I currently have a method for detecting a card in an image and for the most part it works when the lighting is fairly consistent and the background is very calm.
Here is
The art of image processing is (in my 10+ years experience) just that: an art. No single answer exists, and there is always more than one way to do it. And it will definitely fail in some cases.
In my experience of working on automatically detecting features in medical images, it takes a long time to build to reliable algorithm, but in hindsight the best result is obtained with a relative simple algorithm. However, it takes a lot of time to get to this simple algorithm.
To get to this, the general approach is always the same:
That having said, I think that you need to answer these questions during the tuning of the algorithm:
These will make the requirements and assumptions on the image to acquire. Assumptions on which you can rely are very strong: they make the algorithm fast, robust and simple if you choose the right ones. Also let these requirements and assumptions be part of the testing database.
So what would I choose? Based on the three images you provided I would start with something like:
The algorithm then would look like:
cornerHarris
.Try to build parallelograms with one corner per each quadrant by combining points from each quadrant. Create a fitness function which gives higher score to:
This fitness function gives a lot of tuning possibilities later on.
Return the parallelogram with the highest score.
So why using corner-detection instead of a hough-transform to do line detection? In my opinion the hough-transform is (next to being slow) quite sensitive to patterns in the background (which is what you see in your first image -- it detects a stronger line in the background then of the card), and it cannot handle a little curved lines that well, unless you use a larger bin size which will worsen the detection.
Good luck!
A more general approach would definitely be something like Rutger Nijlunsing suggested in his answer. However, in your case, at least for the provided sample images, a very simple approach like morphological opening followed by thresholding, contour processing and convexhull would yield the result you want. Use a scaled down version of the images for processing so that you don't have to use a large kernel for morphological operations. Below are the images processed this way.
pyrDown(large, rgb0);
pyrDown(rgb0, rgb0);
pyrDown(rgb0, rgb0);
Mat small;
cvtColor(rgb0, small, CV_BGR2GRAY);
Mat morph;
Mat kernel = getStructuringElement(MORPH_ELLIPSE, Size(11, 11));
morphologyEx(small, morph, MORPH_OPEN, kernel);
Mat bw;
threshold(morph, bw, 0, 255.0, CV_THRESH_BINARY | CV_THRESH_OTSU);
Mat bdry;
kernel = getStructuringElement(MORPH_ELLIPSE, Size(3, 3));
erode(bw, bdry, kernel);
subtract(bw, bdry, bdry);
// do contour processing on bdry
This approach will not work in general, so I would strongly recommend something like Rutger suggested.