[FFmpeg-devel] A very basic DCT question

Baptiste Coudurier baptiste.coudurier
Fri Sep 19 21:38:37 CEST 2008


Michael Niedermayer wrote:
> On Fri, Sep 19, 2008 at 11:46:41AM -0700, Roman Shaposhnik wrote:
>> On Fri, 2008-09-19 at 18:56 +0200, Michael Niedermayer wrote:
>>> 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? 
>> well, my loud mouth got me into trouble again ;-) Remember the
>> discussion around MediaLib? this exercise is part of trying
>> to see how good of fit it can be for open source and I'm
>> trying to gather the data first (it actually distracts me
>> a bit from the DV stuff I promised to deliver, so I hope
>> to get it done ASAP).
>>> 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 ..
>> constants where? Suppose you have DCT_foo (which is closed
>> sourced and part of the vendor library) that follows an
>> exact definition of what DCT is supposed to do with its
>> input. It gives you the output that, as far as FFmpeg
>> is concerned, appears to be scaled by a factor of 1/8x.
>> So the question is: can its output be consumed as 
>> is by the rest of FFmpeg's machinery?
>> Just to be clear -- I'm not proposing any kind of extensions
>> or anything like that. If the answer is "no, you have to
>> scale it back by a factor of 8x" -- that's fine too.
>> For now, I have three libraries like that: IPP, medialib,
>> and the one from AMD.
> Why dont you write a dct that is better and faster and outputs things
> scaled correctly the way lavc expects? :)

Humm, and what if the scaled by 8 values do not fit in DCTELEM ? Would
it be better to avoid the scaling ? Im thinking about > 8bits supporting
codecs like h264, jpeg2k and dnxhd.


Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
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