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Bug introduced in 10.0 and fixed in 10.1


I'm sorry if this is a duplicate, but I don't even know how to look for it.

I had this line of code that worked on a Notebook I wrote on Mathematica 9.0:

BarLegend[{colorf[#] &, {0, 30}}]

colorf is a Blend going from 0 to 30 (40 actually, but I don't think it really matters).

I just installed Mathematica 10.0, and it no longer works: the bar is all red (i.e. colorf[0]). Why is this happening?

edit: colorf is defined as follows

colorf=Blend[{{0, Red}, {20, Yellow}, {40, Green}}, Round[#, 0.1]] &;
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  • $\begingroup$ colorf = Blend[{Red, Blue}, #] &; BarLegend[{colorf, {0, 30}}] works fine in V. 10.0.2. -- maybe it is the definition of colorf and using colof[#] &? $\endgroup$
    – gwr
    Dec 11, 2014 at 21:46
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    $\begingroup$ Could you provide all necessary code? @gwr's code works for me as well, so you should provide more information in order to reproduce the problem. $\endgroup$ Dec 11, 2014 at 21:52
  • $\begingroup$ Thanks for asking. I just put it into the question $\endgroup$
    – Rafael
    Dec 11, 2014 at 23:30
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    $\begingroup$ To me this looks like a bug in the scaling of BarLegends with continuous gradients introduced in version 10 (I'm using v10.0.2). $\endgroup$
    – Karsten7
    Dec 12, 2014 at 2:30

2 Answers 2

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To me it looks like there is a bug in the Implementation of BarLegend. When the the number of contours increases there is not only a switch from discrete contours to a continuous gradient (this behavior is documented), but also a change in the scaling (that's the bug).

colorf = Blend[{{0, Red}, {20, Yellow}, {40, Green}}, Round[#, 0.1]] &;
BarLegend[{colorf[#] &, {0, 30}}, #] & /@ Range[15]

enter image description here

One can use Rescale to fix this:

colorf2 = Blend[{{0, Red}, {20, Yellow}, {40, Green}}, 
            Round[Rescale[#, {0, 1}, {0, 40}], 0.1]] &;
BarLegend[{colorf2[#] &, {0, 30}}]

enter image description here

But now one has to be aware that BarLegend with discrete contours is broken:

BarLegend[{colorf2[#] &, {0, 30}}, #] & /@ Range[15]

enter image description here

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  • $\begingroup$ Wow, impressive. Thanks. However, the problem now is that yellow is 15, when it should be 20... Shall I just divide the parameter by 40? $\endgroup$
    – Rafael
    Dec 12, 2014 at 10:57
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    $\begingroup$ Finally I solved the 15->yellow problem by doing BarLegend[{colorf[#*30] &, {0, 30}}]. Not the cleanest of solutions, but does the trick pretty well and doesn't need to modify the color function. I'm chosing this answer for completeness. Thanks to both! $\endgroup$
    – Rafael
    Dec 12, 2014 at 11:12
  • $\begingroup$ @Rafael You can also use BarLegend[{colorf2[# 30/40] &, {0, 30}}]. $\endgroup$
    – Karsten7
    Dec 12, 2014 at 11:24
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Blend was modified in version 10. It appears that Blend is now scaled to {0, 1}.

colorf = Blend[{{0, Red}, {0.5, Yellow}, {1, Green}}, Round[#, 0.01]] &;

BarLegend[{colorf, {0, 30}}]

enter image description here

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    $\begingroup$ It seems to be no issue of Blend, but a bug in the scaling of BarLegend. See me answer and try Graphics[Table[{colorf[x], Disk[{x, 0}]}, {x, 1, 40}]]. $\endgroup$
    – Karsten7
    Dec 12, 2014 at 2:34
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    $\begingroup$ Interesting, both answers have good points but there seems to be indeed a bug as BarLegend[{colorf, {0, 30}},n] will give false answers for small n as shown in @Karsten 7's answer. BTW why is the Round[#, 0.001] needed? $\endgroup$
    – gwr
    Dec 12, 2014 at 10:05
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    $\begingroup$ @gwr In my opinion the Round is not needed, but it's in the code provided by the OP and I left it in the code to show that it's not the reason for the problem. $\endgroup$
    – Karsten7
    Dec 12, 2014 at 10:23
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    $\begingroup$ The purpose of round, as far as I remember, had to do with the main plot (to which this was but the legend). I think it was creating too many colors and thus leaking memory or something (there were thousands of points, nearly everyone with its own color) $\endgroup$
    – Rafael
    Dec 12, 2014 at 11:05

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