Mailman 3 May 2012 - scikit-image

Re: Bizarre error
by Stéfan van der Walt 01 May '12

01 May '12

On Mon, Apr 30, 2012 at 8:51 PM, Tony Yu <tsyu80(a)gmail.com> wrote: > Actually I'm not too surprised that you can't reproduce this since you never > got a failure with the SSIM tests. But, yes, this is very strange. Good point :) Do you have any more information on your system? Linux, 64-bit? Standard processor? NumPy compiled from source? Stéfan

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Re: Bizarre error
by Tony Yu 30 Apr '12

30 Apr '12

On Mon, Apr 30, 2012 at 11:43 PM, Stéfan van der Walt <stefan(a)sun.ac.za>wrote: > Hey Tony > > On Mon, Apr 30, 2012 at 8:01 PM, Tony Yu <tsyu80(a)gmail.com> wrote: > > Can anyone reproduce this? (Script below will print an array when it > fails; > > otherwise it exits without printing anything) > > I've run this script a few times, and I don't see anything. Very strange! > > Stéfan > Actually I'm not too surprised that you can't reproduce this since you never got a failure with the SSIM tests. But, yes, this is very strange. -Tony

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Re: Bizarre error
by Stéfan van der Walt 30 Apr '12

30 Apr '12

Hey Tony On Mon, Apr 30, 2012 at 8:01 PM, Tony Yu <tsyu80(a)gmail.com> wrote: > Can anyone reproduce this? (Script below will print an array when it fails; > otherwise it exits without printing anything) I've run this script a few times, and I don't see anything. Very strange! Stéfan

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Bizarre error
by Tony Yu 30 Apr '12

30 Apr '12

Sorry for the cryptic title, but I don't know a better way to describe it. While I was playing around with the PR to add a structural similarity function <https://github.com/scikits-image/scikits-image/pull/119>, I found that summing over an array doesn't give consistent results on my system (example script below). The example script is pulled from the structural similarity function. The test is run in a loop because the error appears randomly---running the test with the *exact* same arrays will fail on one attempt but not another. Random observations: - the failure is more likely to occur with large arrays - the failure *does not occur* without the subtraction of `ux` (see script) - this seems like some sort of memory alignment error (maybe something to do with `as_strides`, which is used by view_as_windows) Can anyone reproduce this? (Script below will print an array when it fails; otherwise it exits without printing anything) I know this is probably more of numpy issue (if it's reproducible), but I want to try to reproduce it before rewriting the example without calling `view_as_windows`. Thanks, -Tony #~~~ # requires somewhat-recent master because of call to view_as_windows. import sys import numpy as np from skimage.util.shape import view_as_windows def test_func(X, Y, win_size=7): XW = view_as_windows(X, (win_size, win_size)) ux = np.mean(np.mean(XW, axis=2), axis=2) XWM = XW - ux[..., None, None] # Repeat *exact same calculation* and check that they give the same value. vx1 = np.sum(np.sum(XWM, axis=2), axis=2) vx2 = np.sum(np.sum(XWM, axis=2), axis=2) if not np.allclose(vx1, vx2): diff = vx1 - vx2 print diff[np.nonzero(diff)] sys.exit() X = np.random.uniform(size=(30, 30)) Y = np.random.uniform(size=(30, 30)) for i in xrange(100): test_func(X, Y)

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