使用libjpeg讀取.jpeg文件時出錯

Question

當FILE對象成功讀取輸入圖像文件時，在以下代碼中的jpeg_read_header()函數中jpeg_read_header() NULL指針異常。

#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 )函數調用的值是：

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     

我哪里出錯了？ 這是我第一次使用圖像庫。

Answer 1

全局狀態200表示（根據libjpeg源代碼）：

#define DSTATE_START 200 /* after create_decompress */

調用jpeg_stdio_src僅對准備解壓縮器結構有效，即，它分配內部cinfo->src緩沖區並初始化其他解壓縮器狀態。

換句話說，JPEG文件解析尚未開始：因此您在這里沒有問題。

你至少需要執行jpeg_read_header以確保cinfo結構充滿了所有的元數據信息（ image_width ， image_height ， jpeg_color_space等）。 如果在該步驟出現問題，則可能與您的JPEG文件有關，該文件可能已損壞（？）。

您是否看到類似在Stderr上Not a JPEG file打印Not a JPEG file內容？