From dd003b6a280038c3cb271cf7da953dba833972a9 Mon Sep 17 00:00:00 2001 From: Willem Jan Palenstijn Date: Sat, 8 Jun 2013 21:51:08 +0200 Subject: COMMON: Add basic documentation for RDFT --- common/rdft.h | 36 +++++++++++++++++++++++++++++++++++- 1 file changed, 35 insertions(+), 1 deletion(-) (limited to 'common') diff --git a/common/rdft.h b/common/rdft.h index 3386940404..76e95c363a 100644 --- a/common/rdft.h +++ b/common/rdft.h @@ -20,7 +20,7 @@ * */ -// Based on eos' (I)RDFT code which is in turn +// Based on xoreos' (I)RDFT code which is in turn // Based upon the (I)RDFT code in FFmpeg // Copyright (c) 2009 Alex Converse @@ -44,6 +44,40 @@ namespace Common { * * Used in engines: * - scumm + * + * + * It has four modes: + * + * Below, n = 1 << bits + * + * (I)DFT_R2C: + * input: + * n real floats + * output: + * n/2 complex floats (stored as real part followed by imag part). + * + * The output represents the first half of the (I)DFT of the input. + * If F is the complex (I)DFT of the input, then + * output[0] = F[0] + i * F[n/2] and + * output[k] = F[k] for k = 1 .. n/2-1. + * Note that F[0] and F[k] are real since the input is real, and + * the remaining values of F can be reconstructed from symmetry if desired. + * + * (I)DFT_C2R: + * input: + * n/2 complex floats + * output: + * n real floats + * + * The input encodes a complex vector x of length n that has the + * required symmetry to have a real (I)DFT: + * x[0] = Re(input[0]) + * x[k] = input[k] for k = 1 .. n/2-1 + * x[n/2] = Im(input[0]) + * x[k] = conj(input[n-k]) for k = n/2+1 .. n-1 + * The output is then the real (I)DFT of x, divided by 2. + * + * TODO: Is this division by 2 intentional? */ class RDFT { -- cgit v1.2.3