FilterBoxBorderGetBufferSize is not monotonically increasing with image size

Hello,

I am having some trouble with the buffer sizes returned by FilterBoxBorderGetBufferSize.

According to the documentation, roiSize is the "Maximum size of the destination image ROI". As I understand it the buffer size returned should be valid for any destination image ROI smaller or equal to roiSize. This is important for me since I am allocating a buffer once (for a fixed filter size) and reusing it many times on images that are smaller than some known bound. After some testing however, I found out that the returned buffer size is sometimes larger for smaller images. Here is an example:

	int bufferSizeA = -1;
	int bufferSizeB = -1;
	
	const auto filterSize = IppiSize{ 7, 7 };
	
	const auto imageSizeA = IppiSize{ 13, 13 };
	const auto imageSizeB = IppiSize{ 14, 14 };
	
	ippiFilterBoxBorderGetBufferSize(imageSizeA, filterSize, ipp8u, 1, &bufferSizeA));
	ippiFilterBoxBorderGetBufferSize(imageSizeB, filterSize, ipp8u, 1, &bufferSizeB));

When I run it with the initial release of IPP2018 I get bufferSizeA = 3205 and bufferSizeB = 3184. Is this a bug or is my understanding of roiSize wrong?

I wrote a small program to try out various filter and image sizes. It seems there is always a drop when the width and/or height of the image is exactly twice the corresponding dimension of the filter. If that is true I suppose I could detect it and circumvent it by getting the buffer sizes for nearby image sizes and taking the maximum.

Any advice would be greatly appreciated.