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Change pixel x,y location with mathematical function

asked 2016-09-09 09:47:39 -0500

n.argirop gravatar image

As the title suggests, I have an image, whose pixels I want to move based on a mathematical function. The code I use right now is consisted of two loops. And because of the nested loop, the process takes forever, -to be exact more than two minutes most of the times-, having in mind that each image has 12000 pixels to run through the loops. Is there another way to do this? I looked through the documentation but couldn't find anything.

imgcor = np.zeros(img.shape, dtype=img.dtype)
                for f in range(rowc):
                    for k in range(colc):
                        offX = k + (f*b*c*(math.sin(math.radians(a))))
                        offY = f + (f*b*d*(math.cos(math.radians(a))))
                        imgcor[f, k] = img[int(offY)%rowc, int(offX)%colc]

The a,b,c,d values are certain parameters that are assigned earlier in the script.

P.S. I am using Python2.7 and opencv 2.4

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Normally an algorithm like this should be very fast. If you have constant parameters, try to precompute the most you can: m=b*math.sin(math.radians(a)), and then offX=k+(f*c*m). I think the sinus function takes the most time in your function.

kbarni gravatar imagekbarni ( 2016-09-09 11:13:26 -0500 )edit

@kbarni Thanks for the reply! I thought about after I posted my question and tried it but the differences are -if not zero- at least very small (maybe less than a second).

n.argirop gravatar imagen.argirop ( 2016-09-09 12:21:50 -0500 )edit

That's strange.

I implemented a similar method for optical corrections (two nested for loops) in C++. It has more complex operations to get the pixel coordinates.

I benchmarked it, it can process a huge 80 megapixel RGB photo in 250ms (1/4s) on my laptop.

Anyway, for further optimisations, you can parallelize the outer loop (I don't know how it's done in Python). My parallelized function processed the same image in 150ms (1/7s).

kbarni gravatar imagekbarni ( 2016-09-12 08:47:59 -0500 )edit

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answered 2016-09-09 13:35:35 -0500

Have a look at cv::remap

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Asked: 2016-09-09 09:47:39 -0500

Seen: 157 times

Last updated: Sep 09 '16