SP implement log, exp, none boosting methods

Author: breznak

src/examples/mnist/MNIST_SP.cpp

@@ -72,8 +72,8 @@ class MNIST {
 
 void setup() {
 
-  input.initialize({28, 28,1}); 
-  columns.initialize({28, 28, 16}); //1D vs 2D no big difference, 2D seems more natural for the problem. Speed-----, Results+++++++++; #columns HIGHEST impact. 
+  input.initialize({28, 28, 1}); 
+  columns.initialize({28, 28, 8}); //1D vs 2D no big difference, 2D seems more natural for the problem. Speed-----, Results+++++++++; #columns HIGHEST impact. 
   sp.initialize(
     /* inputDimensions */             input.dimensions,
     /* columnDimensions */            columns.dimensions,

src/htm/algorithms/SpatialPooler.cpp

@@ -758,8 +758,14 @@ void applyBoosting_(const UInt i,
 		    const vector<Real>& actualDensity,
 		    const Real boost,
 	            vector<Real>& output) {
-  //output[i] = exp((targetDensity - actualDensity[i]) * boost); //exponential boosting, default for Numenta
-  output[i] = log(actualDensity[i]) / log(targetDensity);
+
+  if(boost == SpatialPooler::BOOSTING_DISABLED) { //boosting disabled, skip
+    return;
+  } else if(boost == SpatialPooler::BOOSTING_LOG) {
+    output[i] = log2(actualDensity[i]) / log2(targetDensity);
+  } else { //exponential boost
+    output[i] = exp((targetDensity - actualDensity[i]) * boost);
+  } 
 }
 
 

src/htm/algorithms/SpatialPooler.hpp

@@ -63,6 +63,9 @@ static const int DISABLED = -1; //value denoting a feature is disabled
 class SpatialPooler : public Serializable
 {
 public:
+  static const constexpr Real BOOSTING_DISABLED = 0.0f;
+  static const constexpr Real BOOSTING_LOG      = -1234.0f;
+
   SpatialPooler();
   SpatialPooler(const vector<UInt> inputDimensions, const vector<UInt> columnDimensions,
                 UInt potentialRadius = 16u, Real potentialPct = 0.5f,
@@ -185,13 +188,21 @@ class SpatialPooler : public Serializable
         boost. Shorter values make it potentially more unstable and
         likely to oscillate.
 
-  @param boostStrength A number greater or equal than 0, used to
-        control boosting strength. 
-	No boosting is applied if it is set to 0.0, (runs faster due to skipped code).
-        The strength of boosting increases as a function of boostStrength.
-        Boosting encourages columns to have similar activeDutyCycles as their
-        neighbors, which will lead to more efficient use of columns. However,
-        too much boosting may also lead to instability of SP outputs.
+  @param boostStrength A number used to control boosting strength coeficient, 
+	 and mode of operation based on (reserved) values:
+	- `== SpatialPooler::BOOSTING_DISABLED`:
+	No boosting is applied if it is set, (runs faster due to skipped code).
+	- `== SpatialPooler::BOOSTING_LOG`:
+	Logarithmic boosting is used.
+	- `> 0.0`: 
+	Exponential boosting (default in Numenta) selected. `boostStrength` is 
+	used as a multiplacation constant.
+        The strength of boosting increases as a function of `boostStrength`.
+
+        Boosting encourages columns to have similar `activeDutyCycles` (aka. activation
+        frequencies) as their neighbors, which will lead to more efficient use of columns. 
+	This helps achieving the target sparsity of the output.
+	However, too much boosting may also lead to instability of SP outputs.
 
 
   @param seed Seed for our random number generator. If seed is < 0