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bugs - How to improve the recognition quality when TextRecognize work on single character


Bug introduced in 11.0 and persisting through 11.3



From this answer, I doubt the capability to work on single character. So I give some test to verify this possibility. You can get my test imgs by this code


imgs = Binarize[
    Import[#]] & /@ {"https://i.stack.imgur.com/PvuFe.png", 
   "https://i.stack.imgur.com/bXHyv.png", 
   "https://i.stack.imgur.com/6Uxpo.png"};


Note the TextRecognize[#, "Character"] & /@ imgs will get nothing. We can get a example from the documentation in Examples/Applications, that indicate the appropriate mask maybe can improve the performance to get a character, but I don't very like this method. Because it is hard to build a mask for characters "i","j" like 


TextRecognize[#, 
   Masking -> MorphologicalTransform[#, "BoundingBoxes", Infinity], 
   RecognitionPrior -> "Character"] & /@ imgs


{{H,1,O},{m,Y},{d,d}}






  • Any workaround that can improve the recognition quality when TextRecognize work on single character


Or



  • If we want to improve the recognition quality by mask, how to build correct mask?


I desire to make my this answer better by TextRecognize.



Answer



I felt that I miss some simple way to unite closely located components and finally I found it: ImageForestingComponents (thanks to this answer)!




  • It is unfortunate that a link to this function isn't included in the "See Also" drop-down list neither on the Docs page for ComponentMeasurements, nor MorphologicalComponents, nor MorphologicalTransform. That's why I wasn't able to find it quickly...


I'll show how it can be used on the most problematic case with letter "i" which is formed by two not connected clusters of points:


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


image



With horizontal radius 1 and vertical radius 6 we get a segmentation where our letter "i" is counted as a single component:



ImageForestingComponents[i, Automatic, {1, 6}] // Colorize


image



Using ComponentMeasurements we can get the bounding boxes of our characters dropping the background:


c = ComponentMeasurements[ImageForestingComponents[i, Automatic, {1, 6}], 
  "BoundingBox", #"ConvexCoverage" < .9 &]



{2 -> {{66., 125.}, {79., 161.}}, 3 -> {{46., 61.}, {84., 98.}}}

HighlightImage[i, {Yellow, Rectangle @@@ c[[All, 2]]}]


image



TextRecognize accepts a set of Rectangle primitives as a Mask (it is documented under the Examples ► Options ► Masking sub-subsection):


TextRecognize[i, Masking -> Rectangle @@@ c[[All, 2]], RecognitionPrior -> "Character"]



{"i", "O"}

That's all. :^)


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