At the white regions of this image I want to add a few random arrows. I would like to realize two different of adding the random arrows:

  • The arrows may not leave the white region
  • The arrows have to leave the white region


 img = Import["ExampleData/spikey2.png"]

enter image description here

How can you do this with Mathematica?

In particular I would like to know how to detect the white regions, and how to store this information. If I give you an arrow can you tell me whether it crosses non-white parts of the image?

There is not much space for the arrows in this picture. It would be good to add a bit of white canvas to create some space for the arrows.

  • 3
    $\begingroup$ How about showing what you tried, and more importantly, what you expect to actually see. $\endgroup$
    – bill s
    Commented Apr 10, 2013 at 19:28
  • $\begingroup$ I made a mistake in may question. I said I the arrows have to leave the white region, but in fact they should not. I corrected this. Sorry for that $\endgroup$
    – sjdh
    Commented Apr 11, 2013 at 6:00
  • $\begingroup$ @bills I don't how to find the white regions in an image. So I don't know where to start. $\endgroup$
    – sjdh
    Commented Apr 11, 2013 at 6:01

2 Answers 2


Extending s0rce with ImageValuePosition (new in 9.0).

Module[{w, h},
 {w, h} = ImageDimensions@img;
    Arrowheads[.08], Opacity[.7],
    Map[Arrow[{#, # - .5 (# - .5 {w, h})}] &,
     RandomSample[ImageValuePositions[img, {1, 1, 1}], 20]]},
   AspectRatio -> Automatic,
   ImageSize -> {w, h},
   PlotRange -> {{0, w}, {0, h}}]]


Arrows end half way to center.


Here is one way:

img = Import["ExampleData/spikey2.png"];
arrowstarts = RandomChoice[ImageValuePositions[img, {1, 1, 1}], 10]
arrowends = RandomChoice[ImageValuePositions[img, {0, 0, 0}, 0.75], 10]
arrows = Graphics[
   MapThread[Arrow[{#1, #2}] &, {arrowstarts, arrowends}], 
   AspectRatio -> Automatic, ImageSize -> {150, 135}, 
   Background -> Transparent, PlotRange -> {{0, 150}, {0, 135}}];
ImageCompose[img, arrows]


  • $\begingroup$ Isn't your graphics missing PlotRange? Graphics without it displays max and min values and nothing further, which makes the composition a bit off, doesn't it. $\endgroup$
    – BoLe
    Commented Apr 10, 2013 at 20:40
  • $\begingroup$ @BoLe, um it appears to work, I just ran my code again on a fresh kernel and it worked. $\endgroup$
    – s0rce
    Commented Apr 10, 2013 at 21:48
  • 1
    $\begingroup$ It works, it composes the images but not correctly. To see this change 10 to 1 in both RandomChoice and run a few times if necessary. $\endgroup$
    – BoLe
    Commented Apr 10, 2013 at 21:57
  • $\begingroup$ @BoLe, you're right! fixed with ImageValuePositions and PlotRange. $\endgroup$
    – s0rce
    Commented Apr 10, 2013 at 23:48

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