This wiki provide simple example how to use tesseract-ocr API (v3.02.02) in C++. It is expected that tesseract-ocr is correctly installed including all dependecies. It is expected that user is familiar with C++, compiling and linking program on his platform, though basic compilation examples are included for beginners with Linux.
More details about tesseract-ocr API can found at baseapi.h
- Basic example
- example
- Result iterator example
- Orientation and script detection (OSD) example
- Example of iterator over the classifier choices for a single symbol
- Compiling C++ API programs on Linux
- C-API in python
- Example using the C-API in a C program
Basic example
Code:
#include <tesseract/baseapi.h>
#include <leptonica/allheaders.h>
int main()
{
char *outText;
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
// Initialize tesseract-ocr with English, without specifying tessdata path
if (api->Init(NULL, "eng")) {
fprintf(stderr, "Could not initialize tesseract.\n");
exit(1);
}
// Open input image with leptonica library
Pix *image = pixRead("/usr/src/tesseract-3.02/phototest.tif");
api->SetImage(image);
// Get OCR result
outText = api->GetUTF8Text();
printf("OCR output:\n%s", outText);
// Destroy used object and release memory
api->End();
delete [] outText;
pixDestroy(&image);
return 0;
}
Program must be linked to tesseract-ocr library and leptonica library.
If you want to restrict recognition to a sub-rectangle of the image - call SetRectangle(left, top, width, height) after SetImage. Each SetRectangle clears the recogntion results so multiple rectangles can be recognized with the same image. E.g.
api->SetRectangle(30, 86, 590, 100);
GetComponentImages example
Pix *image = pixRead("/usr/src/tesseract-3.02/phototest.tif");
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
api->Init(NULL, "eng");
api->SetImage(image);
Boxa* boxes = api->GetComponentImages(tesseract::RIL_TEXTLINE, true, NULL, NULL);
printf("Found %d textline image components.\n", boxes->n);
for (int i = 0; i < boxes->n; i++) {
BOX* box = boxaGetBox(boxes, i, L_CLONE);
api->SetRectangle(box->x, box->y, box->w, box->h);
char* ocrResult = api->GetUTF8Text();
int conf = api->MeanTextConf();
fprintf(stdout, "Box[%d]: x=%d, y=%d, w=%d, h=%d, confidence: %d, text: %s",
i, box->x, box->y, box->w, box->h, conf, ocrResult);
}
Result iterator example
There is posibility to get confidence value and BoundingBox per word from ResultIterator:
Pix *image = pixRead("/usr/src/tesseract-3.02/phototest.tif");
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
api->Init(NULL, "eng");
api->SetImage(image);
api->Recognize(0);
tesseract::ResultIterator* ri = api->GetIterator();
tesseract::PageIteratorLevel level = tesseract::RIL_WORD;
if (ri != 0) {
do {
const char* word = ri->GetUTF8Text(level);
float conf = ri->Confidence(level);
int x1, y1, x2, y2;
ri->BoundingBox(level, &x1, &y1, &x2, &y2);
printf("word: '%s'; \tconf: %.2f; BoundingBox: %d,%d,%d,%d;\n",
word, conf, x1, y1, x2, y2);
delete[] word;
} while (ri->Next(level));
}
Of course there is posibility to use other PageiteratorLevel.
Orientation and script detection (OSD) example
const char* inputfile = "/usr/src/tesseract-3.02/eurotext.tif";
tesseract::Orientation orientation;
tesseract::WritingDirection direction;
tesseract::TextlineOrder order;
float deskew_angle;
PIX *image = pixRead(inputfile);
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
api->Init("/usr/src/tesseract-3.02/", "eng");
api->SetPageSegMode(tesseract::PSM_AUTO_OSD);
api->SetImage(image);
api->Recognize(0);
tesseract::PageIterator* it = api->AnalyseLayout();
it->Orientation(&orientation, &direction, &order, &deskew_angle);
printf("Orientation: %d;\nWritingDirection: %d\nTextlineOrder: %d\n" \
"Deskew angle: %.4f\n",
orientation, direction, order, deskew_angle);
Explanation for result codes are in publictypes.h
Example of iterator over the classifier choices for a single symbol
Pix *image = pixRead("/usr/src/tesseract-3.02/phototest.tif");
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
api->Init(NULL, "eng");
api->SetImage(image);
api->SetVariable("save_blob_choices", "T");
api->SetRectangle(37, 228, 548, 31);
api->Recognize(NULL);
tesseract::ResultIterator* ri = api->GetIterator();
tesseract::PageIteratorLevel level = tesseract::RIL_SYMBOL;
if(ri != 0) {
do {
const char* symbol = ri->GetUTF8Text(level);
float conf = ri->Confidence(level);
if(symbol != 0) {
printf("symbol %s, conf: %f", symbol, conf);
bool indent = false;
tesseract::ChoiceIterator ci(*ri);
do {
if (indent) printf("\t\t ");
printf("\t- ");
const char* choice = ci.GetUTF8Text();
printf("%s conf: %f\n", choice, ci.Confidence());
indent = true;
} while(ci.Next());
}
printf("---------------------------------------------\n");
delete[] symbol;
} while((ri->Next(level)));
}
Compiling C++ API programs on Linux
Including and linking to Tesseract's API is done in a standard Linux way. To compile a basic program against the API, you can use a command like this:
g++ -o myprogram myprogram.cpp -llept -ltesseract
If Tesseract is installed in an unusual place, you can specify the include and lib directories explicitly with g++'s -I and -L flags, like this:
g++ -o myprogram myprogram.cpp -I/home/nick/local/include/tesseract -L/home/nick/local/lib -llept -ltesseract
C-API in python
Tesseract-ocr from version 3.02.02 provide C-API. This enable to use tesseract-ocr shared library in python (and other languages that can use C libraries):
import os
import ctypes
lang = "eng"
filename = "/usr/src/tesseract-ocr/phototest.tif"
libname = "/usr/local/lib64/libtesseract.so.3"
TESSDATA_PREFIX = os.environ.get('TESSDATA_PREFIX')
if not TESSDATA_PREFIX:
TESSDATA_PREFIX = "../"
tesseract = ctypes.cdll.LoadLibrary(libname)
tesseract.TessVersion.restype = ctypes.c_char_p
tesseract_version = tesseract.TessVersion()
api = tesseract.TessBaseAPICreate()
rc = tesseract.TessBaseAPIInit3(api, TESSDATA_PREFIX, lang)
if (rc):
tesseract.TessBaseAPIDelete(api)
print("Could not initialize tesseract.\n")
exit(3)
text_out = tesseract.TessBaseAPIProcessPages(api, filename, None, 0)
result_text = ctypes.string_at(text_out)
print 'Tesseract-ocr version', tesseract_version
print result_text
Example of passing python file object to C-API can be found at pastebin.
Example using the C-API in a C program
The C-API can of course also be used by regular C programs, as in this very basic example.
#include <stdio.h>
#include <allheaders.h>
#include <capi.h>
void die(const char *errstr) {
fputs(errstr, stderr);
exit(1);
}
int main(int argc, char *argv[]) {
TessBaseAPI *handle;
PIX *img;
char *text;
if((img = pixRead("img.png")) == NULL)
die("Error reading image\n");
handle = TessBaseAPICreate();
if(TessBaseAPIInit3(handle, NULL, "eng") != 0)
die("Error initialising tesseract\n");
TessBaseAPISetImage2(handle, img);
if(TessBaseAPIRecognize(handle, NULL) != 0)
die("Error in Tesseract recognition\n");
if((text = TessBaseAPIGetUTF8Text(handle)) == NULL)
die("Error getting text\n");
fputs(text, stdout);
TessDeleteText(text);
TessBaseAPIEnd(handle);
TessBaseAPIDelete(handle);
pixDestroy(&img);
return 0;
}
On Linux you can compile it as you would build a program using the C++ API.
