Find outer checkerboard corners - opencv

Question

In the below picture, I have the 2D locations of the green points and I want to calculate the locations of the red points, or, as an intermediate step, I want to calculate the locations of the blue points. All in 2D.
Of course, I do not only want to find those locations for the picture above. In the end, I want an automated algorithm which takes a set of checkerboard corner points to calculate the outer corners.
I need the resulting coordinates to be as accurate as possible, so I think that I need a solution which does not only take the outer green points into account, but which also uses all the other green points' locations to calculate a best fit for the outer corners (red or blue).
If OpenCV can do this, please point me into that direction.

Answer 1

In general, if all you have is the detection of some, but not all, the inner corners, the problem cannot be solved. This is because the configuration is invariant to translation - shifting the physical checkerboard by whole squares would produce the same detected corner position on the image, but due to different physical corners.
Further, the configuration is also invariant to rotations by 180 deg in the checkerboard plane and, unless you are careful to distinguish between the colors of the squares adjacent each corner, to rotations by 90 deg and reflections with respect the center and the midlines.
This means that, in addition to detecting the corners, you need to extract from the image some features of the physical checkerboard that can be used to break the above invariance. The simplest break is to detect all 9 corners of one row and one column, or at least their end-corners. They can be used directly to rectify the image by imposing the condition that their lines be at 90 deg angle. However, this may turn out to be impossible due to occlusions or detector failure, and more sophisticated methods may be necessary.
For example, you can try to directly detect the chessboard edges, i.e. the fat black lines at the boundary. One way to do that, for example, would be to detect the letters and numbers nearby, and use those locations to constrain a line detector to nearby areas.
By the way, if the photo you posted is just a red herring, and you are interested in detecting general checkerboard-like patterns, and can control the kind of pattern, there are way more robust methods of doing it. My personal favorite is the "known 2D crossratios" pattern of Matsunaga and Kanatani.

Answer 2

I solved it robustly, but not accurately, with the following solution:
Find lines with at least 3 green points closely matching the line. (thin red lines in pic)
Keep bounding lines: From these lines, keep those with points only to one side of the line or very close to the line.
Filter bounding lines: From the bounding lines, take the 4 best ones/those with most points on them. (bold white lines in pic)
Calculate the intersections of the 4 remaining bounding lines (none of the lines are perfectly parallel, so this results in 6 intersections, of which we want only 4).
From the intersections, remove the one farthest from the average position of the intersections until only 4 of them are left.
That's the 4 blue points.
You can then feed these 4 points into OpenCV's findPerspectiveTransform function to find a perspective transform (aka a homography):
Point2f* srcPoints = (Point2f*) malloc(4 * sizeof(Point2f));
std::vector<Point2f> detectedCorners = CheckDet::getOuterCheckerboardCorners(srcImg);
for (int i = 0; i < MIN(4, detectedCorners.size()); i++) {
srcPoints[i] = detectedCorners[i];
}
Point2f* dstPoints = (Point2f*) malloc(4 * sizeof(Point2f));
int dstImgSize = 400;
dstPoints[0] = Point2f(dstImgSize * 1/8, dstImgSize * 1/8);
dstPoints[1] = Point2f(dstImgSize * 7/8, dstImgSize * 1/8);
dstPoints[2] = Point2f(dstImgSize * 7/8, dstImgSize * 7/8);
dstPoints[3] = Point2f(dstImgSize * 1/8, dstImgSize * 7/8);
Mat m = getPerspectiveTransform(srcPoints, dstPoints);
For our example image, the input and output of findPerspectiveTranform looks like this:
input
(349.1, 383.9) -> ( 50.0, 50.0)
(588.9, 243.3) -> (350.0, 50.0)
(787.9, 404.4) -> (350.0, 350.0)
(506.0, 593.1) -> ( 50.0, 350.0)
output
( 1.6 -1.1 -43.8 )
( 1.4 2.4 -1323.8 )
( 0.0 0.0 1.0 )
You can then transform the image's perspective to board coordinates:
Mat plainBoardImg;
warpPerspective(srcImg, plainBoardImg, m, Size(dstImgSize, dstImgSize));
Results in the following image:
For my project, the red points that you can see on the board in the question are not needed anymore, but I'm sure they can be calculated easily from the homography by inverting it and then using the inverse for back-tranforming the points (0, 0), (0, dstImgSize), (dstImgSize, dstImgSize), and (dstImgSize, 0).
The algorithm works surprisingly reliable, however, it does not use all the available information, because it uses only the outer points (those which are connected with the white lines). It does not use any data of the inner points for additional accuracy. I would still like to find an even better solution, which uses the data of the inner points.