I have to implement DCT-II and its inverse which is equivalently given by a forward DCT-III. All from real to real.
I have been checking some paper where it is specified how to do it. However, I already have kissfft as a dependency so I can use FFT and iFFT. I have implemented DCT-II using FFT and an extra space of 4N. I found three different ways to do it here.
For DCT-III, I wounder if I can do the inverse in a clever way (as in the last link) or if I have to implement it using the formula.