Maybe it's worth mentioning that astropy has also done a bit of (incomplete) work on an nddata class that adds things like metadata, error and mask information to ndarray. I think it's a wrapper rather than a subclass. This might not be how you would do it for skimage but it's just FYI (not particularly astronomy specific)... http://docs.astropy.org/en/latest/nddata/index.html Just regarding "sampling" vs "resolution", IMHO the common use of resolution to describe pixel density in computing is somewhat erroneous. There isn't really a good substitute term for describing the intrinsic analogue (eg. optical) resolution of an image or waveform before it's digitized (bandwidth?) and (as someone else pointed out) the digitized sampling density/spacing doesn't necessarily tell you how much detail you have anyway. Cheers, JAmes. On 23/09/13 12:43, Thomas A Caswell wrote:

On Mon, Sep 23, 2013 at 2:24 AM, Almar Klein <almar.klein@gmail.com <mailto:almar.klein@gmail.com>> wrote:

As a skimage (very happy) user, I must say I find **very** convenient that vanilla ndarrays are accepted throughout the libary, because image metadata is a problem best kept away from the calculus, except in issues of the 'sampling case' type, where I agree it is nicer to have it as a clear function argument.

Just to be clear, I think skimage should *always* accept vanilla ndarrays, and I agree that functions that can deal with anisotropy should have an explicit (and optional) "sampling" argument.

I think my suggestion would be that such a function should check if the array has a sampling attribute, and use that if the sampling argument is not given. In that way, we could do "im = imerode(imdilate(my_image_with_sampling_attr)", and things would just work in the correct manner.

My concern with this proposal is now the user has two independent ways to pass in the same information. If inconsistent information is passed in the library has to make a decision about which one takes precedence. This adds to the complexity of the function and can only lead to confusion on the part of the users when they think the precedence should have the opposite order.

If you have a function `f(array, meta_data)` then I would add a thin wrapper for the sub-classed arrays as such: `s_f(sub_classed_array): return f(sub_classed_array, sub_classed_array.get_metadata())`. This adds a lot more functions, but they can probably be auto-generated and don't add much complexity. Maybe make them class methods of the sub-array class so they do not pollute the namespace.

Tom

- Almar

...

On 22 September 2013 05:57, Juan Nunez-Iglesias <jni.soma@gmail.com <mailto:jni.soma@gmail.com>> wrote:

On Sun, Sep 22, 2013 at 1:20 PM, Jaime Fern�ndez del R�o <jaime.frio@gmail.com <mailto:jaime.frio@gmail.com>> wrote:

So a pixel replicated TIFF image has twice the resolution, as per the above official definition, not the same resolution with twice the sampling.

Even in this case, what we call sampling/spacing is the *inverse* of that definition of resolution.

I think strictly speaking, the term "resolution" is a measure for visibility of detail. The term is used a lot to simply denote the *number* of pixels, e.g. 800x600, 1024x768, etc. Spacing/sampling is really just a a measure (usually in mm) that specifies the distance between two pixels. If you know the number of pixels, and the sampling, you know the image size in physical units.

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-- Thomas A Caswell PhD Candidate University of Chicago Nagel and Gardel labs tcaswell@uchicago.edu <mailto:tcaswell@uchicago.edu> jfi.uchicago.edu/~tcaswell <http://jfi.uchicago.edu/~tcaswell> o: 773.702.7204

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