PolarAxis unable to set negative rlimits

[mikeingold]

• are you running newest version (version from docs) ?
• can you reproduce the bug with a fresh environment ? (]activate --temp; add Makie)
• What platform + GPU are you on?

julia> versioninfo()
Julia Version 1.9.4
Commit 8e5136fa29 (2023-11-14 08:46 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: 32 × AMD Ryzen 9 3950X 16-Core Processor
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-14.0.6 (ORCJIT, znver2)
  Threads: 16 on 32 virtual cores
Environment:
  JULIA_NUM_THREADS = 16

Issue

It looks like PolarAxis does not support setting negative r limits or plotting lines with negative values. Polar plots are used frequently in electrical engineering to present things like antenna gain patterns, measured on a dB scale (Example).

I demonstrated this with the MWE below, plotting a fake gain pattern in linear and dB scales on both a regular Axis and a PolarAxis. The polar plot in dB scale won't display any of the line because it's always <= 0. If I try to set rlimits to the appropriate range for this plot, e.g. rlimits=(-40,0) the r axis looks like a mess with no ticks remaining.

Minimum Working Example

using CairoMakie

pow2db(x) = 20log10(x)

theta_prime = pi
angle(a,b) = min(abs(a-b), 2π-abs(a-b))
gain(theta) = abs(sinc(2.0*angle(theta,theta_prime)))

thetas = range(0, 2pi, length=361)

# Axis plots in linear and dB scale
f_xy = Figure()
ax_xy_lin = Axis(f_xy[1,1], title="Gain", xlabel="Theta [rad]")
lines!(ax_xy_lin, thetas, gain.(thetas))
ax_xy_dB = Axis(f_xy[1,2], title="Gain in dB", xlabel="Theta [rad]")
lines!(ax_xy_dB, thetas, pow2db.(gain.(thetas)))
save("f_xy.png", f_xy)

# PolarAxis plots in linear and dB scale
f_polar = Figure()
ax_polar_lin = PolarAxis(f_polar[1,1], title="Gain")
lines!(ax_polar_lin, thetas, gain.(thetas))
ax_polar_dB = PolarAxis(f_polar[1,2], title="Gain in dB")
lines!(ax_polar_dB, thetas, pow2db.(gain.(thetas)))
save("f_polar.png", f_polar)

[ffreyer]

We experimented with offsetting radii so that r_in > 0 ends up at r_out = 0 to increase the space a plot limited to a ring (sector) occupies. (I.e. with rmax > rmin >> 0.) That's what radial_distortion_threshold is about. With r_out = r_in - r0 it sets a limit for how large (r_min - r0) / (r_max - r0) is allowed to get, calculating the r0.

Ultimately this won't help you, since it is restricted to r0 > 0. I also don't think you can hack this by setting r0 in ax.scene.transformation.transform_func, because of how the updates are set up and because PolarAxis probably assumes positive radii at times.

But there is still some functionality for this in the background and I think replacing radial_distortion_threshold with directly setting r0 wouldn't be a bad idea. It seems easier to understand to me and more useful.

If you want to try making these changes you could remove

Makie.jl/src/makielayout/blocks/polaraxis.jl

Lines 225 to 230 in 531d42d

	# To keep the inner clip radius below a certain fraction of the outer clip
	# radius we map all r > r0 to 0. This computes that r0.
	radius_at_origin = map(po.blockscene, po.target_rlims, po.radial_distortion_threshold) do (rmin, rmax), max_fraction
	# max_fraction = (rmin - r0) / (rmax - r0) solved for r0
	return max(0.0, (rmin - max_fraction * rmax) / (1 - max_fraction))
	end

and make radius_at_origin (the r0) an attribute of the PolarAxis by adding it here

Makie.jl/src/makielayout/types.jl

Lines 1690 to 1696 in 531d42d

	"The direction of rotation. Can be -1 (clockwise) or 1 (counterclockwise)."
	direction::Int = 1
	"The angular offset for (1, 0) in the PolarAxis. This rotates the axis."
	theta_0::Float32 = 0f0
	"Controls the argument order of the Polar transform. If `theta_as_x = true` it is (θ, r), otherwise (r, θ)."
	theta_as_x::Bool = true
	"The relative margins added to the autolimits in r direction."

The radial_distortion_threshold further above should be removed as well.