From 160879c100baca7caedc44786cc17de52c2a472d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E6=AD=A6=E5=A3=AB=E9=A3=8E=E5=BA=A6=E7=9A=84=E7=89=9B?=
 <1085968073@qq.com>
Date: Wed, 24 Jul 2024 20:29:13 +0800
Subject: [PATCH] Merge pull request #25807 from spdfghi:4.x

Search in two directions when try to add new quad in addOuterQuad #25807

In ChessBoardDetector::addOuterQuad, previous code try to connect new quad with inner quad, if possible, but only search for one direction. I have made  three test images, one is normal(a.jpg), one lossed an outer quad(b.jpg), and then i flipped it vertically(c.jpg). Only last one fails. I fixed it by check two directions and row/col.

Here is the test code and images:
```
Mat img;
vector<Point2f> corners;
auto size = cv::Size(6, 6);
img = imread("D:/tmp/a.jpg", 0);
std::cout<<cv::findChessboardCorners(img, size, corners)<<"\n";
std::cout << corners.size() << "\n";
img = imread("D:/tmp/b.jpg", 0);
std::cout<<cv::findChessboardCorners(img, size, corners)<<"\n";
std::cout << corners.size() << "\n";
img = imread("D:/tmp/c.jpg", 0);
std::cout<<cv::findChessboardCorners(img, size, corners)<<"\n";
std::cout << corners.size() << "\n";
```
![a](https://github.com/opencv/opencv/assets/92856207/0dc7f5bf-7637-4333-9a9f-ec4ede790027)
a
![b](https://github.com/opencv/opencv/assets/92856207/39793485-ca0c-44c0-b44d-a593d36c1888)
b
![c](https://github.com/opencv/opencv/assets/92856207/2e7789c8-cfa5-438c-9530-2862a8a3741f)
c
---
 modules/calib3d/src/calibinit.cpp | 48 +++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 15 deletions(-)

diff --git a/modules/calib3d/src/calibinit.cpp b/modules/calib3d/src/calibinit.cpp
index 060ee5e158..1767c38981 100644
--- a/modules/calib3d/src/calibinit.cpp
+++ b/modules/calib3d/src/calibinit.cpp
@@ -1152,22 +1152,40 @@ int ChessBoardDetector::addOuterQuad(ChessBoardQuad& quad, std::vector<ChessBoar
             // have to set exact corner
             q.corners[j] = quad.corners[i];
 
-            // now find other neighbor and add it, if possible
-            int next_i = (i + 1) & 3;
-            int prev_i = (i + 3) & 3; // equal to (j + 1) & 3
-            ChessBoardQuad* quad_prev = quad.neighbors[prev_i];
-            if (quad_prev &&
-                quad_prev->ordered &&
-                quad_prev->neighbors[i] &&
-                quad_prev->neighbors[i]->ordered )
+            // set row and col for next step check
+            switch (i)
             {
-                ChessBoardQuad* qn = quad_prev->neighbors[i];
-                q.count = 2;
-                q.neighbors[prev_i] = qn;
-                qn->neighbors[next_i] = &q;
-                qn->count += 1;
-                // have to set exact corner
-                q.corners[prev_i] = qn->corners[next_i];
+            case 0:
+                q.col = quad.col - 1; q.row = quad.row - 1; break;
+            case 1:
+                q.col = quad.col + 1; q.row = quad.row - 1; break;
+            case 2:
+                q.col = quad.col + 1; q.row = quad.row - 1; break;
+            case 3:
+                q.col = quad.col - 1; q.row = quad.row + 1; break;
+            }
+
+            // now find other neighbor and add it, if possible
+            for (int k = 1; k <= 3; k += 2)
+            {
+                int next_i = (i + k) % 4;
+                int prev_i = (i + k + 2) % 4;
+                ChessBoardQuad* quad_prev = quad.neighbors[prev_i];
+                if (quad_prev &&
+                    quad_prev->ordered &&
+                    quad_prev->neighbors[i] &&
+                    quad_prev->neighbors[i]->ordered &&
+                    std::abs(quad_prev->neighbors[i]->col - q.col) == 1 &&
+                    std::abs(quad_prev->neighbors[i]->row - q.row) == 1)
+                {
+                    ChessBoardQuad* qn = quad_prev->neighbors[i];
+                    q.count = 2;
+                    q.neighbors[prev_i] = qn;
+                    qn->neighbors[next_i] = &q;
+                    qn->count += 1;
+                    // have to set exact corner
+                    q.corners[prev_i] = qn->corners[next_i];
+                }
             }
         }
     }