sliding window std filter for a one channel image

Question

I would like to apply "std filter" with a fixed patch size to a single channel image.
That is I want out[i,j] to equal the std of the pixels values at a neighborhood around img[i,j].

For those of you who are familiar with Matlab, I'm looking for the equivalent of

>> out = nlfilter( img, [P P], @std );

Is there a way to do this using ippi functions?

I came across ippiMean_StdDev but it seems to work for a single window, and not a sliding window (returning a scalar value rather than an array).
I also saw ippiRectStdDev but the manual states this function is for integral images - and I don't see how this applies in my case.

Does anyone has a working example or a more detailed manual for this?

Answer 1

Finally I figured it out.

1. input image must be in uint8 format
2. need to allocate 2 buffers (32bit float and 64bit float in my case)

3. sizes of arrays:
   input size HxW
   filter size, PxP
   result size H-P+1xW-P+1
   intermidate buffers (32f and 64f) sizes H+1xW+1 (note the plus one for integral image boundary!)

   // first, compute integral and sqIntegral image 
   IppiSize sz; sz.width = W; sz.height = H;
   ippiSqrIntegral_8u32f64f_C1R( uint8ImgPtr, W*sizeof(unsigned char), 
       d32ImgPtr, (W+1)*sizeof(float), 
       d64ImgPtr, (W+1)*sizeof(double), 
       sz, 0, 0 );
   // using the integral images compute the std filter result
   IppiRect rect = { 0, 0, P, P };
   IppiSize dsz; dsz.width = W-P+1; dsz.height = H-P+1;
   ippiRectStdDev_32f_C1R( d32ImgPtr, (W+1)*sizeof(float), 
       d64ImgPtr, (W+1)*sizeof(double), 
       dstPtr, (W-P+1)*sizeof(float), dsz, rect );