Merge pull request #23108 from crackwitz:issue-23107

Usage of imread(): magic number 0, unchecked result

* docs: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()

* samples, apps: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()

* tests: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()

* doc/py_tutorials: check imread() result

doc/py_tutorials/py_core/py_basic_ops/py_basic_ops.markdown

@@ -25,6 +25,7 @@ Let's load a color image first:
 >>> import cv2 as cv
 
 >>> img = cv.imread('messi5.jpg')
+>>> assert img is not None, "file could not be read, check with os.path.exists()"
 @endcode
 You can access a pixel value by its row and column coordinates. For BGR image, it returns an array
 of Blue, Green, Red values. For grayscale image, just corresponding intensity is returned.
@@ -173,6 +174,7 @@ from matplotlib import pyplot as plt
 BLUE = [255,0,0]
 
 img1 = cv.imread('opencv-logo.png')
+assert img1 is not None, "file could not be read, check with os.path.exists()"
 
 replicate = cv.copyMakeBorder(img1,10,10,10,10,cv.BORDER_REPLICATE)
 reflect = cv.copyMakeBorder(img1,10,10,10,10,cv.BORDER_REFLECT)

doc/py_tutorials/py_core/py_image_arithmetics/py_image_arithmetics.markdown

@@ -50,6 +50,8 @@ Here \f$\gamma\f$ is taken as zero.
 @code{.py}
 img1 = cv.imread('ml.png')
 img2 = cv.imread('opencv-logo.png')
+assert img1 is not None, "file could not be read, check with os.path.exists()"
+assert img2 is not None, "file could not be read, check with os.path.exists()"
 
 dst = cv.addWeighted(img1,0.7,img2,0.3,0)
 
@@ -76,6 +78,8 @@ bitwise operations as shown below:
 # Load two images
 img1 = cv.imread('messi5.jpg')
 img2 = cv.imread('opencv-logo-white.png')
+assert img1 is not None, "file could not be read, check with os.path.exists()"
+assert img2 is not None, "file could not be read, check with os.path.exists()"
 
 # I want to put logo on top-left corner, So I create a ROI
 rows,cols,channels = img2.shape

doc/py_tutorials/py_core/py_optimization/py_optimization.markdown

@@ -37,6 +37,7 @@ of odd sizes ranging from 5 to 49. (Don't worry about what the result will look
 goal):
 @code{.py}
 img1 = cv.imread('messi5.jpg')
+assert img1 is not None, "file could not be read, check with os.path.exists()"
 
 e1 = cv.getTickCount()
 for i in range(5,49,2):