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I have an image segmentation and I'd like to label with a SwatchLegend[]. Here's a MWE:

SwatchLegend[Colorize[ImageData[Rasterize@Graphics@Annulus[], "Bit"]], {"0", "1"}]

The second example from the Colorize reference page:

cols = Range[Max[labels] + 1];
Legended[Colorize[labels], 
SwatchLegend[Map[RGBColor, ImageData[Colorize[{cols}]][[1]]], cols]]

example

Is there a solution that doesn't entail manually extracting the colors? How would one label the color regions with callouts or other labeling techniques?

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Maybe this way?

Legended[
 Colorize[ImageData[Binarize[Rasterize@Graphics@Annulus[]]]],
 SwatchLegend[
  Map[RGBColor, ImageData[Colorize[{{0, 1}}]][[1]]], {"0", "1"}]
 ]

enter image description here

I think Colorize expects a rank-two array of integers for two-dimensional images. Colorize is typically used as postprocessor of MorphologicalComponents and that returns precisely such matrices. Your ImageData was a of rank 3, the lowest level being for the color channels. So that might have provoked Mathematica to assume the image were 3D.

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  • $\begingroup$ That's exactly what I was hoping to avoid, but I think it might be the only way :( $\endgroup$ – M.R. Apr 26 '18 at 22:09
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Using Stefan's answer to this question:

Colorize; (* force autoloading *)

img = Import["https://i.stack.imgur.com/e6aAs.png"]

test image

labels = MorphologicalComponents[img];
ll = Select[Union[Flatten[labels]], Positive];

cols = RGBColor[Image`ColorOperationsDump`hashcolor[#]/255] & /@ ll;

Legended[Colorize[labels, ImageSize -> Medium], SwatchLegend[cols, ll]]

colorization with legends

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