使用 libjpeg 读取 .jpeg 文件时出错

Question

当输入图像文件被 FILE 对象成功读取时，我在以下代码中的函数 jpeg_read_header() 中出现 NULL 指针异常。

#include "jpeglib.h"
#include <iostream>

using namespace std;



void decode_frame(char *filename)
{
    unsigned char* raw_image;
    JSAMPROW row_pointer[1];
    unsigned long location = 0;

    struct jpeg_error_mgr jerr;
    struct jpeg_decompress_struct cinfo ;

    FILE *infile = fopen(filename, "rb" );

    if (infile == NULL )
    {
        printf("Error opening jpeg file %s\n!", filename );
        return -1;
    }
    cinfo.err = jpeg_std_error(&jerr);

    /* create decompressor */
    jpeg_create_decompress(&cinfo);

    /* this makes the library read from infile */
    jpeg_stdio_src(&cinfo, infile );

    /* read jpeg header */
    jpeg_read_header(&cinfo, TRUE);

    /* decompress */
    jpeg_start_decompress(&cinfo);

    /*allocate memory */
    raw_image = (unsigned char*)malloc( cinfo.output_width*cinfo.output_height*cinfo.num_components );

    /* now actually read the jpeg into the raw buffer */
    row_pointer[0] = (unsigned char *)malloc( cinfo.output_width*cinfo.num_components );
    /* read scanlines */
    while (cinfo.output_scanline < cinfo.output_height) {
        jpeg_read_scanlines( &cinfo, row_pointer, 1 );
        for( int i=0; i < cinfo.image_width*cinfo.num_components;i++) 
            raw_image[location++] = row_pointer[0][i];
    }  

    /* clean up */
    jpeg_finish_decompress(&cinfo);
    jpeg_destroy_decompress(&cinfo);
    fclose( infile );
    free( row_pointer[0] );
}

int main(){
    char *f = "Example.jpg";
    decode_frame(f);
    return 0;
}

cinfo alfter jpeg_stdio_src(&cinfo, infile )函数调用的值为：

cinfo{err=0x0020f8fc mem=0x021cb320 progress=0x00000000 ...}
progress             0x00000000 {progress_monitor=??? pass_counter=??? pass_limit=??? ...}
client_data          0xcccccccc  void *
is_decompressor      1 
global_state         200    
src 0x021c4480       {next_input_byte=0x00000000 <Bad Ptr> bytes_in_buffer=0 init_source=0x686ccb50 ...}    
image_width          0  
image_height         0  
num_components       0
jpeg_color_space     JCS_UNKNOWN    
out_color_space      JCS_UNKNOWN
scale_num            0      
scale_denom          0      
output_gamma         0.00000000000000000    
buffered_image       0  
raw_data_out         0
dct_method           JDCT_ISLOW     

我哪里出错了？这是我第一次使用图片库。

Answer 1

全局状态200表示（根据libjpeg源码）：

#define DSTATE_START 200 /* after create_decompress */

调用jpeg_stdio_src只是为了准备解压器结构，即分配内部cinfo->src缓冲区并初始化其他解压器状态。

也就是说 JPEG 文件解析还没有开始：所以这里没有问题。

您至少需要执行jpeg_read_header 以确保cinfo 结构填充了所有元数据信息（image_width、image_height、jpeg_color_space 等）。如果该步骤出现问题，这可能与您的 JPEG 文件损坏有关 (?)。

你看到像Not a JPEG file这样的东西印在stderr上了吗？