如何使用Python解析JPEG文件中的複雜結構？

Question

JPEG文件的尾部包含一個複雜的結構，如下列：

FFD8........................... 
............................... 
............FFD9 1C01 0000 .... 
............................... 

struct definations in C file are: 
    typedef struct 
    { 
     short wYear; 
     short wMonth; 
     short wDayOfWeek; 
     short wDay; 
     short wHour; 
     short wMinute; 
     short wSecond; 
     short wMilliseconds; 
    }SYSEMTTIME; 
    typedef struct 
    { 
     int nOcrResult;  
     char szPlateText[16]; 
     char szPlateColor[8]; 
     char szCarColor[8]; 
     RECT rtPlate;   
    } OCR_PLATE; 
    typedef struct 
    { 
     unsigned int size; 
     unsigned char nLane[4]; 
     unsigned char nImageFalgs[4]; 
     unsigned int nRandom[4]; 
     unsigned char nIndex[4]; 
     unsigned char nImageIndex[4]; 
     unsigned char nTotalCount[4]; 
     unsigned char nTrigerNow[4]; 
     unsigned char nCarSpeed[4]; 
     unsigned char nLimitSpeed[4]; 
     unsigned char nDelayFrame[4]; 
     OCR_PLATE  struPlate[4]; 
     SYSTEMTIME  stTime; 
     unsigned int szFlags; 
    } IMAGE_CAPTURE_INFO; 

而在Python中，我一直在使用CTYPE的庫寫了一些類：

 class POINT(Structure): 
      _fields_ = [("x", c_int),("y", c_int)] 
     class RECT(Structure): 
      _fields_ = [("left", c_int),("top", c_int),("right", c_int), 
         ("bottom",  c_int)] 
     class OCR_PLATE(Structure):   
      _fields_ = [("nOcrResult", c_int), 
         ("szPlateText", c_char * 16), 
         ("szPlateColor", c_char * 8), 
         ("szCarColor", c_char * 8), 
         ("rtPlate", RECT)] 
     class SYSTEMTIME(Structure):  
      _fields_ = [("wYear", c_short), 
         ("wMonth", c_short), 
         ("wDayOfWeek", c_short), 
         ("wDay", c_short), 
         ("wHour", c_short), 
         ("wMinute", c_short), 
         ("wSecond", c_short), 
         ("wMilliseconds", c_short)] 
     class IMAGE_CAPTURE_INFO(Structure): 
      _fields_ = [("size", c_uint), 
         ("nLane", c_ubyte * 4), 
         ("nImageFalgs", c_ubyte * 4), 
         ("nRandom", c_uint * 4), 
         ("nIndex", c_ubyte * 4), 
         ("nImageIndex", c_ubyte * 4), 
         ("nTotalCount", c_ubyte * 4), 
         ("nTrigerNow", c_ubyte * 4), 
         ("nCarSpeed", c_ubyte * 4), 
         ("nLimitSpeed", c_ubyte * 4), 
         ("nDelayFrame", c_ubyte * 4), 
         ("struPlate", OCR_PLATE * 4), 
         ("stTime", SYSTEMTIME), 
         ("szFlags", c_uint)] 

但如何讀取數據從JPEG圖像文件中以上述結構的形式？

Answer 1

您需要知道該結構在文件中的偏移量。
假設它位於文件末尾：

data = open("filename.jpg", "rb").read() 
offset = len(data) - sizeof(IMAGE_CAPTURE_INFO) 
myStructure = IMAGE_CAPTURE_INFO.from_buffer(data, offset)