How much should we support time based traits?

[Tokazama]

I brought this up on Slack and thought it would be good to have a formal issue for it. We have some basic support for identifying the time axis in ImageAxes and we have nimages and assert_timedim_last. However, very common (and convenient) to have some additional methods for interacting with and describing time. Personally I'm interested in having:

• sampling_rate: number of samples per second
• duration: length of time the data describes
• onset: first time point

I'm sure there could be others but at this point I think it would make the most sense to focus on traits/methods that are universally helpful for interacting with data that has a time axis.

Is this something we should have in JuliaImages and if so, should this be a part of ImageCore or ImageAxes?

[timholy]

I am of the growing opinion that we should only support what we can be sure of, and in this package there is no way to know whether an array even has a time dimension.

With this philosophy, the implementation of assert_timedim_last in this package has the wrong fallback: it should be an error, rather than a default-pass. I recognize that may be an indication that I'm using the wrong philosophy, but to current-me the name suggests that it provides a guarantee that the last dimension corresponds to time. Which you can't guarantee without an axis that has been flagged as being of the time type.

[Tokazama]

Just to clarify, does that mean you think this package should only have functionality related to spatial and color components of images? Does this mean we should move time related functionality out of this package? I'm assuming if we did so we would still want whatever package that contains it as a dependency in Images.jl and other related packages.

[timholy]

I guess I'm saying this package should define "stubs" but that the default might be to throw an error. (I'm not sure I'm entirely happy with my previous choices.)

[Tokazama]

So you mean something like...

"""
    sampling_rate

...
"""
sampling_rate

...and just let upstream define methods?

I think in an ideal world we would be able to depend on another package that is dedicated to time series analysis and defines these methods using AxisArrays or a generic interface we can just inherit from. However, I'm not aware of any well established package that fits this description (most are domain specific and we would be inheriting only the syntax at this point AFIK). From there I just assumed that images is work with space and time axes and that's my use case, so here I am. I'm pretty open to whatever people think is best as long as it doesn't mean waiting five months for a counsel of experts on time series analysis to agree on syntax.

[johnnychen94]

Could it be defining these symbols in ImagesAPI, and then reexporting (and possibly extending) them from ImageCore?

[Tokazama]

Could it be defining these symbols in ImagesAPI, and then reexporting (and possibly extending) them from ImageCore?

This was kind of my original intention when I suggested ImagesAPI. I figured there would be a lot very generic syntax we'd want common across the ecosystem but that the exact way it interacts with structures would be variable enough that we just need a common API package. However, I understand that's also a lot of what ImageCore is for right now.

I'm up for starting a PR wherever this feature is appropriate right now and if it seems reasonable I could also take care of some of the rough spots in the syntax/API with stuff like assert_timedim_last.

[johnnychen94]

If you need this feature eagerly, perhaps a better solution is to add them to another repository (say VideoCore.jl :P ) you take control of. When codes are stabilized and when we're realizing they're another core of the ecosystem of the JuliaImages, that's probably the right time to merge these two packages. But by then we might just want to keep two cores.

The issue I see is that some scattered traits in ImageCore.jl aren't enough to support the whole images processing tasks of time series, while you don't have the bigger plan in your mind right now (am I interpreting it right?) I think that's @timholy was worried about.

[Tokazama]

If you need this feature eagerly, perhaps a better solution is to add them to another repository (say VideoCore.jl :P )

I'm totally open to that and was actually going to just define my own stuff in another package but @baggepinnen brought up temporal filtering on slack. I thought others might be interested in being able to reference a common set of methods.

some scattered traits in ImageCore.jl aren't enough to support the whole images processing tasks of time series

I completely agree with this but I don't think it's necessarily the issue with including methods that reference the time domain. Most methods in ImageCore.jl don't support the whole of any processing task. Most are just needed across a wide variety of tasks and make sense to have in the core package, right? However, I can see how hosting core methods concerning time may be outside of the scope of JuliaImages.

while you don't have the bigger plan in your mind right now

I don't have bigger plans for a dedicated time series package in JuliaImages but I always have bigger plans ;)

[timholy]

Sorry for the delay, I'm back to this now.

Having these as traits might make sense, but I'd prefer something that composes more broadly. Already timeaxis gets you almost all the way there. Setup:

using ImageAxes, Unitful
using AxisArrays: AxisArray
const mm = u"mm"
const s = u"s"
img = AxisArray(rand(3, 3, 20), (:y, :x, :time), (1.2mm, 0.8mm, 0.15s), (0mm, 10mm, 0.8s))

Then, for your three items in your top post:

sampling_rate(img) = 1/step(timeaxis(img).val)
duration(img) = (ax = timeaxis(img); last(ax.val) - first(ax.val))
onset(img) = first(timeaxis(img).val)

When things are this easy, there's a bit of a downside in a huge explosion of exported names. Can we first ask what needs to be changed to make timeaxis immediately useful? To me, the most obvious is that the ax.val construct is ugly; most likely we should support first, last, and step on Axis objects.

[Tokazama]

I really like the idea of taking this back to some fundamental interactions on Axis. I'll open an issue over there and if it seems like people are happy with the idea I'll make a PR.

[timholy]

Now that JuliaArrays/AxisArrays.jl#176 is released, should this be closed?

[Tokazama]

tl;dr: If the answer we've converged on here is that JuliaImages should only worry about an axis interface that provides sufficient information to implement this, then it should be closed.

I still think there is a place for specific time based methods (although it might not be here). While your previously proposed methods solve the majority of cases, there are instances where sampling isn't continuous along the axis. There are also a bunch of other things that I haven't brought up that might be useful, like timestamps.

Time is a pretty fundamental component of many non image related analyses and I think we could leverage a lot of other efforts. There seems to be a lot of interest in Julia for getting behind some sort of standard for this (see this discourse thread for reference) so I think the long term plan should be depending on some other effort. In order for this to work we'd need to use a generalizable array approach like AxisArrays (instead of a structure with a time axis referenced like TimeSeries.jl does). I would love to get involved in those conversation if only to convince people to adopt a standard like AxisArrays but this last effort has me convinced that we're not ready for this.

Perhaps once there's a stronger consensus on how to approach named dimensions and labeled axes I could revisit this, but for now I think this is the best we're going to do.