Quick guide to images on jupyter

[alanedelman]

Students want a basic understanding of how to display images
e.g.

img = imread(foo)

The following display a picture

img
img[1:10,:]
img[:,1:10]

but not

img[1:10,1:10]

This drives people crazy

What should I tell them? or where can I point them?

[timholy]

I don't use Jupyter so I've not personally put much effort into making it work well, and it looks like no one else has either. But here's what's going on: img[1:10,1:10] returns an Array, and Jupyter is not set up to display all Arrays as an image (otherwise zeros(3,3) would produce a black square rather than the printout of a matrix 😄). I didn't remember that img[:,1:10] gives back an Image; that's probably unintentional, it should give an array too.

getindexim(img, 1:10, 1:10) preserves the Image metadata, and that will display a subregion as an Image.

I'm not saying this is ideal; see #173 for discussion. We can probably reconsider this by adding yet-another-AbstractArray layer, but I do have a few reservations about that until we get triangular dispatch---the dispatch already gets hairy sometimes just with Image{RGB{Float32}, 2, Array{RGB{Float32}, 2}}...

[timholy]

I didn't remember that img[:,1:10] gives back an Image; that's probably unintentional, it should give an array too.

This actually turns out to be a new consequence of JuliaLang/julia#10525 and my definition of similar.

[timholy]

Ah, there's one other thing we could do that would avoid an extra layer. The following should turn on display for any AbstractArray{C<:Colorant}:

Base.mimewritable{C<:Colorant}(::MIME"image/png", img::AbstractArray{C}) = sdims(img) == 2 && timedim(img) == 0

Base.writemime{C<:Colorant}(stream::IO, mime::MIME"image/png", img::AbstractArray{C}; kwargs...) =
    writemime(stream, mime, Image(img, spatialorder=["x","y"]); kwargs...)

If you want, I can add this and tag a release just for that. Or you can put this in a file that everyone loads.

[timholy]

(I should say I can't test because my own IJulia produces no output whatsoever, not even of numbers.)

[SimonDanisch]

@timholy that's actually how I do it in GLVisualize and works well for me :)
I do it the other way around though:
visualize(image::AbstractImage) = visualize(data(image))
I probably shouldn't since the visualization might need some information stored in the properties.
Good that we talked about this :D

[timholy]

OK, I fixed my IJulia (there must have been a conflict between the new Conda infrastructure and an old pip-installed version) and verified that the above work. I pushed and tagged a new release. Did not merge into master, since that's focused on migrating to FileIO, but a Pkg.update("Images") should fix this for your students and hopefully drive them less crazy.

@SimonDanisch, we'll want to put the same fix into ImageMagick.jl.