Kifu: Go game record (kifu) generator
Goban and Grid Detection:
1. The input image is filtered and thresholded and a big quadrilateral shape is selected, sorting contours detected with cvFindContours() from OpenCV.
2. The selection is rectified with cvGetPerspectiveTransform() and cvWarpPerspective().
3. cvHoughLines2() find lines in the rectified selection and we compute the intersections for each detected line segment.
4. The intersection is first validated when the angle formed by the intersection point and the line extremities is near 90 degrees; and other detected intersections that are less distant than ~92% of the expected grid spacing are discarded. Finally each 'cell' or group is validated when the 4 angles for the 4 intersections are around 90 degrees.
5. Intersections can be mapped back to the pre-warped coordinates and vote for the original point, and we restart at step 2 with the quadrilateral found with the next threshold value.
6. The grid map is built using original point vote results, missing intersections can be evaluated mirroring relative polar coordinates.
7. Finally, we can validate the grid, or restart at step 1 with another threshold, or rectify the image again with the exact grid corners coordinates known.
Image change detection:
Using vertical and horizontal RGB components sums of a thresholded difference between a reference image and the current one,
it is easy to compute the image coordinates of a played stone
Stone detection:
When a stone is played it overlaps 1 grid square on a corner, 2 on the borders and 4 in the center.
Computing horizontal and vertical pixel sums for each grid cell or around each intersection can tell where the stone is played and reveal the color of the stone, being darker or brighter than the empty intersection region.
The intersection can also be seen on the difference image when the stone is white, that could also be used to detect the stone color.
Methods for mapping the coordinates:
"2.2 Perspective transformation with two vanishing points"
(pages 2 and 3, equations 7 and 10)
http://cipa.icomos.org/fileadmin/papers/potsdam/2001-21-gf01a.pdf
Inverse homography and plane image rectification (page 14)
http://www-prima.imag.fr/jlc/Courses/2002/DEA-IVR.VO/DEA-IVR.VO.S2.pdf
"Inferring Projective Mappings" (page 3)
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.9.7803
with related java and C source code here: http://www.developpez.net/forums/showthread.php?t=591698
Links:
http://www.sourceforge.net/projects/kifu
http://opencvlibrary.sourceforge.net/
Photointerpretation and Small Scale Stereoplotting with Digitally Rectified Photographs with Geometrical constraints:
http://cipa.icomos.org/fileadmin/papers/potsdam/2001-21-gf01a.pdf
Vision par Ordinateur:
http://www-prima.imag.fr/jlc/Courses/2002/DEA-IVR.VO/DEA-IVR.VO.S2.pdf
Projective Mappings for Image Warping:
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.9.7803
http://sciences.ch/htmlfr/geometrie/geometrieprojective01.php
