Robust card detection/persecutive correction OpenCV

Question

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

Answer 1

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.