Pyramidal Histogram Of Oriented Gradients - Trilinear interpolation [closed]

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Hello im struggling with an implementation of this article: https://goo.gl/8mpIuq

I performed bilinear interpolation over the histogram bins and the results are better with this interpolation, however on page 2 its also mentioned that a trilinear interpolation is added when the pyramid level reaches level 2. I have read this answer HOG Trilinear Interpolation of Histogram Bins and I completely understand the formula behind trilinear interpolation over 2x2 block sizes, but in this article we have a 3x3 block size and 7x7 on pyramid level 3, because these block sizes yield the best results.

The main point about trilinear interpolation is that each pixel in a cell contributes to its local cell by a weight which is defined as the position in each block. I don't know how to represent the location of a pixel in 3x3 block size or what kind of formula should i use.

Thank you for all your help!