message = joutput_message;
     jpeg_create_decompress(&jinfo);

     image->data = NULL;

     if (setjmp(jerr.jmp)) goto bail;

     jpeg_mem_src(&jinfo, img_data, img_size);

     if (jpeg_read_header(&jinfo, TRUE) != JPEG_HEADER_OK) goto bail;

     jinfo.dct_method = JDCT_FLOAT; // change this to JDCT_ISLOW on Android/iOS  

     if (!jpeg_start_decompress(&jinfo)) goto bail;

     if (jinfo.num_components !=  && jinfo.num_components != ) goto bail;

     image->data = new (std::nothrow) unsigned char[jinfo.output_width * jinfo.output_height * jinfo.output_components];
     if (!image->data) goto bail;

     {
         JSAMPROW ptr = image->data;
         while (jinfo.output_scanline < jinfo.output_height)
         {
             if (jpeg_read_scanlines(&jinfo, &ptr, ) != ) goto bail;

             ptr += jinfo.output_width * jinfo.output_components;
         }
     }

     if (!jpeg_finish_decompress(&jinfo)) goto bail;

     image->bpp = jinfo.output_components;
     image->width = jinfo.output_width;
     image->height = jinfo.output_height;

     jpeg_destroy_decompress(&jinfo);

     return true;

 bail:
     jpeg_destroy_decompress(&jinfo);
     if (image->data) delete[] image->data;

     return false;
 }

 struct ImageData {
     unsigned char *pixels;
     long  width;
     long height;
 };

 int TestImage(string strSrcImageName, string strDstImageName)
 {
     //read
     struct jpeg_decompress_struct cinfo_decompress;
     FILE* infile;
     int row_stride;
     struct jpeg_error_mgr jerr;

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

     cinfo_decompress.err = jpeg_std_error(&jerr);
     jpeg_create_decompress(&cinfo_decompress);
     jpeg_stdio_src(&cinfo_decompress, infile);
     int ret = jpeg_read_header(&cinfo_decompress, TRUE);
     if (ret != JPEG_HEADER_OK) return -;
     jpeg_start_decompress(&cinfo_decompress);
     row_stride = cinfo_decompress.output_width * cinfo_decompress.output_components;
     int buffer_height = ;
     JSAMPARRAY buffer = (JSAMPARRAY)malloc(sizeof(JSAMPROW) * buffer_height);
     buffer[] = (JSAMPROW)malloc(sizeof(JSAMPLE) * row_stride);
     //JSAMPARRAY buffer = (*cinfo_decompress.mem->alloc_sarray)((j_common_ptr)&cinfo_decompress, JPOOL_IMAGE, row_stride, 1);
     ImageData *imageData;
     imageData = new ImageData;
     imageData->width = cinfo_decompress.output_width;
     imageData->height = cinfo_decompress.output_height;

     imageData->pixels = new unsigned char[cinfo_decompress.output_width * cinfo_decompress.output_height * cinfo_decompress.output_components];
     long counter = ;

     while (cinfo_decompress.output_scanline < cinfo_decompress.output_height) {
         jpeg_read_scanlines(&cinfo_decompress, buffer, );
         memcpy(imageData->pixels + counter, buffer[], row_stride);
         counter += row_stride;
     }

     jpeg_finish_decompress(&cinfo_decompress);
     jpeg_destroy_decompress(&cinfo_decompress);

     fclose(infile);

     //write
     unsigned char* image_buffer;
     int image_height = cinfo_decompress.output_height;
     int image_width = cinfo_decompress.output_width;
     FILE * outfile;
     JSAMPROW row_pointer[];
     int row_stride_dst;
     struct jpeg_compress_struct cinfo_compress;
     cinfo_compress.err = jpeg_std_error(&jerr);
     jpeg_create_compress(&cinfo_compress);

     if ((outfile = fopen(strDstImageName.c_str(), "wb")) == NULL) {
         fprintf(stderr, "can't open %s\n", strDstImageName);
         //exit(1);
         return -;
     }

     jpeg_stdio_dest(&cinfo_compress, outfile);

     cinfo_compress.image_width = image_width;
     cinfo_compress.image_height = image_height;
     cinfo_compress.input_components = ;
     cinfo_compress.in_color_space = JCS_EXT_RGB;

     int quality = ;
     jpeg_set_defaults(&cinfo_compress);
     jpeg_set_quality(&cinfo_compress, quality, TRUE);
     jpeg_start_compress(&cinfo_compress, TRUE);

     row_stride_dst = image_width * ;

     image_buffer = new unsigned char[row_stride_dst * cinfo_compress.image_height];
     memcpy(image_buffer, imageData->pixels, row_stride_dst * cinfo_compress.image_height);

     while (cinfo_compress.next_scanline < cinfo_compress.image_height) {
         row_pointer[] = &image_buffer[cinfo_compress.next_scanline * row_stride_dst];
         jpeg_write_scanlines(&cinfo_compress, row_pointer, );
     }

     jpeg_finish_compress(&cinfo_compress);
     fclose(outfile);
     jpeg_destroy_compress(&cinfo_compress);

     if (imageData) {
         delete imageData;
         imageData = NULL;
     }

     if (image_buffer)
         delete[] image_buffer;

     return ;
 }

 int main(int argc, char* argv[])
 {
     string strImageName = "data/srcImg/moon.jpg";
     int flag1 = read_JPEG_file(strImageName);
     if (flag1 == ) cout << "read ok!" << endl;
     else cout << "read error!" << endl;

     strImageName = "2.bmp";
     int flag2 = write_JPEG_file(strImageName, );
     if (flag2 == ) cout << "write ok!" << endl;
     else cout << "write error!" << endl;

     string strSrcImageName = "data/srcImg/moon.jpg";
     string strDstImageName = "b.jpg";
     int flag3 = TestImage(strSrcImageName, strDstImageName);
     if (flag3 == ) cout << "test ok!" << endl;
     else cout << "test error!" << endl;

     return ;
 }