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Metode Otsu

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Review A Threshold Selection Method from Gray-Level Histograms



Pengantar Metode Otsu

Metode otsu merupakan metode memilih batas ambang (threshold) secara otomatis dari tingkat keabu-abuan  histogram  melalui  analisis  diskriminan.  Analisis  diskriminan  tersebut diharapkan dapat  memaksimalkan  pemisahan  objek  (foreground)  dan  latar  belakang (background).
Citra memiliki  gray level  antara  rentang nilai 1 sampai L, dimana L=255. Sedangkan k merupakan nilai  ambang  yang  berada  diantara  nilai  tersebut.  Dengan  kata  lain,  background bernilai [1,,k] dan  foreground  bernilai [k+1,…,L]. Probabilitas setiap piksel, zeroth cumulative moment, first cumulative  moment, total nilai  mean, serta nilai ambang k  ditunjukkan berturutturut pada Persamaan 1, 2, 3, 4 dan 5.





Kelebihan metode otsu antara lain:

  • Metode yang diusulkan lebih sederhana karena pencarian threshold hanya memanfaatkan, zeroth cumulative moment, first cumulative moment, dan total nilai mean pada histogram
  • Menganalisa mean level kelas dan evaluasi kelas pemisahan Berdasarkan Gambar 1.e dan 1.f pada hasil penelitian Nobuyuki Otsu, terlihat jelas bahwa metode ini mampu membedakan objek dan background yang memiliki derau dengan tingkat gray level yang hampir mendekati warna objek.
  • Pemilihan optimal threshold stabil karena tidak berdasarkan nilai turunan untuk suatu kasus yang tidak global (local)




Kemahanan metode otsu antara lain:

  • Berdasarkan Gambar 3.e dan 3.f, terdapat detail objek yang hilang.
  • Objek kurang smooth


Saran perbaikan antara lain:

  • Metode  thresholding  dapat  dilakukan  dengan  mengumpulkan  cluster  pada  area  yang memiliki  similarity  sama  dan  distance  yang  saling  berdekatan  dengan  mengembangkan dendogram/tingkat  keserupaan  gray  level  dari  keabu-abuan  berdasarkan  inter  class  dan intra class sehingga pemisahan antar objek dan foreground lebih sempurna



Referensi:
Otsu, Nobuyuki, 1979, “A Threshold Selection Method from Gray-Level Histograms” IEEE Vol.9

No.1 Hal.62-68.
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