Cropping out an image from an existing image

Question

I would like to crop out an image from an existing image. I've taken an image and applied monochrome on it with threshold 98% using imagemagick (is this doable in openCV?)

The resulting image is this:

enter image description here

Now from this Image I would like to crop out another image so that the final image looks like this:

enter image description here

Question How can I do this in OpenCV? Note, the only reason I want to crop the image is so that I can use this answer to get the part of the text. If there is no need to crop out a new image and instead just concentrate on black part of the image to begin with, that would be great.

It seems that you are trying to resize an image instead of crop it, is that correct? — Boyko Perfanov, Apr 01 '13 at 13:52
@perfanoff birdy wants to crop it. Stack Overflow is resizing the images. [Before](http://i.stack.imgur.com/MKcXt.png). [After](http://i.stack.imgur.com/hMTnv.png). — Drew Dormann, Apr 01 '13 at 14:06
@perfanoff that is correct. thanks for clarification. I am not trying to resize the image. — birdy, Apr 01 '13 at 14:12

score 7 · Accepted Answer · edited May 23 '17 at 10:27

If the text at the top and at the bottom are the regions that you want to crop out, if they are always at the same location the solution is easy: just set a ROI that ignores those areas:

#include <cv.h>
#include <highgui.h>

int main(int argc, char* argv[])
{
    cv::Mat img = cv::imread(argv[1]);
    if (img.empty())
    {
        std::cout << "!!! imread() failed to open target image" << std::endl;
        return -1;        
    }

    /* Set Region of Interest */

    int offset_x = 129;
    int offset_y = 129;

    cv::Rect roi;
    roi.x = offset_x;
    roi.y = offset_y;
    roi.width = img.size().width - (offset_x*2);
    roi.height = img.size().height - (offset_y*2);

    /* Crop the original image to the defined ROI */

    cv::Mat crop = img(roi);
    cv::imshow("crop", crop);
    cv::waitKey(0);

    cv::imwrite("noises_cropped.png", crop);

    return 0;
}

Output image:

If the position of the black rectangle, which is your area of interest, is not present on a fixed location then you might want to check out another approach: use the rectangle detection technique:

On the output above, the area you are interested will be 2nd largest rectangle in the image.

On a side note, if you plan to isolate the text later, a simple cv::erode() could remove all the noises in that image so you are left with the white box & text. Another technique to remove noises is to use cv::medianBlur().You can also explore cv::morphologyEx() to do that trick:

cv::Mat kernel = cv::getStructuringElement(cv::MORPH_ELLIPSE, cv::Size(7, 7), cv::Point(3, 3));
cv::morphologyEx(src, src, cv::MORPH_ELLIPSE, kernel);

A proper solution might even be a combination of these 3. I've demonstrated a little bit of that on Extract hand bones from X-ray image.

The title will always be around the same location. Meaning it will always be on top of the image. How can I set the ROI that ignores the top? I'm new to opencv...is there a sample program that shows ignoring some ROI of the image based on color... — birdy, Apr 01 '13 at 17:53
ok thanks. I tried the `morphologyEx` technique and got this image: http://imageshack.us/a/img16/5506/screenshot20130401at203.png now i guess i need to figure out the coordinates where the big rectangle starts... — birdy, Apr 01 '13 at 18:06
The rectangle detection technique is exactly what I need since the rectangle location won't always be the same. I will use that answer to come up with a solution. thanks — birdy, Apr 01 '13 at 18:26
Is there something that can be done in c++ to avoid typing `cv::` in front of all variables and rather just type `Mat Point ` etc. — birdy, Apr 01 '13 at 19:11
Yes, after the `#include` section, add: `using namespace cv;` — karlphillip, Apr 01 '13 at 21:03

score 1 · Answer 2 · answered Apr 01 '13 at 17:04

A simple solution: scan lines from top down, bottom up, left-right and right-left. Terminate when the number of dark pixels in the line exceeds 50% of the total amount of pixels in the line. This will give you the xmin, xmax, ymin, ymax coordinates to bound your cropping rectangle.

Cropping out an image from an existing image

2 Answers2

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