From 7c4b873ba316d90dd618e62ec5d157f1950ab80b Mon Sep 17 00:00:00 2001
From: Lukas-Alexander Weber
 <32765578+lukasalexanderweber@users.noreply.github.com>
Date: Wed, 24 Mar 2021 16:02:20 +0100
Subject: [PATCH] Merge pull request #19762 from lukasalexanderweber:master
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* Error Message for SURF if not implemented

In OpenCV 4.5.1

import cv2 as cv
cv.xfeatures2d_SURF.create

will not create an AttributeError, even if the function is excluded (no nonfree option)

In Line 305 (now 306) however ´finder = FEATURES_FIND_CHOICES[args.features]()´ will raise an

error: OpenCV(4.5.1) ..\opencv_contrib\modules\xfeatures2d\src\surf.cpp:1029: error: (-213:The function/feature is not implemented) This algorithm is patented and is excluded in this configuration; Set OPENCV_ENABLE_NONFREE CMake option and rebuild the library in function 'cv::xfeatures2d::SURF::create'

So we should check with cv.xfeatures2d_SURF.create() correctly if SURF is available
---
 samples/python/stitching_detailed.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/samples/python/stitching_detailed.py b/samples/python/stitching_detailed.py
index cd3f063e35..7bf5a9ac0d 100644
--- a/samples/python/stitching_detailed.py
+++ b/samples/python/stitching_detailed.py
@@ -29,8 +29,9 @@ BA_COST_CHOICES['no'] = cv.detail_NoBundleAdjuster
 
 FEATURES_FIND_CHOICES = OrderedDict()
 try:
+    cv.xfeatures2d_SURF.create() # check if the function can be called
     FEATURES_FIND_CHOICES['surf'] = cv.xfeatures2d_SURF.create
-except AttributeError:
+except (AttributeError, cv.error) as e:
     print("SURF not available")
 # if SURF not available, ORB is default
 FEATURES_FIND_CHOICES['orb'] = cv.ORB.create