Example how to use tessseract C-API in python with cffi

[Zdenko Podobny]

Hi all,

I publish my test / example how to use tesseract C-API in python3 via cffi[1]. 

I am aware of pytesseract module, which seems to be widely used. It is wrapping tesseract executable, so IMO it could have some limitation e.g. from point of performance (it using disk operation for input and output).

 

It is in form of jupyter notebook[3] (github is able to show it, but not run ;-)) so you can interactively view what is happening.

My aim is not to create new tesseract python wrapper (I do not have a time for it, and I am not able to create nice python code as pytesseract  has :-) ) so it is not robust: I just did it on windows 64 bit, but IMO is should be possible with small modification to use in Linux and Mac. If needed I can add 32bit windows libs...

Personally I would like have python tesseract and leptonica module using directly its API... I know that James Barlow already started to wrapping leptonica, but it is (not yet?) available as independent module (it is part of OCRmyPDF). 

Anyway I hope this will help somebody.

[1] https://github.com/zdenop/SimpleTesseractPythonWrapper  

[2] https://pypi.org/project/pytesseract/

[3] https://github.com/zdenop/SimpleTesseractPythonWrapper/blob/master/SimpleTesseractPythonWrapper.ipynb 

[4] https://github.com/jbarlow83/OCRmyPDF/tree/master/src/ocrmypdf/lib  

Zdenko

[Guru Govindan]

Hi Thanks a lot for the great post. I am trying to use cffi wrapper in my project as the pytesseract was really slow.

I use opencv to read my image and I am trying to use your cffi setup and initiate. But I the tesseract is not recognizing any text. 

import ctypes

import os

import cffi

import cv2

tessdata = "/home/vagrant/reelz_base/shared/tessdata/"

ffi = cffi.FFI()

ffi.cdef("""

typedef signed char             l_int8;

typedef unsigned char           l_uint8;

typedef short                   l_int16;

typedef unsigned short          l_uint16;

typedef int                     l_int32;

typedef unsigned int            l_uint32;

typedef float                   l_float32;

typedef double                  l_float64;

typedef long long               l_int64;

typedef unsigned long long      l_uint64;

typedef int l_ok; /*!< return type 0 if OK, 1 on error */

typedef struct TessBaseAPI TessBaseAPI;

typedef struct ETEXT_DESC ETEXT_DESC;

typedef struct TessPageIterator TessPageIterator;

typedef struct TessResultIterator TessResultIterator;

typedef int BOOL;

typedef enum TessOcrEngineMode  {

    OEM_TESSERACT_ONLY          = 0,

    OEM_LSTM_ONLY               = 1,

    OEM_TESSERACT_LSTM_COMBINED = 2,

    OEM_DEFAULT                 = 3} TessOcrEngineMode;

typedef enum TessPageSegMode {

    PSM_OSD_ONLY               =  0,

    PSM_AUTO_OSD               =  1,

    PSM_AUTO_ONLY              =  2,

    PSM_AUTO                   =  3,

    PSM_SINGLE_COLUMN          =  4,

    PSM_SINGLE_BLOCK_VERT_TEXT =  5,

    PSM_SINGLE_BLOCK           =  6,

    PSM_SINGLE_LINE            =  7,

    PSM_SINGLE_WORD            =  8,

    PSM_CIRCLE_WORD            =  9,

    PSM_SINGLE_CHAR            = 10,

    PSM_SPARSE_TEXT            = 11,

    PSM_SPARSE_TEXT_OSD        = 12,

    PSM_COUNT                  = 13} TessPageSegMode;

typedef enum TessPageIteratorLevel {

     RIL_BLOCK    = 0,

     RIL_PARA     = 1,

     RIL_TEXTLINE = 2,

     RIL_WORD     = 3,

     RIL_SYMBOL    = 4} TessPageIteratorLevel;    

TessPageIterator* TessBaseAPIAnalyseLayout(TessBaseAPI* handle);

TessPageIterator* TessResultIteratorGetPageIterator(TessResultIterator* handle);

BOOL TessPageIteratorNext(TessPageIterator* handle, TessPageIteratorLevel level);

BOOL TessPageIteratorBoundingBox(const TessPageIterator* handle, TessPageIteratorLevel level,

                                 int* left, int* top, int* right, int* bottom);

const char* TessVersion();

TessBaseAPI* TessBaseAPICreate();

void  TessBaseAPIDelete(TessBaseAPI* handle);

size_t TessBaseAPIGetOpenCLDevice(TessBaseAPI* handle, void **device);

void  TessBaseAPISetInputName( TessBaseAPI* handle, const char* name);

const char* TessBaseAPIGetInputName(TessBaseAPI* handle);

void  TessBaseAPISetInputImage(TessBaseAPI* handle, struct Pix* pix);

int   TessBaseAPIGetSourceYResolution(TessBaseAPI* handle);

BOOL  TessBaseAPISetVariable(TessBaseAPI* handle, const char* name, const char* value);

BOOL  TessBaseAPISetDebugVariable(TessBaseAPI* handle, const char* name, const char* value);

void  TessBaseAPIPrintVariables(      const TessBaseAPI* handle, FILE* fp);

BOOL  TessBaseAPIPrintVariablesToFile(const TessBaseAPI* handle, const char* filename);

int   TessBaseAPIInit1(TessBaseAPI* handle, const char* datapath, const char* language, TessOcrEngineMode oem,

                                         char** configs, int configs_size);

int   TessBaseAPIInit2(TessBaseAPI* handle, const char* datapath, const char* language, TessOcrEngineMode oem);

int   TessBaseAPIInit3(TessBaseAPI* handle, const char* datapath, const char* language);

int TessBaseAPIInit4(TessBaseAPI* handle, const char* datapath, const char* language, TessOcrEngineMode mode,

   char** configs, int configs_size,

   char** vars_vec, char** vars_values, size_t vars_vec_size,

   BOOL set_only_non_debug_params);

   

void TessBaseAPISetImage(TessBaseAPI* handle, const unsigned char* imagedata, int width, int height,

                                             int bytes_per_pixel, int bytes_per_line);

   

int TessBaseAPIRecognize(TessBaseAPI* handle, ETEXT_DESC* monitor);

TessResultIterator* TessBaseAPIGetIterator(TessBaseAPI* handle);

BOOL   TessResultIteratorNext(TessResultIterator* handle, TessPageIteratorLevel level);

char*  TessResultIteratorGetUTF8Text(const TessResultIterator* handle, TessPageIteratorLevel level);

float  TessResultIteratorConfidence(const TessResultIterator* handle, TessPageIteratorLevel level);

char*  TessBaseAPIGetUTF8Text(TessBaseAPI* handle);

const char*  TessResultIteratorWordFontAttributes(const TessResultIterator* handle, BOOL* is_bold, BOOL* is_italic,

                                                              BOOL* is_underlined, BOOL* is_monospace, BOOL* is_serif,

                                                              BOOL* is_smallcaps, int* pointsize, int* font_id);

BOOL TessResultIteratorWordIsFromDictionary(const TessResultIterator* handle);

BOOL TessResultIteratorWordIsNumeric(const TessResultIterator* handle);

BOOL TessResultIteratorSymbolIsSuperscript(const TessResultIterator* handle);

BOOL TessResultIteratorSymbolIsSubscript(const TessResultIterator* handle);

BOOL TessResultIteratorSymbolIsDropcap(const TessResultIterator* handle);

void   TessBaseAPIEnd(TessBaseAPI* handle);

""")

tess_libname = "/usr/local/lib/libtesseract.so.4"

if os.path.exists(tess_libname):

    tesseract = ffi.dlopen(tess_libname)

api = None

tesseract_version = ffi.string(tesseract.TessVersion())

print('Tesseract-ocr version', tesseract_version.decode('utf-8'))

api = tesseract.TessBaseAPICreate()

c_ubyte_p = ctypes.POINTER(ctypes.c_ubyte)

lang="eng"

oem = tesseract.OEM_DEFAULT

tesseract.TessBaseAPIInit3(api, tessdata.encode(), lang.encode())

img = cv2.imread("/data/framecache/testing.jpg")

tesseract.TessBaseAPISetDebugVariable(api, "tesseract_char_whitelist", "ABCDEFGHIJKLMNOPQRSTUVWXYZ")

tesseract.TessBaseAPISetDebugVariable(api, "tessedit_pageseg_mode", "8")

tesseract.TessBaseAPISetVariable(api, "image_default_resolution", "72")

tesseract.TessBaseAPISetImage(api, ffi.cast("unsigned char*", img.ctypes.data), img.shape[0], img.shape[1], 1, img.shape[1])

# tesseract.TessBaseAPIRecognize(api, ffi.NULL)

utf8_text = ffi.string(tesseract.TessBaseAPIGetUTF8Text(api)).decode('utf-8')

print utf8_text

[Zdenko Podobny]

My code use PIL to open image files.

You need to convert cv image structure to pix or send the data to  tesseract[1] with 

void SetImage(const unsigned char* imagedata, int width, int height, int bytes_per_pixel, int bytes_per_line)[2]

[1] https://stackoverflow.com/questions/8115368/converting-cvmat-for-tesseract  
[2] https://github.com/tesseract-
ocr/tesseract/blob/3e7144e79654d9da7a180a3c51e843afb4a77491/src/api/baseapi.h#L333

If you have problem, post your code and image.

Zdenko

ut 2. 4. 2019 o 8:46 Guru Govindan <gurunatha...@gmail.com> napísal(a):

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[Zdenko Podobny]

OK. I have more time to look at you code and I see there few problems:

• Whitelist does not work at tesseract 4.0 (search for more details in forum/issue tracker)
• Setting variable: there is no variable like "image_default_resolution" in tesseract 4.x - you need to check return value of command. Try something like this:

# Example to set variable

var = "user_defined_dpi"

value = "250"

ret_val = tesseract.TessBaseAPISetVariable(api, var.encode(), value.encode())

print(f"SetVariable for {var}={value} {'was sucessfull' if ret_val else 'failed'}.")

# Example to get int variable

dpi = ffi.new('int *')

ret_val = tesseract.TessBaseAPIGetIntVariable(api, var.encode(), dpi)

print((f"Can not get int value for '{var}'.", f"{var}: {dpi[0]}.")[ret_val])

• Setting data to tesseract: did you check if there in no error? Try to get thresholded (binarized) image from tesseract to check that:

thresholded_pix = tesseract.TessBaseAPIGetThresholdedImage(api)

if thresholded_pix == ffi.NULL:

    print(thresholded_pix)

    print("There is no image in tesseract api. Did you use SetImage function?")

else:

    filename = "thresholded_pix.png"

    if leptonica.pixWrite(filename.encode(), thresholded_pix, leptonica.IFF_PNG):

        print("There was problem to save thresholded image to disk.")

    else:

        print(f"Thresholded image to disk as '{filename}'.")

• and then you will see that your usage of  TessBaseAPISetImagedoes not what you expect. Following code worked for my test images, but  I do not guarantee it will for for all types of image (RGBA, grays scale, binary):

img = cv2.imread("test.jpg", flags=cv2.IMREAD_COLOR)

tesseract.TessBaseAPISetImage(api,

    ffi.cast("unsigned char*", img.ctypes.data),

    img.shape[1], img.shape[0], img.shape[2], int(img.size/img.shape[0]))

I hope this will help you. Please share you result, especially if you make this code more robust.

Zdenko

ut 2. 4. 2019 o 8:54 Zdenko Podobny <zde...@gmail.com> napísal(a):

[Guru Govindan]

Thanks a lot for your response and example. I will try today. I dont have much experience with cffi. So I decided to write my tesseract python interface with just ctypes.

The following is my code for the same. It seems to work. So for loading the tesseract library it takes about 160ms and recognizing the text with (psm 6) takes about 140ms. These are the results with tessdata_best.

So in order to get better performance I decided to create 100 instances of my class and rotate them in an array. That way my recognizing time is under ~140ms.

But I definetely want to try with cffi as it is a cleaner interface to understand and somehow feel might be better in performance.

import cv2

import ctypes

import ctypes.util

from datetime import datetime

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 = "/usr/local/lib/libtesseract.so.4"

        cls._lib = lib = ctypes.CDLL(lib_path)

        # source:

        # https://github.com/tesseract-ocr/tesseract/blob/95ea778745edd1cdf6ee22f9fe653b9e061d5708/src/api/capi.h

        lib.TessBaseAPICreate.restype = cls.TessBaseAPI

        lib.TessBaseAPIDelete.restype = None # void

        lib.TessBaseAPIDelete.argtypes = (

            cls.TessBaseAPI,) # handle

        lib.TessBaseAPIInit3.argtypes = (cls.TessBaseAPI, ctypes.c_char_p, ctypes.c_char_p) 

        lib.TessBaseAPISetImage.restype = None

        lib.TessBaseAPISetImage.argtypes = (cls.TessBaseAPI, ctypes.c_void_p, ctypes.c_int, ctypes.c_int, ctypes.c_int, ctypes.c_int)    

        lib.TessBaseAPISetVariable.argtypes = (cls.TessBaseAPI, ctypes.c_char_p, ctypes.c_char_p)

        lib.TessBaseAPIGetUTF8Text.restype = ctypes.c_char_p

        lib.TessBaseAPIGetUTF8Text.argtypes = (

            cls.TessBaseAPI,)

    def __init__(self, language='eng_best', datapath=None, lib_path=None):

        if self._lib is None:

            self.setup_lib(lib_path)

        self._api = self._lib.TessBaseAPICreate()

        print "initializing tesseract!!!!"

        if self._lib.TessBaseAPIInit3(self._api, datapath, language):

            print "Tesseract initialization failed!!"

            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

        print "bytes per line={}".format(bytes_per_line)

        self._lib.TessBaseAPISetImage(self._api,

                                      imagedata, width, height,

                                      bytes_per_pixel, bytes_per_line)

    def set_variable(self, key, val):

        self._check_setup()

        self._lib.TessBaseAPISetVariable(self._api, key, val)

    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')

def convert_to_grayscale(image_data):

    return cv2.cvtColor(image_data, cv2.COLOR_BGR2GRAY)

# a method to make it look similar to tesslib.py

def tesseract_process_image2(tess, frame_piece):

    grayscaled = len(frame_piece.frame.shape) == 2

    if not grayscaled:

        image_data = convert_to_grayscale(frame_piece.frame)

    height, width = frame_piece.frame.shape

    tess.set_variable("tesseract_char_whitelist", frame_piece.whitelist)

    tess.set_variable("tessedit_pageseg_mode", str(frame_piece.psm))

    # tess.set_variable("user_words_suffix", "user-data")

    # tess.set_variable("user_pattern_suffix", "user-pattern")

    tess.set_variable("image_default_resolution", "70")

    tess.set_image(frame_piece.frame.ctypes, width, height, 1)

    text = tess.get_utf8_text()

    return text.strip()

class FramePiece(object):

  def __init__(self, img, whitelist):

    self.frame = img

    self.whitelist = whitelist if whitelist else "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890"

    self.psm = 6

# overloaded method for view page

def tesseract_process_image(tess, frame, whitelist=None):

    frame_piece = FramePiece(frame, whitelist)

    return tesseract_process_image2(tess, frame_piece)

############################### TESTING ######################

if __name__ == '__main__':

    img = cv2.imread('/data/framecache/testing.jpg')

    height, width, depth = img.shape

    print datetime.utcnow()

    tess = Tesseract()

    print "ocr image Start:{}".format(datetime.utcnow())

    frame_piece = FramePiece(img)

    res = tesseract_process_image2(tess, frame_piece)

    print res

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