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While images can often be represented as plain Arrays, sometimes additional information about the "meaning" of each axis of the array is needed. For example, in a 3-dimensional MRI scan, the voxels may not have the same spacing along the z-axis that they do along the x- and y-axes, and this fact should be accounted for during the display and/or analysis of such images. Likewise, a movie has two spatial axes and one temporal axis; this fact may be relevant for how one performs image processing.

The ImageAxes package (which is incorporated into Images) combines features from AxisArrays and SimpleTraits to provide a convenient representation and programming paradigm for dealing with such images.


If you want to directly use ImageAxes, add it via the package manager.


Names and locations

The simplest thing you can do is to provide names to your image axes:

using ImageAxes
img = AxisArray(reshape(1:192, (8,8,3)), :x, :y, :z)

As described in more detail in the AxisArrays documentation, you can now take slices like this:

slz = img[Axis{:z}(2)]
slx = img[Axis{:x}(2)]
sly = img[Axis{:y}(2)]

You can also give units to the axes:

using ImageAxes, Unitful
const mm = u"mm"
img = AxisArray(reshape(1:192, (8,8,3)),

which specifies that x and y have spacing of 1mm and z has a spacing of 3mm, as well as the location of the center of each voxel.

Temporal axes

Any array possessing an axis Axis{:time} will be recognized as having a temporal dimension. Given an array A,

using ImageAxes, Unitful
const s = u"s"
img = AxisArray(reshape(1:9*300, (3,3,300)),

you can retrieve its temporal axis with

ax = timeaxis(img)

and index it like

img[ax(4)]  # returns the 4th "timeslice"

You can also specialize methods like this:

using ImageAxes, SimpleTraits
@traitfn nimages(img::AA) where {AA<:AxisArray;  HasTimeAxis{AA}} = length(timeaxis(img))
@traitfn nimages(img::AA) where {AA<:AxisArray; !HasTimeAxis{AA}} = 1

where the pre-defined HasTimeAxis trait will restrict that method to arrays that have a timeaxis. A more complex example is

using ImageAxes, SimpleTraits, Statistics

@traitfn meanintensity(img::AA) where {AA<:AxisArray; !HasTimeAxis{AA}} = mean(img)
@traitfn function meanintensity(img::AA) where {AA<:AxisArray; HasTimeAxis{AA}}
    ax = timeaxis(img)
    n = length(ax)
    intensity = zeros(eltype(img), n)
    for ti in 1:n
        sl = view(img, ax(ti))
        intensity[ti] = mean(sl)

and, when appropriate, it will return the mean intensity at each timeslice.

Custom temporal axes

Using SimpleTraits's @traitimpl, you can add Axis{:t} or Axis{:scantime} or any other name to the list of axes that have a temporal dimension:

using ImageAxes, SimpleTraits
@traitimpl TimeAxis{Axis{:t}}

Note this declaration affects all arrays throughout your entire session. Moreover, it should be made before calling any functions on array-types that possess such axes; a convenient place to do this is right after you say using ImageAxes in your top-level script.