[FFmpeg-devel] [PATCH 4/4] avcodec/faandct: use more accurate constants

[Michael Niedermayer]

On Fri, Nov 13, 2015 at 11:42:31AM -0500, Ganesh Ajjanagadde wrote:
> This guarantees a "best effort precision".
> 
> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> ---
>  libavcodec/faandct.c | 29 +++++++++++++----------------
>  1 file changed, 13 insertions(+), 16 deletions(-)
> 
> diff --git a/libavcodec/faandct.c b/libavcodec/faandct.c
> index 473bc1d..c8ea328 100644
> --- a/libavcodec/faandct.c
> +++ b/libavcodec/faandct.c
> @@ -31,23 +31,20 @@
>  
>  typedef float FLOAT;
>  
> -//numbers generated by simple c code (not as accurate as they could be)
> -/*
> -for(i=0; i<8; i++){
> -    printf("#define B%d %1.20llf\n", i, (long double)1.0/(cosl(i*acosl(-1.0)/(long double)16.0)*sqrtl(2)));
> -}
> +/* numbers generated by arbitrary precision arithmetic followed by truncation
> +to 36 fractional digits (enough for a 128-bit IEEE quad, see /usr/include/math.h
> +for this approach). This guarantees a "best effort precision".
>  */
> -#define B0 1.00000000000000000000
> -#define B1 0.72095982200694791383 // (cos(pi*1/16)sqrt(2))^-1
> -#define B2 0.76536686473017954350 // (cos(pi*2/16)sqrt(2))^-1
> -#define B3 0.85043009476725644878 // (cos(pi*3/16)sqrt(2))^-1
> -#define B4 1.00000000000000000000 // (cos(pi*4/16)sqrt(2))^-1
> -#define B5 1.27275858057283393842 // (cos(pi*5/16)sqrt(2))^-1
> -#define B6 1.84775906502257351242 // (cos(pi*6/16)sqrt(2))^-1
> -#define B7 3.62450978541155137218 // (cos(pi*7/16)sqrt(2))^-1
> -
> -
> -#define A1 0.70710678118654752438 // cos(pi*4/16)
> +#define B0 1.000000000000000000000000000000000000L
> +#define B1 0.720959822006947913789091890943021267L // (cos(pi*1/16)sqrt(2))^-1
> +#define B2 0.765366864730179543456919968060797734L // (cos(pi*2/16)sqrt(2))^-1
> +#define B3 0.850430094767256448766702844371412325L // (cos(pi*3/16)sqrt(2))^-1
> +#define B4 1.000000000000000000000000000000000000L // (cos(pi*4/16)sqrt(2))^-1
> +#define B5 1.272758580572833938461007018281767032L // (cos(pi*5/16)sqrt(2))^-1
> +#define B6 1.847759065022573512256366378793576574L // (cos(pi*6/16)sqrt(2))^-1
> +#define B7 3.624509785411551372409941227504289587L // (cos(pi*7/16)sqrt(2))^-1

iam not sure how portable x.yL is
otherwise should be ok

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Its not that you shouldnt use gotos but rather that you should write
readable code and code with gotos often but not always is less readable
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 181 bytes
Desc: Digital signature
URL: <http://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20151114/63210f3c/attachment.sig>