Ggtcf

enter image description hereI have a scanned copy of a document as an image submitted by the user, it covers only 40% of the paper's height. I want to crop only that part, how to achieve this.
It is not necessary that the required form will always be on the top of the paper, it can be anywhere and the rest is blank white paper, how to crop that part?

The scanned copy I have got using scanner made in python only, so it has little black dots in the page.

You can consider the steps below to crop the blank or the none blank part:

cv::namedWindow("result", cv::WINDOW_FREERATIO);
cv::Mat img = cv::imread(R"(xbNQF.png)"); // read the image

// main code starts from here
cv::Mat gray; // convert the image to gray and put the result in gray mat
cv::cvtColor(img, gray, cv::COLOR_BGR2GRAY); // img -> gray
// threshold the gray image to remove the noise and put the result again in gray image
// it will convert all the background to black and all the text and fields to white
cv::threshold(gray, gray, 150, 255, cv::THRESH_BINARY_INV);

// now enlage the text or the inpout text fields
cv::dilate(gray, gray, cv::getStructuringElement(cv::MORPH_RECT, cv::Size(15, 3)));
// now clean the image, remove unwanted small pixels
cv::erode(gray, gray, cv::getStructuringElement(cv::MORPH_RECT, cv::Size(3, 3)));

// find all non zero to get the max y
cv::Mat idx; // the findNonZero() function will put the result in this mat
cv::findNonZero(gray, idx); // pass the mat idx to the function
// now iterate throgh the idx to find the max y
double maxY = 0; // this will keep the max y, init value is 0
for (int i=0; i<idx.rows; ++i) 
 cv::Point pnt = idx.at<cv::Point>(i);
 if (pnt.y > maxY) // if this Y is greater than the last Y, copy it to the last Y
 maxY = pnt.y; // this
 


// crop the none blank (upper) part
// NOTE: from this point you can also crop the blank part
// (0,0) means start form left-top, (gray.cols, int(maxY+5)) means 
// whidth the same as the original image, and the height is
// the maxY + 5, 5 here means let give some margin the cropped image
// if you don't want, then you can delete it.
cv::Mat submat = img(cv::Rect(0, 0, gray.cols, int(maxY+5)));
cv::imshow("result", submat);

cv::waitKey();

And it is the result:

Hope it helps!

Update:
If you interest to all min, max of the (x, y), then search like this:

double maxX = 0, minX = std::numeric_limits<double>::max();
double maxY = 0, minY = std::numeric_limits<double>::max();
for (int i=0; i<idx.rows; ++i) 
 cv::Point pnt = idx.at<cv::Point>(i);
 if (pnt.x > maxX) 
 maxX = pnt.x;
 
 if (pnt.x < minX) 
 minX = pnt.x;
 
 if (pnt.y > maxY) 
 maxY = pnt.y;
 
 if (pnt.y < minY) 
 minY = pnt.y;
 

So then you can crop anywhere of the image once you have these points.