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For example, using ExampleData[{"TestImage", "Girl2"}]:

Mathematica graphics

what's a general way to make the background transparent? I've tried various combinations of EdgeDetect and Threshold and ImageAdd but can't figure it out. Thank you.

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Maybe something useful here: stackoverflow.com/questions/8041703/… –  Chris Degnen Aug 7 '12 at 20:40
@ChrisDegnen: that is a very useful post in general but simply running it on the girl image instead of the one given doesn't work because it's not a single colored background. Plus, parts of the girl herself have the same color as the background... –  Philip Maymin Aug 7 '12 at 22:26
@Verde: I'd be just as happy with a white background, it's not about the transparency issues of that post per se, just how to remove the background without affecting the head of the girl. Thanks. –  Philip Maymin Aug 7 '12 at 22:28
@Verbeia -- I, uh, um... don't know. That's certainly where I was aiming for. I guess I don't really know the difference between the two sites. Do you suggest I take it down from here and put it up there? PS that is great code at the link, but it also doesn't work immediately on the girl, even moving the locator around. –  Philip Maymin Aug 8 '12 at 11:01
You can flag to have it migrated if you want. Your question is about programming so it is on topic on StackOverflow, but really any Mathematica-related question is better off on the Mathematica specific site now. –  Verbeia Aug 8 '12 at 12:03
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migrated from dsp.stackexchange.com Aug 15 '12 at 8:05

This question came from our site for practitioners of the art and science of signal, image and video processing.

2 Answers

f[im_, n_] := Nest[Erosion[Dilation[#, 2], 2] &, im, n];
(*Get your image and Crop Nuisances*)
i1 = ImageTake[ExampleData[{"TestImage", "Girl2"}], 2 {10, -10}, {10, -10}];
(*Get edges and extend edges to border*)
b3 = Erosion[ColorNegate@f[EdgeDetect[i1, 1], 10], 1];
(*identify area& Get background pixels*)
b5 = Position[#, SortBy[Tally@Flatten@#, -#[[2]] &][[1, 1]]] &@ MorphologicalComponents@b3;
(*make a mask*)
b6 = Array[{0, 0, 0} &, Reverse@ImageDimensions@i1];
(b6[[##]] = {1, 1, 1}) & @@@ b5;
b7 = ColorNegate@ImageMultiply[ColorNegate@Blur@Dilation[Image@b6, 1], ColorNegate@i1]

Mathematica graphics

Mathematica graphics


It works quite well mostly

Mathematica graphics Mathematica graphics

The problems are due to the background not being connected or not being the biggest morphological component.

Edit Using RM's suggestion

Mathematica graphics

Not perfect, but better. (blondes still can't make it unharmed)

f[im_, n_] := Nest[Erosion[Dilation[#, 1], 1] &, im, n];
(*Get your image and Crop Nuisances*)
  i1 = ImageTake[j, 2 {10, -10}, {10, -10}];
  (*Get edges and extend edges to border*)
  b3 = Erosion[ColorNegate@f[EdgeDetect[i1, 1], 1], 1];
  b4 = (MorphologicalComponents@b3);
  (*identify area& Get background pixels*)
  (*select the component with most pixels at the border*)
  bckg = SortBy[
     Tally@Flatten@Join[#[[{1, -1}]], (Transpose@#)[[{1, -1}]]] &@
      b4, -#[[2]] &][[1, 1]];
  (*Its mean color*)
  meanBkg = Mean@Extract[ImageData[i1], Position[#, bckg] &@b4];
  (*identify other bckgnd components*)
  bckgs = 
       If[#1 < .08, #2, Sequence @@ {}] &, #] &@(EuclideanDistance[
        meanBkg, #] & /@ (Mean@
          Extract[ImageData[i1], Position[b4, #]] & /@ 
  (*replace by main bckgnd color*)
  b4 = b4 /. (Rule[#, bckg] & /@ bckgs);
  (*get all bckgnds together*)
  b5 = Position[b4, bckg];
  (*make a mask*)
  b6 = Array[{0, 0, 0} &, Reverse@ImageDimensions@i1];
  (b6[[##]] = {1, 1, 1}) & @@@ b5;
  b7 = ColorNegate@
    ImageMultiply[ColorNegate@Blur@Dilation[Image@b6, 1], 
     ColorNegate@i1]), {j, h}]
share|improve this answer
cool graphic - helps a lot to see how it's working! –  cormullion Aug 16 '12 at 17:42
What is b4? Not defined. –  Philip Maymin Aug 20 '12 at 5:06
@PhilipMaymin Sorry, it was a leftover of a previous version. Corrected. Thanks! –  belisarius Aug 20 '12 at 5:11
your algorithm doesn't like blondes and golden hair? Looks like one more kicked into the CW bin... soon anyway :) –  rm -rf Aug 20 '12 at 5:18
aha! I hadn't actually noticed that the figures in the first column have backgrounds with a radial lighting effect. Pretty impressive. –  rm -rf Aug 20 '12 at 8:45
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============>> 2-liner <<============

Let's import the image and crop boundary artifacts:

img = ImageCrop[ExampleData[{"TestImage", "Girl2"}], {200, 200}]

enter image description here

We have quite uniform background. So with Mathematica functions such as RegionBinarize where you can specify background test pixel, your task is about just a few lines:

      ColorNegate@RegionBinarize[img, {{10, 190}}, 0.1]], 3], 3], 2]]

enter image description here

Below is just a deeper insight on how it all owrks on pixel value level.

============>> Deeper Insight <<============

Now lets get the ImageData and see how the RGB pixel values are distributed on a diagonal across the image:

data = ImageData[img];
ListPlot[Transpose[data[[#, #]] & /@ Range[200]]]

enter image description here

Because background is located above approximately 0 to 60, i'll pick number 30 (1st tuning parameter) as a test pixel and plot EuclideanDistance of all other pixels from that test pixel:

ListPlot[EuclideanDistance[#, data[[30, 30]]] & /@ (data[[#, #]] & 
/@Range[200]), Filling -> 0]

enter image description here

Based on this we can say that roughly background is everything below threshold 0.1 (2nd tuning parameter) and replace all those pixels with white {1,1,1} pixel:

ndata = data /. {x_, y_, z_} /; 
    EuclideanDistance[{x, y, z}, data[[30, 30]]] < .1 -> {1, 1, 1};

Based on that create mask and to avoid pixelation at the edge Blur it at some radius value (3rd tuning parameter)

nimg = Blur[ColorNegate@Erosion[DeleteSmallComponents[
     Dilation[ColorNegate@Binarize[Image[ndata], .99], 3]], 3], 5]

enter image description here

And finally mask the original image by adding the thresholded version of it:

ImageAdd[img, nimg]

enter image description here

In the same way you could add an alpha channel to background making it transparent. To play with the whole procedure make an app to tune in the 3 parameters described above. Note the little piece of background in the left bottom corner is removed now by tuning background pixel parameter.

Manipulate[ImageAdd[img, Blur[ColorNegate@Erosion[DeleteSmallComponents[
      Dilation[ColorNegate@Binarize[Image[data /. {x_, y_, z_} /; 
             EuclideanDistance[{x, y, z}, data[[px, px]]] < th -> {1, 
             1, 1}], .99], 3]], 3], bl]]
 , {{px, 30, "background"}, 1, 60, 1, Appearance -> "Labeled", ImageSize -> Small}
 , {{th, .1, "threshold"}, .01, .2, Appearance -> "Labeled", ImageSize -> Small}
 , {{bl, 2, "blur"}, 0, 10, 1, Appearance -> "Labeled", ImageSize -> Small}]

enter image description here

Additional parameters to add could be Binarize function ranges. Binarize[image,{Subscript[t, 1],Subscript[t, 2]}] creates a binary image by replacing all values in the range Subscript[t, 1] through Subscript[t, 2] with 1 and others with 0. ~ from Documentation.

share|improve this answer
There is a piece of background under your right armpit :) –  belisarius Aug 15 '12 at 9:38
@Verde removed now by playing with background pixel parameter ;-) –  Vitaliy Kaurov Aug 15 '12 at 9:46
+1 Nothing better than a clean armpit –  belisarius Aug 15 '12 at 9:57
you can make your two liner a one liner easily... –  rm -rf Aug 15 '12 at 19:34
Great answer, Vitaliy - I'm working through it and appreciating and learning from it. But a question: what do you mean by "Because background is located above approximately 0 to 60"? –  cormullion Aug 16 '12 at 17:41
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