在python中使用csepes和cv2的tesseract 3.02的C API

Question

我试图在python中使用带有ctypes和cv2的Tesseract 3.02.Tesseract提供了一组暴露的C风格API,其中一个如下:

TESS_API void  TESS_CALL TessBaseAPISetImage(TessBaseAPI* handle, const unsigned char* imagedata, int width, int height, int bytes_per_pixel, int bytes_per_line);

到目前为止,我的代码如下:

tesseract = ctypes.cdll.LoadLibrary('libtesseract302.dll')
api = tesseract.TessBaseAPICreate()
tesseract.TessBaseAPIInit3(api, '', 'eng')
imcv = cv2.imread('test.bmp')
w, h, d = imcv.shape
ret = tesseract.TessBaseAPISetImage(api, ctypes.c_char_p(str(imcv.data)), w, h, d, w * d)
#ret = 44 here

最后一行返回错误代码44,我在Tesseract提供的errcode.h中找不到任何地方.我不确定我在这里做错了什么.

我发现了类似的问题如何使用ctypes和tesseract 3.0.2识别数据而不是文件名？但问题仍未解决.我也知道https://code.google.com/p/python-tesseract/,我深入研究了这个项目的源代码,但无法找到我需要的信息.

我可以通过调用来确认test.bmp中的图像是否合法且可读cv2.imshow.同样的图像可以通过命令行上的Tesseract进行OCR.

Answer 1

默认restype值为c_int,并且整数的默认参数转换也是c_int.您将在网络上找到假设具有32位平台的示例sizeof(int) == sizeof(void *).这绝不是一个很好的假设.要在转换为Python整数和从Python整数转换时保护64位指针不被截断,请设置函数指针argtypes和restype.无论如何,这是一个好主意,因为它允许ctypes ArgumentError在使用错误类型或数量的参数时引发.

如果您不想为每个函数定义原型,那么至少设置TessBaseAPICreate.restype为不透明的指针类型.

下面ctypes的定义是基于报头API/capi.h.为方便起见,我将API打包成了一个Tesseract类.

import sys
import cv2
import ctypes
import ctypes.util

if sys.platform == 'win32':
    LIBNAME = 'libtesseract302'
else:
    LIBNAME = 'tesseract'

class TesseractError(Exception):
    pass

class Tesseract(object):
    _lib = None
    _api = None

    class TessBaseAPI(ctypes._Pointer):
        _type_ = type('_TessBaseAPI', (ctypes.Structure,), {})

    @classmethod
    def setup_lib(cls, lib_path=None):
        if cls._lib is not None:
            return
        if lib_path is None:
            lib_path = ctypes.util.find_library(LIBNAME)
            if lib_path is None:
                 raise TesseractError('tesseract library not found')
        cls._lib = lib = ctypes.CDLL(lib_path)

        # source:
        # https://github.com/tesseract-ocr/tesseract/
        #         blob/3.02.02/api/capi.h

        lib.TessBaseAPICreate.restype = cls.TessBaseAPI

        lib.TessBaseAPIDelete.restype = None # void
        lib.TessBaseAPIDelete.argtypes = (
            cls.TessBaseAPI,) # handle

        lib.TessBaseAPIInit3.argtypes = (
            cls.TessBaseAPI, # handle
            ctypes.c_char_p, # datapath
            ctypes.c_char_p) # language

        lib.TessBaseAPISetImage.restype = None
        lib.TessBaseAPISetImage.argtypes = (
            cls.TessBaseAPI, # handle
            ctypes.c_void_p, # imagedata
            ctypes.c_int,    # width
            ctypes.c_int,    # height
            ctypes.c_int,    # bytes_per_pixel
            ctypes.c_int)    # bytes_per_line

        lib.TessBaseAPIGetUTF8Text.restype = ctypes.c_char_p
        lib.TessBaseAPIGetUTF8Text.argtypes = (
            cls.TessBaseAPI,) # handle

    def __init__(self, language='eng', datapath=None, lib_path=None):
        if self._lib is None:
            self.setup_lib(lib_path)
        self._api = self._lib.TessBaseAPICreate()
        if self._lib.TessBaseAPIInit3(self._api, datapath, language):
            raise TesseractError('initialization failed')

    def __del__(self):
        if not self._lib or not self._api:
            return
        if not getattr(self, 'closed', False):
            self._lib.TessBaseAPIDelete(self._api)
            self.closed = True

    def _check_setup(self):
        if not self._lib:
            raise TesseractError('lib not configured')
        if not self._api:
            raise TesseractError('api not created')

    def set_image(self, imagedata, width, height,
                  bytes_per_pixel, bytes_per_line=None):
        self._check_setup()
        if bytes_per_line is None:
            bytes_per_line = width * bytes_per_pixel
        self._lib.TessBaseAPISetImage(self._api,
                                      imagedata, width, height,
                                      bytes_per_pixel, bytes_per_line)

    def get_utf8_text(self):
        self._check_setup()
        return self._lib.TessBaseAPIGetUTF8Text(self._api)

    def get_text(self):
        self._check_setup()
        result = self._lib.TessBaseAPIGetUTF8Text(self._api)
        if result:
            return result.decode('utf-8')

用法示例:

if __name__ == '__main__':
    imcv = cv2.imread('ocrtest.png')
    height, width, depth = imcv.shape

    tess = Tesseract()
    tess.set_image(imcv.ctypes, width, height, depth)
    text = tess.get_text()

    print text.strip()

我在Linux上用libtesseract.so.3测试了这个.请注意,cv2.imread返回NumPy数组.它有一个ctypes包含_as_parameter_钩子的属性,设置为c_void_p指向数组的指针.另请注意,问题中显示的代码具有转置的宽度和高度.它应该是h, w, d = imcv.shape.

ocrtest.png:

输出:

I am trying to use Tesseract 3.02 with ctypes and cv2 in python. Tesseract
provides a DLL exposed set of C style APIs, one of them is as following: