It's looping from data to data+WIDTH*HEIGHT filling the memory with fillValue. I'd guess this is optimised code, because the loop runs backwards, i.e. from n to 0, which is arguably more efficient than looping from 0 to n, and they're not just using memset(). I'd also guess it was unnecessarily optimised, based on what the programmer thought would run slowly rather than based on hard data from a profiler.