ENH: Aligning streams on a power of 2 grid with respect to a decoding tile size

Code/Common/otbImageRegionSquareTileSplitter.h

@@ -124,7 +124,7 @@ public:
   itkGetMacro(TileDimension, unsigned int);
 
 protected:
-  ImageRegionSquareTileSplitter() : m_SplitsPerDimension(0U), m_TileDimension(0), m_TileSizeAlignment(16) {}
+  ImageRegionSquareTileSplitter() : m_SplitsPerDimension(0U), m_TileDimension(0), m_TileSizeAlignment(0), m_TileHint(2048) {}
   virtual ~ImageRegionSquareTileSplitter() {}
   void PrintSelf(std::ostream& os, itk::Indent indent) const;
 
@@ -135,6 +135,7 @@ private:
   itk::FixedArray<unsigned int, VImageDimension> m_SplitsPerDimension;
   unsigned int m_TileDimension;
   unsigned int m_TileSizeAlignment;
+  unsigned int m_TileHint;
 };
 
 } // end namespace otb

Code/Common/otbImageRegionSquareTileSplitter.txx

@@ -33,8 +33,23 @@ ImageRegionSquareTileSplitter<VImageDimension>
   unsigned int theoricalNbPixelPerTile = region.GetNumberOfPixels() / requestedNumber;
   unsigned int theoricalTileDimension = static_cast<unsigned int> (vcl_sqrt(static_cast<double>(theoricalNbPixelPerTile)) );
 
-  // Take the next multiple of m_TileSizeAlignment (eventually generate more splits than requested)
-  m_TileDimension = (theoricalTileDimension + m_TileSizeAlignment - 1) / m_TileSizeAlignment * m_TileSizeAlignment;
+  m_TileDimension = m_TileHint;
+
+  // If we have less than one jpeg 2000 tile, split in sub-tiles
+  if(theoricalTileDimension < m_TileHint)
+    {
+    while(m_TileDimension > theoricalTileDimension)
+      {
+      m_TileDimension/=2;
+      }
+    }
+    else
+      {
+      while(2*m_TileDimension < theoricalTileDimension)
+        {
+        m_TileDimension*=2;
+        }
+      }
 
   // Minimal tile size is m_TileSizeAlignment * m_TileSizeAlignment
   if (m_TileDimension < m_TileSizeAlignment)
@@ -78,26 +93,47 @@ ImageRegionSquareTileSplitter<VImageDimension>
     itkExceptionMacro("Asked for split number " << i << " but region contains only " << numPieces << " splits");
     }
 
-  // Compute the split index in the streaming grid
-  unsigned int remaining = i;
+  // First, find the megatile index
+
+  unsigned int tilesPerMegaTilesPerDim = m_TileHint / m_TileDimension;
+
+  if(tilesPerMegaTilesPerDim == 0)
+    {
+    tilesPerMegaTilesPerDim =1;
+    }
+
+  unsigned int tilesPerMegaTiles = tilesPerMegaTilesPerDim*tilesPerMegaTilesPerDim;
+
+  unsigned int remaining = i / tilesPerMegaTiles;
   for (unsigned int j = VImageDimension - 1; j > 0; --j)
     {
-    splitIndex[j] = remaining / m_SplitsPerDimension[VImageDimension - 1 - j];
-    remaining = remaining % m_SplitsPerDimension[VImageDimension - 1 - j];
+    splitIndex[j] = tilesPerMegaTilesPerDim * remaining / (m_SplitsPerDimension[VImageDimension - 1 - j] / tilesPerMegaTilesPerDim);
+    remaining = remaining % (m_SplitsPerDimension[VImageDimension - 1 - j] / tilesPerMegaTilesPerDim);
     }
-  splitIndex[0] = remaining;
+  splitIndex[0] = tilesPerMegaTilesPerDim * remaining;
 
+  // Now splitIndex contains the megaTile index
+  remaining = i % tilesPerMegaTiles;
+  for (unsigned int j = VImageDimension - 1; j > 0; --j)
+    {
+    splitIndex[j] += remaining / tilesPerMegaTilesPerDim;
+    remaining = remaining % (tilesPerMegaTilesPerDim);
+    }
+  splitIndex[0]+= remaining;
+  
+  // Now split index contains the tile index
+    
   // Transform the split index to the actual coordinates
   for (unsigned int j = 0; j < VImageDimension; ++j)
     {
     splitRegion.SetIndex(j, region.GetIndex(j) + m_TileDimension * splitIndex[j]);
     splitRegion.SetSize(j, m_TileDimension);
     }
-
-  // Handle the borders
-  splitRegion.Crop(region);
-
-  return splitRegion;
+    
+    // Handle the borders
+    splitRegion.Crop(region);
+    
+    return splitRegion;
 }
 
 /**