QRDetector: tune distance and angle checks for FinderPatternSets

axxel · axxel · commit 3787aca92c25 · 2025-05-20T10:34:07.000+02:00
Turned out the scaling of the squaredDistance with the square of the finder pattern size ratio was skewing the cosine calculation too much, which could lead to accepting degenerate triangles like a,b,c on a line. This could cause the alignment pattern detection to find the same points for different alignment patterns and that in turn could result in RegressionLine creations with two identical points, which caused an assert to fail (see second issue in #951).
diff --git a/core/src/qrcode/QRDetector.cpp b/core/src/qrcode/QRDetector.cpp
@@ -113,14 +113,16 @@ FinderPatternSets GenerateFinderPatternSets(FinderPatterns& patterns)
 
 	auto sets            = std::multimap<double, FinderPatternSet>();
 	auto squaredDistance = [](const auto* a, const auto* b) {
-		// The scaling of the distance by the b/a size ratio is a very coarse compensation for the shortening effect of
+		// The scaling of the distance based on the b/a size ratio is a very coarse compensation for the shortening effect of
 		// the camera projection on slanted symbols. The fact that the size of the finder pattern is proportional to the
 		// distance from the camera is used here. This approximation only works if a < b < 2*a (see below).
 		// Test image: fix-finderpattern-order.jpg
-		return dot((*a - *b), (*a - *b)) * std::pow(double(b->size) / a->size, 2);
+		// Originally, I scaled the squaredDistance with the (b/a)^2 ratio but that could skew the cosine calculation
+		// below too much, resulting in the acceptance of degenerate triangles (a, b and c on a line).
+		return dot((*a - *b), (*a - *b)) * double(b->size) / a->size;
 	};
-	const double cosUpper = std::cos(45. / 180 * 3.1415); // TODO: use c++20 std::numbers::pi_v
-	const double cosLower = std::cos(135. / 180 * 3.1415);
+	const double cosUpper = std::cos(60. / 180 * 3.1415); // TODO: use c++20 std::numbers::pi_v
+	const double cosLower = std::cos(120. / 180 * 3.1415);
 
 	int nbPatterns = Size(patterns);
 	for (int i = 0; i < nbPatterns - 2; i++) {
@@ -162,7 +164,7 @@ FinderPatternSets GenerateFinderPatternSets(FinderPatterns& patterns)
 					moduleCount < 21 * 0.9 || moduleCount > 177 * 1.5) // moduleCount may be overestimated, see above
 					continue;
 
-				// Make sure the angle between AB and BC does not deviate from 90° by more than 45°
+				// Make sure the angle between AB and BC does not deviate from 90° too much
 				auto cosAB_BC = (distAB2 + distBC2 - distAC2) / (2 * distAB * distBC);
 				if (std::isnan(cosAB_BC) || cosAB_BC > cosUpper || cosAB_BC < cosLower)
 					continue;
