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

[Ganesh Ajjanagadde]

On Fri, Nov 13, 2015 at 6:17 PM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
> 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

Technically C89, see e.g
http://port70.net/~nsz/c/c89/c89-draft.html#3.1.3.1, wikipedia, or
http://en.cppreference.com/w/cpp/language/floating_literal.
Nevertheless, will wait for comments especially from people who use
Microsoft stuff.

>
> [...]
> --
> 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
>
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