[FFmpeg-devel] [PATCH 3/4] avcodec/ppc/fdctdsp: use more accurate constants

[Ganesh Ajjanagadde]

On Mon, Nov 16, 2015 at 1:58 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> On Mon, Nov 16, 2015 at 1:20 PM, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
>> On Mon, Nov 16, 2015 at 6:24 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>>> On Mon, Nov 16, 2015 at 9:48 AM, Daniel Serpell <dserpell at gmail.com> wrote:
>>>> Hi!,
>>>>
>>>> El Fri, Nov 13, 2015 at 11:42:30AM -0500, Ganesh Ajjanagadde escribio:
>>>>> Whoever wrote this stuff had a pretty bad libm - digits differ pretty
>>>>> quickly.
>>>>
>>>> They where correctly rounded to 24bit precision. I don't know if that
>>>> was intentional, so I can't comment on the correctness of the patch.
>>>
>>> Unless I am off here, doing things at 24 bits makes very little sense:
>>> float itself offers 32 bits
>>
>> Actually floats offer 23-bits of precision for the significand (with 8
>> for mantissa and one sign bit) - 7-8 digits, so any more precision
>> than that couldn't be stored accurately anymore, so when using single
>> precision like this code, more precision is not needed.
>
> Point taken. However, the general remark still applies: precision is
> being lost unnecessarily at an intermediate computation stage since it
> is anyway a static computation.
>
>> Maybe it was even specifically wanted to have values that are
>> represented accurately, but I do not know the history of this code
>> either.

pushed

>>
>> - Hendrik
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