converting cv::Mat for tesseract - c++

Question

I'm using OpenCV to extract a subimage of a scanned document and would like to use tesseract to perform OCR over this subimage.
I found out that I can use two methods for text recognition in tesseract, but so far I wasn't able to find a working solution.
A.) How can I convert a cv::Mat into a PIX*?
(PIX* is a datatype of leptonica)
Based on vasiles code below, this is essentially my current code:
cv::Mat image = cv::imread("c:/image.png");
cv::Mat subImage = image(cv::Rect(50, 200, 300, 100));
int depth;
if(subImage.depth() == CV_8U)
depth = 8;
//other cases not considered yet
PIX* pix = pixCreateHeader(subImage.size().width, subImage.size().height, depth);
pix->data = (l_uint32*) subImage.data;
tesseract::TessBaseAPI tess;
STRING text;
if(tess.ProcessPage(pix, 0, 0, &text))
{
std::cout << text.string();
}
While it doesn't crash or anything, the OCR result still is wrong. It should recognize one word of my sample image, but instead it returns some non-readable characters.
The method PIX_HEADER doesn't exist, so I used pixCreateHeader, but it doesn't take the number of channels as an argument. So how can I set the number of channels?
B.) How can I use cv::Mat for TesseractRect() ?
Tesseract offers another method for text recognition with this signature:
char * TessBaseAPI::TesseractRect (
const UINT8 * imagedata,
int bytes_per_pixel,
int bytes_per_line,
int left,
int top,
int width,
int height
)
Currently I am using the following code, but it also returns non-readable characters (although different ones than from the code above.
char* cr = tess.TesseractRect(
subImage.data,
subImage.channels(),
subImage.channels() * subImage.size().width,
0,
0,
subImage.size().width,
subImage.size().height);

Answer 1

tesseract::TessBaseAPI tess;
cv::Mat sub = image(cv::Rect(50, 200, 300, 100));
tess.SetImage((uchar*)sub.data, sub.size().width, sub.size().height, sub.channels(), sub.step1());
tess.Recognize(0);
const char* out = tess.GetUTF8Text();

Answer 2

For Anybody using the JavaCPP presets of OpenCV/Tesseract, here is what works
Mat img = imread("file.jpg");
Mat gray = new Mat();
cvtColor(img, gray, CV_BGR2GRAY);
// api is a Tesseract client which is initialised
api.SetImage(gray.data().asBuffer(),gray.size().width(),gray.size().height(),gray.channels(),gray.size1())

Answer 3

cv::Mat image = cv::imread(argv[1]);
cv::Mat gray;
cv::cvtColor(image, gray, CV_BGR2GRAY);
PIX *pixS = pixCreate(gray.size().width, gray.size().height, 8);
for(int i=0; i<gray.rows; i++)
for(int j=0; j<gray.cols; j++)
pixSetPixel(pixS, j,i, (l_uint32) gray.at<uchar>(i,j));

Answer 4

First, make a deep copy of your subImage, so that it will be stored in a coninuous memory block:
cv::Mat subImage = image(cv::Rect(50, 200, 300, 100)).clone();
Then, init a PIX headed (I don't know how) with the correct parameters.
// ???? Put your own constructor here.
PIX* pix = new PIX_HEADER(width, height, channels, depth);
OR, create it manually:
PIX pix;
pix.width = subImage.width;
...
Then set the pix data pointer to the subImage data pointer
pix.data = subImage.data;
Finally, make sure your subImage objects does not go out of scope before you finish your work with pix.