Updated geotrellis sigmoidal contrast calculation

Signed-off-by: Grigory Pomadchin <gr.pomadchin@gmail.com>
locationtech · Dec 23, 2020 · b630b12 · b630b12
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - Remove explicit unused Scaffeine dependency from projects [#3314](https://github.com/locationtech/geotrellis/pull/3314)
 - Remove an excessive close after the abort call in S3RangeReader [#3324](https://github.com/locationtech/geotrellis/pull/3324)
+- Fix sigmoidal contrast calculation [#3328](https://github.com/locationtech/geotrellis/pull/3328)
 
 ## [3.5.1] - 2020-11-23
 

diff --git a/raster/src/main/scala/geotrellis/raster/sigmoidal/SigmoidalContrast.scala b/raster/src/main/scala/geotrellis/raster/sigmoidal/SigmoidalContrast.scala
@@ -44,7 +44,9 @@ object SigmoidalContrast {
         (intensity + (1<<(bits-1))) / ((1<<bits)-1)
     }
 
-    val numer = 1/(1+math.exp(beta*(alpha-u))) - 1/(1+math.exp(beta))
+    // see https://legacy.imagemagick.org/Usage/color_mods/#sigmoidal
+    // and https://github.com/locationtech/geotrellis/issues/3326
+    val numer = 1/(1+math.exp(beta*(alpha-u))) - 1/(1+math.exp(beta*alpha))
     val denom = 1/(1+math.exp(beta*(alpha-1))) - 1/(1+math.exp(beta*alpha))
     val gu = math.max(0.0, math.min(1.0, numer / denom))