Merge pull request #18001 from Yosshi999:sift-8bit-descr

* 8-bit SIFT descriptors

* use clearer parameter

* update docs

* propagate type info

* overload function for avoiding ABI-break

* bugfix: some values are undefined when CV_SIMD is absent

modules/features2d/test/test_sift.cpp

@@ -0,0 +1,34 @@
+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html
+
+#include "test_precomp.hpp"
+
+namespace opencv_test { namespace {
+
+TEST(Features2d_SIFT, descriptor_type)
+{
+    Mat image = imread(cvtest::findDataFile("features2d/tsukuba.png"));
+    ASSERT_FALSE(image.empty());
+
+    Mat gray;
+    cvtColor(image, gray, COLOR_BGR2GRAY);
+
+    vector<KeyPoint> keypoints;
+    Mat descriptorsFloat, descriptorsUchar;
+    Ptr<SIFT> siftFloat = cv::SIFT::create(0, 3, 0.04, 10, 1.6, CV_32F);
+    siftFloat->detectAndCompute(gray, Mat(), keypoints, descriptorsFloat, false);
+    ASSERT_EQ(descriptorsFloat.type(), CV_32F) << "type mismatch";
+
+    Ptr<SIFT> siftUchar = cv::SIFT::create(0, 3, 0.04, 10, 1.6, CV_8U);
+    siftUchar->detectAndCompute(gray, Mat(), keypoints, descriptorsUchar, false);
+    ASSERT_EQ(descriptorsUchar.type(), CV_8U) << "type mismatch";
+
+    Mat descriptorsFloat2;
+    descriptorsUchar.assignTo(descriptorsFloat2, CV_32F);
+    Mat diff = descriptorsFloat != descriptorsFloat2;
+    ASSERT_EQ(countNonZero(diff), 0) << "descriptors are not identical";
+}
+
+
+}} // namespace