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    <p>Its an interesting question. I am seeing the same problem. I posted a bug and got a reply. <a href="http://code.opencv.org/issues/1748" rel="noreferrer">http://code.opencv.org/issues/1748</a></p> <p>The solution is simple. The dst array is undefined on creation and the operation changes only those destination array pixels p, for which mask(p)!=0.</p> <p>So the only mechanism that works is to premake dst before addition. I.e.</p> <pre><code>dst = np.zeros(...) dst = cv2.add(a, a, dst=dst, mask=mask) </code></pre> <p>The next release will clear newly created images in operations such as cv2.add, cv2.subtract, cv2.bitwise_and/or/xor - so it will work without problem.</p> <p>my code looks like:</p> <pre><code>import cv2 import numpy as np import time a = np.ones((2,2,3), dtype=np.uint8) print "simple add" t = time.time() for i in range(10000): b = cv2.add(a,a) print "%5.4f seconds" % (time.time()-t) print b print "\nnumpy add" t = time.time() for i in range(10000): b = a+a print "%5.4f seconds" % (time.time()-t) print b # make mask same dimensions but 1 byte deep(not three) mask = np.zeros(a.shape[:-1], dtype=np.uint8) mask[1,1] = 255 print "\nmask", mask.shape print mask print "\nmasked add - uninitialised" t = time.time() for i in range(10000): b = cv2.add(a,a,mask=mask) print "%5.4f seconds" % (time.time()-t) print b print "uninitialised entries are unmodified - so random.\n Inconsistent when run more than once." print "same calc a second time..." b = cv2.add(a,a,mask=mask) print b print "\nmasked add - using preinitialised dst" t = time.time() b = a.copy() for i in range(10000): b = cv2.add(a,a,b,mask=mask) print "%5.4f seconds" % (time.time()-t) print b print "Consistent when run more than once." print "same calc a second time..." b = a.copy() b = cv2.add(a,a,b,mask=mask) print b </code></pre> <p>FYI: timings (10k repeats):</p> <pre><code>cv2.add - no mask 0.0120 seconds cv2.add - with mask 0.0160 seconds np.add 0.0190 seconds cv2.add - uninitialised mask 0.0220 seconds </code></pre> <p>FYI: Submit bugs following instructions here: <a href="http://code.opencv.org/projects/OpenCV/wiki/WikiStart" rel="noreferrer">http://code.opencv.org/projects/OpenCV/wiki/WikiStart</a></p>
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    1. COedited and updated with bug report response.
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