CairoMakie stop splitting line into segments when using varying colour along line

Specific Domains Visualization
1

When you have a line with varying colour, CairoMakie splits the line up into segments defined by your data points. Is there a way to avoid this?

image

code to produce fig:

using CairoMakie
f = Figure()
ax = Axis(f[1, 1])
x = 0:0.2:2*pi
lines!(ax,x,sin.(x),color=x,linewidth=8)
save("test.pdf", f)
2

This is a limitation of Cairo’s line drawing, it can’t draw gradients along lines so you have to draw them segment by segment. I guess one could draw polys instead and manually compute the join geometry. That could be a PR for someone motivated, it would be slower though.

3

Hi thanks very much for your reply!

I assumed it was something like that, but saw the colour changing on a straight segment:

image

I imported the .pdf into inkscape and the coloured segment is one object. Is the issue with curved lines + colour gradients?

4

I mean not along a line that zigzags across the canvas. It can draw a gradient over the whole line but that’s usually not what you want.

5

I see, thank you! I implemented your poly idea, and seems to be working well up until applying the colour. There seems to be a bug with CairoMakie and setting the colour of a poly using a vector of numbers. Unless I’m missing something?

 f = Figure()
 ax = Axis(f[1,1])

 x = [-1,-1,1,1,-1]
 y = [-1,1,1,-1,-1]
 c = [0,2,4,6,8]
 poly!(ax,hcat(x,y),color=c)

 save("test.pdf",f)

Error:

ERROR: BoundsError: attempt to access 1-element Vector{Vector{Point{2, Float32}}} at index [2]
Stacktrace:
  [1] getindex
    @ .\essentials.jl:13 [inlined]
  [2] attr_broadcast_getindex
    @ C:\Users\username\.julia\packages\Makie\VRavR\src\utilities\utilities.jl:174 [inlined]
  [3] macro expansion
    @ C:\Users\username\.julia\packages\Makie\VRavR\src\utilities\utilities.jl:213 [inlined]
  [4] broadcast_foreach(::CairoMakie.var"#56#57"{…}, ::Vector{…}, ::Vector{…}, ::ColorTypes.RGBA{…}, ::Nothing, ::Int64, ::Base.RefValue{…})
    @ Makie C:\Users\username\.julia\packages\Makie\VRavR\src\utilities\utilities.jl:199
  [5] draw_poly(scene::Scene, screen::CairoMakie.Screen{CairoMakie.PDF}, poly::Poly{Tuple{…}}, points_list::Vector{Vector{…}})
    @ CairoMakie C:\Users\username\.julia\packages\CairoMakie\W0SZK\src\overrides.jl:76
  [6] draw_plot(scene::Scene, screen::CairoMakie.Screen{CairoMakie.PDF}, poly::Poly{Tuple{Vector{Vector{Point{2, Float32}}}}})
    @ CairoMakie C:\Users\username\.julia\packages\CairoMakie\W0SZK\src\overrides.jl:21
  [7] cairo_draw(screen::CairoMakie.Screen{CairoMakie.PDF}, scene::Scene)
    @ CairoMakie C:\Users\username\.julia\packages\CairoMakie\W0SZK\src\infrastructure.jl:51
  [8] backend_show(screen::CairoMakie.Screen{CairoMakie.PDF}, io::IOStream, ::MIME{Symbol("application/pdf")}, scene::Scene)
    @ CairoMakie C:\Users\username\.julia\packages\CairoMakie\W0SZK\src\display.jl:90
  [9] (::Makie.var"#1279#1280"{Module, Bool, @Kwargs{}, Figure, MIME{Symbol("application/pdf")}, Scene})(io::IOStream)
    @ Makie C:\Users\username\.julia\packages\Makie\VRavR\src\display.jl:344
 [10] open(::Makie.var"#1279#1280"{…}, ::String, ::Vararg{…}; kwargs::@Kwargs{})
    @ Base .\io.jl:396
 [11] open
    @ .\io.jl:393 [inlined]
 [12] save(file::FileIO.File{…}, fig::Figure; size::Tuple{…}, resolution::Nothing, backend::Module, update::Bool, screen_config::@Kwargs{})
    @ Makie C:\Users\username\.julia\packages\Makie\VRavR\src\display.jl:336
 [13] save(file::FileIO.File{FileIO.DataFormat{:PDF}, String}, fig::Figure)
    @ Makie C:\Users\username\.julia\packages\Makie\VRavR\src\display.jl:308
 [14] save(filename::String, fig::Figure; args::@Kwargs{})
    @ Makie C:\Users\username\.julia\packages\Makie\VRavR\src\display.jl:305
 [15] save(filename::String, fig::Figure)
    @ Makie C:\Users\username\.julia\packages\Makie\VRavR\src\display.jl:302
 [16] top-level scope
    @ REPL[3]:10
Some type information was truncated. Use `show(err)` to see complete types.
6

Fixed the colour issue, but the polygons seem to have some jagged edges which don’t look as nice as lines. I think I will just smooth out my original data around the segment endpoints using interpolations, and then find a way to stretch the individual segments a tiny bit so that they overlap ever so slightly…

edit: any idea where in makie code the line is split up into segments? I’d like to modify how it does that

7

If you see that you’re hitting the mesh drawing which uses patches that are always rasterized without antialiasing. You probably tried using poly with array colors I guess, that would make sense. You’d have to create the polygons in the CairoMakie backend when line drawing is called here