Custom image loader in Julia using MLUtils.jl

New to Julia
1

I am new to Julia and feel a little bit confused about how to implement custom image loader, first I have generated 10 sample images and saved it in the temp folder, then I have defined “ImageFolder” but really got stuck in understanding how to use numobs and getobs since documentation is not very comprehensible for me this is what I got:

using FileIO, ImageIO, ImageCore, MLUtils

struct ImageFolder
    files::Vector{String}
    function ImageFolder(dir::String)
        return new(readdir(dir; join = true))
    end
end


MLUtils.numobs(data::ImageFolder) = data.files
MLUtils.getobs(data::ImageFolder, idx::AbstractVector{<:Integer} = rand(3, 4, length(idx)))

dir = tempname()
mkpath(dir)

@info "Writing random images to $(dir)"

for i in 1:10
    save(joinpath(dir, "$i.jpeg"), colorview(RGB, rand(3, 10, 10)))
end

would appreciate any help, thank you in advance…

Tried to implement numobs and getobs so that MLUtils.getobs(data::ImageFolder, idx::AbstractVector{<:Integer}) should return an Array{Float32,4} of dimension d1xd2xd3xn where d1, d2 are the dimensions of the images and n is equal to length(idx).

2

For usage of a custom DataLoader with images saved on disk, the transfer learning tutorial provides an example: model-zoo/tutorials/transfer_learning at master · FluxML/model-zoo · GitHub

1 Like
3

Thank you for the answer, nevertheless I would like to use numobs and getobs but could not find any comprehensive tutorials, will see if I can come up with something

4

What’s the issue with the above referenced length and getindex approach?
These are aligned with the MLUtils DataLoader extension methodology and provides performance as good as it gets AFAIK.

1 Like
5

Note that the docs for getobs says:

getobs should only be implemented for types where there is a difference between getobs and Base.getindex (such as multi-dimensional arrays).

Same story for numobs. So the usage above is correct. You are of course free to override it instead, but you’ll miss out on the nice [...] indexing syntax and other features Julia provides for types with getindex/length defined.

1 Like
6

Hi all, yes I am aware of the getobs usage only where is a difference, the thing is for me it is required to use getobs and numobs, but anyway I will try to implement the code with both, thanks for the input:)

7

If you implement getobs and numobs like you would implement getindex and length, then you’re likely 90% of the way there. If you feel like the docs are missing some important detail on either, feel free to file an issue or PR.

8

Hi ATR, given a list of files, you can also use mapobs to turn those into a data container of images:

using MLUtils, FileIO

files = [file for file in readdir(DIR; recursive=true) if endswith(file, ".jpeg")]
images = MLUtils.mapobs(FileIO.load, files)

Then you can get individual images like this:

MLUtils.getobs(images, 1)

One more note: where you’re creating some test images, you’re creating arrays with size (3, 10, 10) like one would have in Python to represent a 2D image. In Julia however, images are usually represented as 2-dimensional arrays with an element type like RGB that includes a complete pixel value (e.g. 3 colors).

To convert such a 2D image to a 3D array with the color channels expanded, you can use Images.channelview:

using Images
imagetensors = MLUtils.mapobs(Images.channelview, images)
getobs(imagetensors, 1)
1 Like