Codifying the backend interface

[asinghvi17]

Collecting from the current code of CairoMakie and GLMakie, and my experience writing GRMakie, this is a basic overview of what I think the "backend interface" which all backends must satisfy looks like now. Please correct me (or this post) if there are any incorrect statements here!

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Each backend must implement the Makie backend interface. Let us consider an imaginary backend, ImaginaryMakie.jl. To be compliant with the backend interface, and thus a functioning backend for Makie, this package must implement, at a minimum,

• An ImaginaryBackend <: Makie.AbstractBackend type.
• An ImaginaryMakie.activate!(...) function, which can be called with no arguments (i.e., all arguments should have a default value). This function must be called in the package's __init__ method. It must, at a minimum, set Makie.current_backend[] = b::ImaginaryBackend.
• An ImaginaryScreen <: Makie.AbstractScreen type, which contains everything needed to display a Scene. When the Scene is displayed using an ImaginaryScreen, that screen is pushed to scene.current_screens.
  • An overload for GeometryBasics.widths(scr::ImaginaryScreen)::NTuple{2, Int}
  • An overload for Makie.colorbuffer(screen::ImaginaryScreen)::Matrix{RGB} which quickly rasterizes the screen to an RGB matrix.
• An overload of Makie.backend_display(x::ImaginaryBackend, scene::Scene)::ImaginaryScreen which displays scene to a backend screen. Semantically, this should be used to bring up an interactive screen.
• Overloads of Makie.backend_show(backend::ImaginaryBackend, io::IO, mime::MIME, scene::Scene)::ImaginaryScreen for all MIME types which the backend can handle.
  • Corresponding overloads of Makie.backend_showable(backend::ImaginaryBackend, mime::MIME, scene::Scene) for those MIME types.
• An overload for Base.delete!(screen::ImaginaryScreen, scene::Scene, plot::AbstractPlot).
• Methods to draw the atomic plot types (lines, linesegments, scatter, text, heatmap, image, mesh, surface, volume). An arbitrary plot p can be decomposed into atomics by descending down the tree formed by p.plots and its elements. Some of these atomics (surface, volume) can be ignored if the backend only supports 2D plots, but it is recommended that a warning be added in that case.

When rendering Scenes, the ImaginaryBackend must render a rectangle filled with the background color, the plots in scene.plots, and its children in scene.children. There is no proscribed order in which this must be done, but common sense applies.

Beyond these few methods, Makie is completely blind to how the backend arranges itself. Note that this includes how the atomic plot types are rendered, or if any plot types are rendered before full decomposition into atomics.