Hi Victor,

The biggest problem is that you’re getting bitten by datatypes. Please read the following document:

Specifically, rank_mean is a uint8 image, so only contains integers between 0 and 255. naive_convolve is a float image, with continuous values between 0 and 255. Try this:

In : naive_convolve_float = naive_convolve / 255

In : rank_mean_float = rank_mean / 255

In : plt.imshow(np.abs(naive_convolve_float - rank_mean_float), cmap='magma')
Out: <matplotlib.image.AxesImage at 0x10d098fd0>

Result: There is a smaller difference also. Internally, filters.rank uses a fancy rolling histogram algorithm with integer data values. This means that the result of the rank_mean is only approximately accurate, essentially to within integer rounding (good enough for most real-world uses), while the convolve2d code gives you an exact value (to within floating point error).

Hope this helps!

Juan.

On 2 Nov 2017, 7:54 PM +1100, Poughon Victor <Victor.Poughon@cnes.fr>, wrote:
Hello,

I looks like skimage.filters.rank.mean and scipy.signal.convolve2d don't output exactly the same images. When doing:

image = data.coins()
K = np.ones((11, 11))

rank_mean = rank.mean(image, selem=K)
naive_convolve = convolve2d(image, K, mode="same") / K.sum()

All output pixel are different, with an absolute difference varying randomly between 0 and 1. Of course there's also a massive difference at the border, but that's expected because convolve2d treats image boundaries differently. But even in the center of the image all pixels are different. I've made a test script with an illustrated output image, you can check it out in this gist:

https://gist.github.com/vpoughon/b4afc76ce5dc681fda9d0550d41359d3

Am I doing something wrong?

Thanks,

Victor Poughon

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