Warning: Invalid resolution 0 dpi. Using 70 instead - opencv

Question

I know this theme already exists, but I didn't find any solution for this.
I am trying to detect characters from picture in this code below:
#include <tesseract/baseapi.h>
#include <leptonica/allheaders.h>
#include <opencv2/opencv.hpp>
#include <sstream>
#include <memory>
#include <iostream>
#define path "/home/jovan/Pictures/"
void resize(cv::Mat &img);
PIX *mat8ToPix(const cv::Mat *mat8);
cv::Mat pix8ToMat(PIX *pix8);
int main(int argc, char **argv)
{
// Load image
std::stringstream ss;
ss << path;
ss << argv[1];
cv::Mat im = cv::imread(ss.str() );
if (im.empty())
{
std::cout<<"Cannot open source image!" << std::endl;
return EXIT_FAILURE;
}
resize(im);
cv::Mat gray;
cv::cvtColor(im, gray, CV_BGR2GRAY);
// Pass it to Tesseract API
tesseract::TessBaseAPI tess;
tess.Init(NULL, "eng", tesseract::OEM_DEFAULT);
tess.SetPageSegMode(tesseract::PSM_SINGLE_BLOCK);
tess.SetVariable("tessedit_char_whitelist", "QWERTYUIOPASDFGHJKLZXCVBNM");
PIX *image = mat8ToPix(&im);
//tess.SetImage((uchar*)gray.data, gray.cols, gray.rows, 1, gray.cols);
tess.SetImage(image);
// Get the text
char* out = tess.GetUTF8Text();
if(out != nullptr)
std::cout << "here it is: "<< out << std::endl;
cv::imshow("image", im);
cv::imshow("gray", gray);
cv::waitKey();
return 0;
}
void resize(cv::Mat &img)
{
while(img.size().width >= 500 && img.size().height >= 500 )
cv::resize(img, img, cv::Size(img.size().width/2, img.size().height/2) );
}
PIX *mat8ToPix(const cv::Mat *mat8)
{
PIX *pixd = pixCreate(mat8->size().width, mat8->size().height, 8);
for(int y=0; y<mat8->rows; y++)
for(int x=0; x<mat8->cols; x++)
pixSetPixel(pixd, x, y, (l_uint32) mat8->at<uchar>(y,x));
return pixd;
}
cv::Mat pix8ToMat(PIX *pix8)
{
cv::Mat mat(cv::Size(pix8->w, pix8->h), CV_8UC1);
uint32_t *line = pix8->data;
for (uint32_t y = 0; y < pix8->h; ++y)
{
for (uint32_t x = 0; x < pix8->w; ++x)
mat.at<uchar>(y, x) = GET_DATA_BYTE(line, x);
line += pix8->wpl;
}
return mat;
}
whatever picture I put to process I get this on terminal:
$: Warning: Invalid resolution 0 dpi. Using 70 instead.
Does anyone have some solution?
Thanks in advance.

Answer 1

If you know the input image's resolution, you can call pixSetResolution on Leptonica Pix object.
Or use Tesseract API to pass in the value. See
Tess4j - Pdf to Tiff to tesseract - "Warning: Invalid resolution 0 dpi. Using 70 instead."

Answer 2

Maybe it helps: I used EMGU & C#, but I think it must be the same in C++:
ocr.SetVariable("user_defined_dpi", "70");
... and the message should disappear ;)

Answer 3

I had similar issue. Found out from here that dark background in the image is the problem. Inversion of the image colors worked.