between implementations. And in part that will be how they utilise the
hardware available to them.
Essentially though any decoder has to unpack the image data into some
'baseband' format first and then it can be recompressed into another format
from there. A to C through B. In most sane implementations there should be
no loss (although potential rounding errors in YUV/RGB handling may exist)
in the A to B stage. Assuming the baseband intermediate has the capacity to
cope with unpacked video without further compression, rounding or averaging.
Although at this point colourspace could become an issue - I believe H.264
is a YUV-native codec (although it may support RGB also?) so unpacking that
into RGB will change the values slightly.
There is one other big factor in different implementations I can think of
though, and that is feature support. H.264 has a lot of feature-levels
(profiles) which are not universally supported, so I imagine it's possible
to have a properly encoded H.264 file created in a profile that your decoder
can't process. But as far as I can determine that should be more or less a
binary thing - it will work, or it won't.
I'll revise my initial statement to be: I've not heard of any qualitative
differences between H.264 implementations. The most obvious differences
would appear to be in speed.
Regardless, I don't think there's anything to be gained purely in terms of
quality from using the highest bitrates available in Avid's DNxHD codecs -
DNxHD 120 is very capable of accurately reproducing all the available image
data from the Canon DSLR video files, and I'd go so far as to say that DNxHD
36 does a very effective job also, with very minimal loss. Rendering effects
and colour corrections is a completely different matter, of course.
On Wed, Jun 16, 2010 at 1:07 PM, Tod Hopkins <hoplist@hillmanncarr.com>wrote:
>
>
> On Jun 15, 2010, at 3:38 AM, Dylan Reeve wrote:
> > In theory there should be no quality differences in decode methods.
>
> The real world is always more complicated. You must decode TO
> something, so separating the decode from encode is tough. In the real
> world you are really always transcoding. And there is a heck of a
> lot of room for variation and error. And software designers may take
> short cuts for efficiency if quality is not the priority.
>
> In other words, I seriously doubt there are no differences, though
> depending on how demanding you are, you may have to look hard to find
> them. The one likely difference will be decode speed. I'd be shocked
> if all decoders were similar in their decode speed.
>
> Players are clearly different. With all codecs, and especially high
> CPU decodes like H.264, the efficiency of the decode software is key.
> If you can't process the full bandwidth in time, you got bupkiss.
> True of all codec and player combos. Strengths and weaknesses.
>
> I don't hesitate to give high bandwidth H.264 to Mac users, but on
> Windows? Got to be careful. (Yes, this is partially a hardware issue,
> but not entirely). And have you ever compared WMV using Flip4Mac
> versus Windows Media Player. Bleh. Not even a close contest. And
> why does MPEG2 stink in Quicktime player but it's fine in DVD player
> and even better in Windows Media Player?
>
> I pays to test if you really care.
>
> Cheers,
> tod
>
> Tod Hopkins
> Hillmann & Carr Inc.
> todhopkins@hillmanncarr.com <todhopkins%40hillmanncarr.com>
>
>
>
--
Dylan Reeve
http://dylanreeve.com/
[Non-text portions of this message have been removed]
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