[FR] Avoid gratuitous use of transparency about plots.jl HOT 6 OPEN

I implemented this approach, but after testing I don't think it's a complete solution due to #4202: GR and PGFPlotsX draw gridlines on top of axis lines, and so if they overlap the axis line will be drawn with the blended colour.

For example, with Plots 1.40.4 and plot(sin.(0:0.1:2π), yticks=-1:0.5:1, yrange=(-1,1)) we have:

After flattening colours, equivalent to gridalpha = 1, foreground_color_grid = RGBA{Float64}(0.9,0.9,0.9,1.0):

The x-axis is covered by a gridline and is now gray:

This means that we can't currently assume that the transparent gridline is always drawn on the backround and replace it with a grid/background colour blend. It seems to me that the cleanest solution would be to first fix #4202.

from plots.jl.

Sounds reasonable. Would you like to work on it?

from plots.jl.

I'd be interested in working on a patch to remove (default) use of transparency for gridlines. I suggest that we keep the current default values, but replace them when preprocessing arguments, flattening as in the example above:

• Replace foreground_color_grid with a blend of the current foreground_color_grid and background_color, using gridalpha as the ratio.
• Set gridalpha to 1.
• Same for foreground_color_minor_grid and minorgridalpha.

This way, we avoid breaking existing scripts that set one of gridalpha or foreground_color_grid and use the default value for the other.

Flattening grid colours in this way should not change the resulting plot, except that (tiny) grid intersections will now have the same colour as gridlines instead of the slightly darker overlap.

Does this sound like a reasonable approach? Should the behaviour be user-configurable (e.g. via a flattengridalpha parameter defaulting to true)?

from plots.jl.

That sounds like a good plan if you want to have this in a 1.x release.

It would also be fine to just remove the use of transparency on the v2 branch if you can wait for the release.

In any case this does not need to be user configurable.

from plots.jl.

@dd0 That sounds like a good approach.

By the way: I used ColorBlendModes.jl because it had the required compositing formulae readily available. But since it is a quite comprehensive package, I'm not necessarily suggesting to add it as a dependency. The basic compositing formulae to flatten two RGBA values are described in Section 11.3 of the ISO 32000:2008 PDF specification, and originally come from Porter and Duff: Compositing Digital Images (SIGGRAPH '84).

from plots.jl.

We probably also should check the code for locations where color=RGBA(0,0,0,0) is (ab)used to just indicate that something shouldn't be drawn, as that could be another way for transparency to enter a PDF backend.

from plots.jl.