fixed #1764

modules/features2d/src/descriptors.cpp

@@ -134,49 +134,43 @@ static void convertBGRImageToOpponentColorSpace( const Mat& bgrImage, vector<Mat
 
     // Calculate the channels of the opponent color space
     {
-        // (R - G) / sqrt(2)
+        // (R - G)/sqrt(2), but converted to the destination data type
         MatConstIterator_<signed char> rIt = bgrChannels[2].begin<signed char>();
         MatConstIterator_<signed char> gIt = bgrChannels[1].begin<signed char>();
         MatIterator_<unsigned char> dstIt = opponentChannels[0].begin<unsigned char>();
-        float factor = 1.f / sqrt(2.f);
         for( ; dstIt != opponentChannels[0].end<unsigned char>(); ++rIt, ++gIt, ++dstIt )
         {
-            int value = static_cast<int>( static_cast<float>(static_cast<int>(*gIt)-static_cast<int>(*rIt)) * factor );
-            if( value < 0 ) value = 0;
-            if( value > 255 ) value = 255;
-            (*dstIt) = static_cast<unsigned char>(value);
+            float value = 0.5f * (static_cast<int>(*gIt) -
+            					  static_cast<int>(*rIt) + 255);
+            (*dstIt) = static_cast<unsigned char>(value + 0.5f);
         }
     }
     {
-        // (R + G - 2B)/sqrt(6)
+        // (R + G - 2B)/sqrt(6), but converted to the destination data type
         MatConstIterator_<signed char> rIt = bgrChannels[2].begin<signed char>();
         MatConstIterator_<signed char> gIt = bgrChannels[1].begin<signed char>();
         MatConstIterator_<signed char> bIt = bgrChannels[0].begin<signed char>();
         MatIterator_<unsigned char> dstIt = opponentChannels[1].begin<unsigned char>();
-        float factor = 1.f / sqrt(6.f);
         for( ; dstIt != opponentChannels[1].end<unsigned char>(); ++rIt, ++gIt, ++bIt, ++dstIt )
         {
-            int value = static_cast<int>( static_cast<float>(static_cast<int>(*rIt) + static_cast<int>(*gIt) - 2*static_cast<int>(*bIt)) *
-                                          factor );
-            if( value < 0 ) value = 0;
-            if( value > 255 ) value = 255;
-            (*dstIt) = static_cast<unsigned char>(value);
+            float value = 0.25f * (static_cast<int>(*rIt) + static_cast<int>(*gIt) -
+                                   2*static_cast<int>(*bIt) + 510);
+            (*dstIt) = static_cast<unsigned char>(value + 0.5f);
         }
     }
     {
-        // (R + G + B)/sqrt(3)
+        // (R + G + B)/sqrt(3), but converted to the destination data type
         MatConstIterator_<signed char> rIt = bgrChannels[2].begin<signed char>();
         MatConstIterator_<signed char> gIt = bgrChannels[1].begin<signed char>();
         MatConstIterator_<signed char> bIt = bgrChannels[0].begin<signed char>();
         MatIterator_<unsigned char> dstIt = opponentChannels[2].begin<unsigned char>();
-        float factor = 1.f / sqrt(3.f);
+        float factor = 1.f/3.f;
         for( ; dstIt != opponentChannels[2].end<unsigned char>(); ++rIt, ++gIt, ++bIt, ++dstIt )
         {
-            int value = static_cast<int>( static_cast<float>(static_cast<int>(*rIt) + static_cast<int>(*gIt) + static_cast<int>(*bIt)) *
-                                          factor );
-            if( value < 0 ) value = 0;
-            if( value > 255 ) value = 255;
-            (*dstIt) = static_cast<unsigned char>(value);
+            float value = factor * (static_cast<int>(*rIt) + 
+									static_cast<int>(*gIt) + 
+									static_cast<int>(*bIt));
+            (*dstIt) = static_cast<unsigned char>(value + 0.5f);
         }
     }
 }