Use array indexing instead of postincrement: 5% speed increase.
Skip isnan() check for integer data: 35% speed increase.
Use isnan() instead of vtkMath::IsNan() for float data: 12%
Total integer speed increase: 40%
Total float speed increase: 18%
Note that using aligned 32-bit memory copies for RGBA instead
of using four 8-bit memory copies would provide a further 35%
speed increase, at the cost of code readability.