Document how to set alpha values.

[lassepe]

Thank you for this awesome package! I'm a huge fan of grammar of graphics and I'm excited to see a native Julia implementation of this paradigm.

The tutorial and the philosophy pages of the documentation suggest that one can set the transparency of a layer via the alpha keyword of visualize, i.e. something like visualize(Scatter; alpha = 0.5). However, that does not seem to work (anymore?). It is surprisingly hard to figure out what would be the right thing to do to achieve the result.

It would be great if the documentation could be updated in this regard or if the documentation could include a pointer to the relevant information about accepted keyword arguments of visualize (probably pointing to the Makie.jl docs?).

[piever]

visual(Scatter; alpha = 0.5) should work, do you have an example when it doesn't? That being said, alpha is the only attribute that is added compared to Makie. Ideally it should be implemented in Makie, but I guess if that doesn't happen anytime soon one can document it here.

[lassepe]

Here is a minimal example that fails to make the scatter points transparent for

using AlgebraOfGraphics: Scatter, data, draw, mapping, visual
import CairoMakie, ElectronDisplay

df = [(; x, y = sin(x)) for x in (-π):0.1:π]
data(df) * mapping(:x, :y) * visual(Scatter; alpha = 0.5) |> draw

Output:

Package versions:

  [cbdf2221] AlgebraOfGraphics v0.4.6 `https://github.com/JuliaPlots/AlgebraOfGraphics.jl.git#master`
  [13f3f980] CairoMakie v0.6.2 `https://github.com/JuliaPlots/Makie.jl.git:CairoMakie#master`
  [d872a56f] ElectronDisplay v1.0.1

[piever]

Ah, thanks for reporting that, this is definitely a bug. I think AoG has trouble applying alpha if no explicit color is passed, either via mapping or via visual. You can probably work around it by doing visual(color = :black, alpha = 0.5).

I suspect it's difficult to fix this in general from AoG, it probably needs MakieOrg/Makie.jl#84 for a cleaner solution.

[lassepe]

Thank you for the quick response. Setting the color explicitly does the trick. Should I close this issue since it will essentially work once the alpha channel is supported in Makie?

[piever]

I think it's best to keep it open (even if it's not really actionable) as a reminder to update things on this end (remove current partial implementation) once Makie has alpha transparency support.

[ismael-lajaaiti]

Bump. As this issue has been open for awhile and it is unclear for how long it will stay open, could we add a short note about that in the documentation? So even if the issue isn't fixed, new users won't struggle to find the trick?

[jerlich]

alpha is supported in Makie (as of July 2023). Is it an easy fix to pass the alpha on to Makie now?

[vladdez]

We tried to use alpha with Polyand it doesn't work, polygones are not transparent:

dat = DataFrame(; rect = [Rect(0, 0, 1, 2), Rect(1, 2, 2, 2)], group = [1, 2])
layer = mapping(color = :group) * (data(dat) * mapping(:rect) * visual(Poly; alpha = 0.5))
draw(layer)

Is it a bug?

[jkrumbiegel]

Yeah in principle I would expect alpha to be passed on like any other attribute, there's probably some old code left over from when AoG had its own alpha, which now removes it from the list or something.

[jkrumbiegel]

Ah no it does get passed along, poly doesn't respect alpha in the first place

[jkrumbiegel]

I think this issue can be closed as it's not a special AoG attribute anymore and the remaining fixes are on Makie's side

[jkrumbiegel]

MakieOrg/Makie.jl#4614