[FFmpeg-devel] A very basic DCT question

Michael Niedermayer michaelni
Fri Sep 19 18:56:12 CEST 2008

On Fri, Sep 19, 2008 at 09:01:34AM -0700, Roman Shaposhnik wrote:
> Guys,
> I'm going to ask the most basic (perhaps even stupid) question
> of all -- is there a way to hook DCT routines that do NOT
> scale their output by a factor of 8x to dsputil properly?

yes, but why? most likely its a matter of adjusting some constants
in the dct to make it return differently scaled output, this seems
to make more sense ...

> I haven't touched that part of FFmpeg for at least 5 years,
> but last time I did, I had an impression that it was
> possible to tell  DSPContext what scale factor was there.

> And since we are on this subject: can someone, please,
> provide a sufficient background for me to understand
> where did this 8x scale factor come from to begin with?
> Most of the DCT implementations I see around don't
> have anything like that.

higher precission. Throwing 3 bits away because most others do it
did not seem like a good idea. Its not as if these would need any
extra instructions, actually they needed less for at least one dct.


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

In a rich man's house there is no place to spit but his face.
-- Diogenes of Sinope
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