Why does not BarLegend accept user-defined color function?

Question

I encountered a really strange problem with BarLegend. Say you have a user-defined color function:

cf[z_]:=RGBColor[RandomReal[z, 3]]

When you want to use it in BarLegend as below:

BarLegend[{cf, {1, 10}}]

Mathematica will generate a blank bar shown as below: enter image description here

However, after I change the function head in BarLegend to a pure function:

BarLegend[{cf[#]&, {1, 10}}]

Mathematica can give desired result.

I cannot figure out why Mathematica doesn't accept user-defined color function. Could anyone please give a clue?

Does cf = Function[z, RGBColor[RandomReal[z, 3]]] work better? — cormullion, Dec 12 '13 at 14:10

score 12 · Accepted Answer · answered Dec 12 '13 at 15:31

If you look into the source code

ClearAttributes[BarLegend, ReadProtected]
?? BarLegend

And into nested functions (BarLegend,Charting`iBarLegend $\to$ Legending`LegendDump`iColorBandLegend $\to$ Legending`LegendDump`parseColorBand) you can find the following code

Switch[Legending`LegendDump`colorfunction,
_String, ColorData[Legending`LegendDump`colorfunction]/@Legending`LegendDump`values,
_ColorDataFunction, Legending`LegendDump`colorfunction/@Legending`LegendDump`values,
_List, Switch[Legending`LegendDump`fn, Legending`ColorBandLegend,With[{Legending`LegendDump`list=Legending`LegendDump`colorfunction},Blend[Legending`LegendDump`list,#1]&]/@Legending`LegendDump`values,
BarLegend|_, Legending`LegendDump`iter=Max[Length[Legending`LegendDump`colorfunction]-Length[Legending`LegendDump`values],0];Do[Legending`LegendDump`colorfunction=Reverse[Charting`padList[Legending`LegendDump`colorfunction,Length[Legending`LegendDump`colorfunction]-1]],{Legending`LegendDump`iter}];If[OddQ[Legending`LegendDump`iter],Reverse[Legending`LegendDump`colorfunction],Legending`LegendDump`colorfunction]],
Hue|GrayLevel, (Legending`LegendDump`colorfunction[#1]&)/@Legending`LegendDump`values,
_Function, Legending`LegendDump`colorfunction/@Legending`LegendDump`values,
_, Return[{}]]

It shows that the function must have the Function header.

So could you provide a working example please? – Tom Sep 24 '19 at 11:51 — Tom, Sep 24 '19 at 11:51

score 0 · Answer 2 · edited Oct 03 '20 at 08:30

0

With this response I tried:

maxmin = {10, 20};
BarLegend[{Function[x, Hue[0.8 Rescale[x, maxmin]]], maxmin}]

which worked nicely to truncate the Hue function to remove the red at the top

edited Oct 03 '20 at 08:30

AsukaMinato

9,758
1
14
40

answered Oct 03 '20 at 08:09

Meir Bar-Tal

1

Why does not BarLegend accept user-defined color function?

2 Answers2