Fixed issue in ORB detection if firstLevel property is set above 0

Author: terfendail

modules/features2d/include/opencv2/features2d.hpp

@@ -304,10 +304,11 @@ class CV_EXPORTS_W ORB : public Feature2D
     will mean that to cover certain scale range you will need more pyramid levels and so the speed
     will suffer.
     @param nlevels The number of pyramid levels. The smallest level will have linear size equal to
-    input_image_linear_size/pow(scaleFactor, nlevels).
+    input_image_linear_size/pow(scaleFactor, nlevels - firstLevel).
     @param edgeThreshold This is size of the border where the features are not detected. It should
     roughly match the patchSize parameter.
-    @param firstLevel It should be 0 in the current implementation.
+    @param firstLevel The level of pytramid to put source image to. Previous layers are filled
+    with upscaled source image.
     @param WTA_K The number of points that produce each element of the oriented BRIEF descriptor. The
     default value 2 means the BRIEF where we take a random point pair and compare their brightnesses,
     so we get 0/1 response. Other possible values are 3 and 4. For example, 3 means that we take 3

modules/features2d/src/orb.cpp

@@ -673,7 +673,7 @@ class ORB_Impl : public ORB
     void setEdgeThreshold(int edgeThreshold_) { edgeThreshold = edgeThreshold_; }
     int getEdgeThreshold() const { return edgeThreshold; }
 
-    void setFirstLevel(int firstLevel_) { firstLevel = firstLevel_; }
+    void setFirstLevel(int firstLevel_) { CV_Assert(firstLevel >= 0);  firstLevel = firstLevel_; }
     int getFirstLevel() const { return firstLevel; }
 
     void setWTA_K(int wta_k_) { wta_k = wta_k_; }
@@ -1014,7 +1014,7 @@ void ORB_Impl::detectAndCompute( InputArray _image, InputArray _mask,
 
     int level_dy = image.rows + border*2;
     Point level_ofs(0,0);
-    Size bufSize((image.cols + border*2 + 15) & -16, 0);
+    Size bufSize((cvRound(image.cols/getScale(0, firstLevel, scaleFactor)) + border*2 + 15) & -16, 0);
 
     for( level = 0; level < nLevels; level++ )
     {
@@ -1082,8 +1082,11 @@ void ORB_Impl::detectAndCompute( InputArray _image, InputArray _mask,
                 copyMakeBorder(mask, extMask, border, border, border, border,
                                BORDER_CONSTANT+BORDER_ISOLATED);
         }
-        prevImg = currImg;
-        prevMask = currMask;
+        if (level > firstLevel)
+        {
+            prevImg = currImg;
+            prevMask = currMask;
+        }
     }
 
     if( useOCL )
@@ -1194,6 +1197,7 @@ void ORB_Impl::detectAndCompute( InputArray _image, InputArray _mask,
 Ptr<ORB> ORB::create(int nfeatures, float scaleFactor, int nlevels, int edgeThreshold,
            int firstLevel, int wta_k, int scoreType, int patchSize, int fastThreshold)
 {
+    CV_Assert(firstLevel >= 0);
     return makePtr<ORB_Impl>(nfeatures, scaleFactor, nlevels, edgeThreshold,
                              firstLevel, wta_k, scoreType, patchSize, fastThreshold);
 }