Convert image document to black and white with OpenCV

Question

I'm new to OpenCV and image processing and I'M not sure how to solve my problem. I have a photo of document made in iPhone and I want to convert that document to black and white. I tried to use threshold but the text was not so good (a little blurry and unreadable). I'd like to text looks same as on the original image, only black, and background will be white. What can I do? P.S. When I made a photo of part of the document, where text is quite big, then result is ok.

I will be grateful for any help.

Here are the example image I use and the result:

Answer 1

My attemp, maybe a little more readable than yours:

IplImage * pRGBImg  = 0;
pRGBImg = cvLoadImage(input_file.c_str(), CV_LOAD_IMAGE_UNCHANGED);
if(!pRGBImg)
{   
    std::cout << "ERROR: Failed to load input image" << std::endl;
    return -1; 
}   

// Allocate the grayscale image
IplImage * pGrayImg = 0;
pGrayImg = cvCreateImage(cvSize(pRGBImg->width, pRGBImg->height), pRGBImg->depth, 1); 

// Convert it to grayscale
cvCvtColor(pRGBImg, pGrayImg, CV_RGB2GRAY);

// Dilate
cvDilate(pGrayImg, pGrayImg, 0, 0.2);

cvThreshold(pGrayImg, pGrayImg, 30, 255, CV_THRESH_BINARY | CV_THRESH_OTSU);
cvSmooth(pGrayImg, pGrayImg, CV_BLUR, 2, 2);

cvSaveImage("out.png", pGrayImg);

Answer 2

Threshold image is used for different purposes.

If u just want to convert it to b/w image just do this. USing openCV 2.2

cv::Mat image_name = cv::imread("fileName", 0);

the second parameter 0 tells to read a color image as b/w image.

And if you want to save as a b/w image file.

code this

cv::Mat image_name = cv::imread("fileName", 0);
cv::imwrite(image_name, "bw_filename.jpg");

Answer 3

Using Adaptive Gaussian Thresholding is a good idea here. This will also enhance the quality of text written in the image. You can do that by simply giving the command:

AdaptiveThreshold(src_Mat, dst_Mat, Max_value, Adaptive_Thresholding_Method, Thresholding_type, blocksize, C);