jpeg 图片解码库内存地址解码

jpeglib库从内存地址读取数据解码。

最近做项目用到了jpeglib这个库做图像jpg图像的解码操作，需要从内存地址解码数据。看了网上的一些博客有的说是不能从内存读取解码，需要改写接口，但后来我在头文件里找了下，发现了其实并不像一些博主所说的没有。我找到了比如这个接口：jpeg_mem_src就支持从内存地址解码。为了让大家不要被误导，少走弯路，特写这个博客供大家参考。经过我些代码做验证完全没有问题。下面贴上代码供大家参考，如有问题，欢迎留言指正。

//jpeg 解压缩出错处理
struct my_error_mgr
{
    struct jpeg_error_mgr pub;
    jmp_buf setjmp_buffer;
};
typedef struct my_error_mgr* my_error_ptr;

//jpeg 解压缩错误处理函数
METHODDEF(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);
}

typedef struct my_error_mgr* my_error_ptr;
JPG jpg_depress(unsigned char* jpg_buffer, int file_len)
{
    struct jpeg_decompress_struct cinfo;
    //struct jpeg_error_mgr jerr;
    struct my_error_mgr jerr;

    JSAMPARRAY buffer;
    int row_width;
    JPG jpg;
    unsigned char* output_buffer;
    unsigned char* tmp = NULL;

    //cinfo.err = jpeg_std_error(&jerr);
    cinfo.err = jpeg_std_error(&jerr.pub);

    //错误处理
    jerr.pub.error_exit = my_error_exit;

    if (setjmp(jerr.setjmp_buffer))
    {
        //在正常情况下，setjmp将返回0，而如果程序出现错误，即调用my_error_exit
        //然后程序将再次跳转于此，同时setjmp将返回在my_error_exit中由longjmp第二个参数设定的值1
        //并执行以下代码
        std::string error = std::string((char*)cinfo.err->output_message);
        jpeg_destroy_decompress(&cinfo);
        throw error;
        
    }
    // Initialization of JPEG compression objects  
    jpeg_create_decompress(&cinfo);

    /* Specify data source for decompression */
    jpeg_mem_src(&cinfo, jpg_buffer, file_len);

    /* 1.设置读取jpg文件头部，Read file header, set default decompression parameters */
    (void)jpeg_read_header(&cinfo, TRUE);
    
    /* 2.开始数据 Start decompressor */
    //解压缩
    (void)jpeg_start_decompress(&cinfo);

    row_width = cinfo.output_width * cinfo.output_components;
    //不是彩色图 先不处理
    if (cinfo.num_components != 3) {
        //throw std::string("cinfo imput image is not 3 channels.");
        throw invalid_argument("jpeglib cinfo input image is not 3 channels.");
    }
    jpg.w = cinfo.output_width;
    jpg.h = cinfo.output_height;
    
    /* *_width = cinfo.output_width;
     *_hight = cinfo.output_height;
     *_line_width = row_width;*/

     /* 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);
    int size = row_width * cinfo.output_height;

    output_buffer = (unsigned char*)malloc(size);
    //output_buffer = gImageBuffer;
    jpg.data = output_buffer;
    //memset(output_buffer, 0, size);
    tmp = output_buffer;
    //*_output_buffer = output_buffer;
    //UINT8* data = 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;
    }
    (void)jpeg_finish_decompress(&cinfo);
    jpeg_destroy_decompress(&cinfo);
    /* Close files, if we opened them */

    return jpg;
}