Quickstart

Quickstart

If you're comfortable with Julia or have used another image-processing package before, this page may help you get started quickly. If some of the terms or concepts here seem strange, don't worry–-there are much more detailed explanations in the following sections.

Images are just arrays

For most purposes, any AbstractArray can be treated as an image. For example,

using Images

img = rand(640,480)               # a random Float64 image
img = rand(RGB{N0f8}, 256, 256)   # a random RGB image, 8 bits per channel
# select a region-of-interest from a larger image
imgc = img[200:245, 17:42]        # makes a copy
imgv = @view img[200:245, 17:42]  # makes a view
# an image that starts black in the upper left and gets bright in the lower right:
img = reshape(linspace(0,1,10^4), 100, 100)
# a 3d box image
img = zeros(128, 128, 80)
img[20:100, 20:100, 10:70] = 1

Some add-on packages enable additional behavior. For example,

using Images, Unitful, AxisArrays
using Unitful: mm, s

img = AxisArray(rand(256, 256, 100, 50), (:x, :y, :z, :time), (0.4mm, 0.4mm, 1mm, 2s))

defines a 4d image (3 space dimensions plus one time dimension) with the specified name and physical pixel spacing for each coordinate. The AxisArrays package supports rich and efficient operations on such arrays, and can be useful to keep track of not just pixel spacing but the orientation convention used for multidimensional images.

JuliaImages interoperates smoothly with AxisArrays and many other packages. As further examples,

It is very easy to define new array types in Julia–and consequently specialized images or operations–and have them interoperate smoothly with the vast majority of functions in JuliaImages.

Colors, the 0-to-1 intensity scale, and views

In JuliaImages, by default all images are displayed assuming that 0 means "black" and 1 means "white" or "saturated" (the latter applying to channels of an RGB image). Perhaps surprisingly, this 0-to-1 convention applies even when the intensities are encoded using only 8-bits per color channel. JuliaImages uses a special type, N0f8, that interprets an 8-bit "integer" as if it had been scaled by 1/255, thus encoding values from 0 to 1 in 256 steps. N0f8 numbers (standing for Normalized, with 0 integer bits and 8 fractional bits) obey standard mathematical rules, and can be added, multiplied, etc. There are types like N0f16 for working with 16-bit images (and even N2f14 for images acquired with a 14-bit camera, etc.).

This infrastructure allows us to unify "integer" and floating-point images, and avoids the need for special conversion functions that change the value of pixels when your main goal is simply to change the type (numeric precision and properties) used to represent the pixel.

Because images are just arrays, some environments (e.g., IJulia/Jupyter) will display numeric arrays as arrays (using a text representation) but will display 2d arrays that have Colorant elements as images. You can "convert" in the following ways:

img = colorview(Gray, rand(8, 8))          # encodes as Gray{Float64}, so displays as image
img = colorview(RGB, rand(3, 8, 8))        # encodes as a 2d RGB{Float64} array
img = colorview(RGB, rand(N0f8, 3, 8, 8))  # uses only 8 bits per channel
# The following two "convert" between representation as an 8-bit RGB
# image and as a 3×m×n UInt8 array
img = colorview(RGB, normedview(A))
A = rawview(channelview(rand(RGB{N0f8}, 8, 8)))

All of these "conversions" actually create views, meaning that no copies of the underlying storage are made unless you call copy on the result.

Default orientation and storage order

Images are "vertical-major," meaning that when the image is displayed the first index corresponds to the vertical axis. Note that by default, in Julia the first index is also the fastest (i.e., has adjacent storage in memory).

You can use permuteddimsview to "reinterpret" the orientation of a chunk of memory without making a copy, or permutedims if you want a copy.

Arrays with arbitrary indices

If you have an input image and perform some kind of spatial transformation on it, how do pixels/voxels in the transformed image match up to pixels in the input? Through Julia's support for arrays with indices that start at values other than 1, it is possible to allow array indices to represent absolute position in space, making it straightforward to keep track of the correspondence between location across multiple images. More information can be found in Keeping track of location with unconventional indices.

Function categories

See Summary and function reference for more information about each of the topics below. The list below is accessible via ?Images from the Julia REPL. If you've used other frameworks previously, you may also be interested in the Comparison with other image processing frameworks.

Constructors, conversions, and traits:

Contrast/coloration:

Algorithms:

Test images and phantoms (see also TestImages.jl):

See also the excellent ImageFeatures package, which supports a number of algorithms important for computer vision.