Merge pull request #12827 from hrnr:stitching_4

[evolution] Stitching for OpenCV 4.0

* stitching: wrap Stitcher::create for bindings

* provide method for consistent stitcher usage across languages

* samples: add python stitching sample

* port cpp stitching sample to python

* stitching: consolidate Stitcher create methods

* remove Stitcher::createDefault, it returns Stitcher, not Ptr<Stitcher> -> inconsistent API
* deprecate cv::createStitcher and cv::createStitcherScans in favor of Stitcher::create

* stitching: avoid anonymous enum in Stitcher

* ORIG_RESOL should be double
* add documentatiton

* stitching: improve documentation in Stitcher

* stitching: expose estimator in Stitcher

* remove ABI hack

* stitching: drop try_use_gpu flag

* OCL will be used automatically through T-API in OCL-enable paths
* CUDA won't be used unless user sets CUDA-enabled classes manually

* stitching: drop FeaturesFinder

* use Feature2D instead of FeaturesFinder
* interoperability with features2d module
* detach from dependency on xfeatures2d

* features2d: fix compute and detect to work with UMat vectors

* correctly pass UMats as UMats to allow OCL paths
* support vector of UMats as output arg

* stitching: use nearest interpolation for resizing masks

* fix warnings

samples/python/stitching.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+
+'''
+Stitching sample
+================
+
+Show how to use Stitcher API from python in a simple way to stitch panoramas
+or scans.
+'''
+
+from __future__ import print_function
+import cv2 as cv
+import numpy as np
+import argparse
+import sys
+
+modes = (cv.Stitcher_PANORAMA, cv.Stitcher_SCANS)
+
+parser = argparse.ArgumentParser(description='Stitching sample.')
+parser.add_argument('--mode',
+    type = int, choices = modes, default = cv.Stitcher_PANORAMA,
+    help = 'Determines configuration of stitcher. The default is `PANORAMA` (%d), '
+         'mode suitable for creating photo panoramas. Option `SCANS` (%d) is suitable '
+         'for stitching materials under affine transformation, such as scans.' % modes)
+parser.add_argument('--output', default = 'result.jpg',
+    help = 'Resulting image. The default is `result.jpg`.')
+parser.add_argument('img', nargs='+', help = 'input images')
+args = parser.parse_args()
+
+# read input images
+imgs = []
+for img_name in args.img:
+    img = cv.imread(img_name)
+    if img is None:
+        print("can't read image " + img_name)
+        sys.exit(-1)
+    imgs.append(img)
+
+stitcher = cv.Stitcher.create(args.mode)
+status, pano = stitcher.stitch(imgs)
+
+if status != cv.Stitcher_OK:
+    print("Can't stitch images, error code = %d" % status)
+    sys.exit(-1)
+
+cv.imwrite(args.output, pano);
+print("stitching completed successfully. %s saved!" % args.output)