Skip to main content

function construction - Pattern that matches colors


Suppose I'm writing a function that takes a color as a parameter; for example:



drawShape[color_] := Graphics[{Style[Disk[], color]}];

But if the caller inputs an invalid color, bad things happen:


Bad


So I want to use a pattern to define drawShape only for values that are actually colors. Conceptually,


drawShape[color_Color] := ...

The problem is that unlike (say) Lists, Integers, Reals, Complexes, or Graphicses, color objects do not share a Color head. That is,


In[1]:= Red // Head
Out[1]= RGBColor

In[2]:= Hue[0.5] // Head
Out[2]= Hue
In[3]:= GrayLevel[0.5] // Head
Out[3]= GrayLevel
In[4]:= CMYKColor[0, 1, 1, 1/2] // Head
Out[4]= CMYKColor
In[4]:= Opacity[0.5, Purple] // Head
Out[4]= Opacity
In[5]:= Transparent // Head
Out[5]= GrayLevel


So that won't work. I also don't see any ColorQ function, with which I could write drawShape[color_ ? ColorQ] := ....


How can I write a pattern that matches any valid color object? Is there a more robust way than just testing for each of these heads?



Answer



Original method


colorQ = Quiet @ Check[Blend @ {#, Red}; True, False] &;

colorQ /@ {Red, Hue[0.5], GrayLevel[0.5], CMYKColor[0, 1, 1, 1/2], Opacity[0.5, Purple]}



{True, True, True, True, True}

colorQ /@ {17, 1.3, Pi, "not a color", {1, 2, 3}, Hue["bad arg"]}


{False, False, False, False, False, False}

You would use: drawShape[color_?colorQ] := . . .



Inspired by kguler's comment this might also be formulated as:



colorQ = Quiet[Head @ Darker @ # =!= Darker] &;

Or:


colorQ = FreeQ[Quiet @ Darker @ #, Darker] &;

Edit: Darker works on entire Image and Graphics objects and therefore the two forms immediately above will incorrectly return True in these cases. Blend solution is still valid.



Version 10 update and analysis


In version 10 there is a built-in function for this: ColorQ




ColorQ[color] yields True if color is a valid color directive and False otherwise.



A bit of spelunking reveals that the inner definition of this function is (contexts stripped for clarity):


iColorQ[args_?(ColorDirectiveQ[Head[#1[[1]]]] &), opts_] := 
  NumberQ[Quiet[ToColor[args[[1]], XYZColor][[1]]]]

This is very similar to my own method, however the inner definition of ColorDirectiveQ omits Opacity:


iColorDirectiveQ[args_, opts_] := 
 TrueQ[Quiet[
   MatchQ[args[[1]], 

    GrayLevel | RGBColor | CMYKColor | Hue | XYZColor | LUVColor | LABColor | LCHColor]]]

This means that the function will return False for e.g. Opacity[0.5, Purple] where mine returns True.


Comments

Post a Comment

Popular posts from this blog

plotting - How to draw lines between specified dots on ListPlot?

I would like to create a plot where I have unconnected dots and some connected. So far, I have figured out how to draw the dots. My code is the following: ListPlot[{{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4,13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full] I have thought using ListLinePlot command, but I don't know how to specify to the command to draw only selected lines between the dots. Do have any suggestions/hints on how to do that? Thank you. Answer One possibility would be to use Epilog with Line : ListPlot[ {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4, 13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full, Epilog -> { Line[ ...
Post a Comment
Read more

equation solving - Invert and fit implicitly defined curve

I need to fit an implicitly defined curve. I thought I could get some data out of Solve , and then using FindFit . Therefore, I would like to find the relation the parametric curve defined by $F(x,y)=0$: Solve[-(1/2) + 1/2 (0.41202 BesselK[0, 0.1 Sqrt[x^2 + y^2]] + (0.101483 x BesselK[1, 0.1 Sqrt[x^2 + y^2]])/Sqrt[x^2 + y^2]) == 0, y] But I can't get an output: Solve was unable to solve the system with inexact coefficients or the system obtained by direct rationalization of inexact numbers present in the system. Since many of the methods used by Solve require exact input, providing Solve with an exact version of the system may help. >> Edit: In particular, I would like to fit the data coming from the curve with the expression of another curve, and not with a function $f(x)$. In particular, since this clearly looks like a cardioid , I would like it to fit to something like it. What other strategies could I try?
Post a Comment
Read more

dynamic - How can I make a clickable ArrayPlot that returns input?

I would like to create a dynamic ArrayPlot so that the rectangles, when clicked, provide the input. Can I use ArrayPlot for this? Or is there something else I should have to use? Answer ArrayPlot is much more than just a simple array like Grid : it represents a ranged 2D dataset, and its visualization can be finetuned by options like DataReversed and DataRange . These features make it quite complicated to reproduce the same layout and order with Grid . Here I offer AnnotatedArrayPlot which comes in handy when your dataset is more than just a flat 2D array. The dynamic interface allows highlighting individual cells and possibly interacting with them. AnnotatedArrayPlot works the same way as ArrayPlot and accepts the same options plus Enabled , HighlightCoordinates , HighlightStyle and HighlightElementFunction . data = {{Missing["HasSomeMoreData"], GrayLevel[ 1], {RGBColor[0, 1, 1], RGBColor[0, 0, 1], GrayLevel[1]}, RGBColor[0, 1, 0]}, {GrayLevel[0], GrayLevel...
Post a Comment
Read more