Multimedia Course Notes

[Lawrence D'Oliveiro-2]

Been reading these online notes
<http://www.cs.cf.ac.uk/Dave/Multimedia/index_html> for a course on
multimedia. Just noticed on this page
<http://www.cs.cf.ac.uk/Dave/Multimedia/node136.html>, it says:

     Using the traditional (and rather subjective) television measurement
     of lines of horizontal resolution DVD can have 540 lines on a
     standard TV (720/(4/3)) and 405 on a widescreen TV (720/(16/9)).

Actually, that’s wrong. The 4:3 factor is the “Kell factor”, it has
nothing to do with the aspect ratio of the screen. Thus the same 540
value applies to both narrowscreen and widescreen formats.

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[Lawrence D'Oliveiro-2]

Where do those "4:2:2", "4:1:1" and "4:2:0" numbers come from
<http://www.cs.cf.ac.uk/Dave/Multimedia/node196.html>? I know they're
commonly used, but they don't really seem to mean anything.

If the second and third numbers refer to the downsampling factors in the
vertical and horizontal directions respectively, then the numbers should
be "4:2:1", "4:2:0.5" and "4:1:1".

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[Ilya Murav'jov]

Lawrence D'Oliveiro пишет:
> Where do those "4:2:2", "4:1:1" and "4:2:0" numbers come from
> <http://www.cs.cf.ac.uk/Dave/Multimedia/node196.html>? I know they're
> commonly used, but they don't really seem to mean anything.
>
> If the second and third numbers refer to the downsampling factors in the
> vertical and horizontal directions respectively, then the numbers should
> be "4:2:1", "4:2:0.5" and "4:1:1".
>
As far as I understand the second number shows downsampling factor
horizontally (it is equal for both chroma). Third number shows nothing
real, just mapping to a particular schema (IMHO).

Ilya


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[Ben Hutchings-3]

On Mon, 2009-11-09 at 23:15 +1300, Lawrence D'Oliveiro wrote:
> Where do those "4:2:2", "4:1:1" and "4:2:0" numbers come from
> <http://www.cs.cf.ac.uk/Dave/Multimedia/node196.html>? I know they're
> commonly used, but they don't really seem to mean anything.
>
> If the second and third numbers refer to the downsampling factors in the
> vertical and horizontal directions respectively, then the numbers should
> be "4:2:1", "4:2:0.5" and "4:1:1".

It's a ratio between:
- number of luma samples per line, l
- greatest number of chroma samples per line, c1
- least number of chroma samples per line, c2
usually scaled so the largest number is 4.

In practice c2 can either be equal to c1 (no vertical subsampling) or
zero (2x vertical subsampling).

Ben.

--
Ben Hutchings
The generation of random numbers is too important to be left to chance.
                                                            - Robert Coveyou


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[Lawrence D'Oliveiro-2]

Ben Hutchings wrote:

> - least number of chroma samples per line, c2

But this is never zero. Unless you truncate it to an integer.

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[Ben Hutchings-3]

On Tue, 2009-11-10 at 14:32 +1300, Lawrence D'Oliveiro wrote:
> Ben Hutchings wrote:
>
> > - least number of chroma samples per line, c2
>
> But this is never zero. Unless you truncate it to an integer.

Per scan-line, not per line of chroma samples.  To a first
approximation, the sample sites for 2x subsampling look like:

YCY YCY  4:2
Y Y Y Y   :0

Ben.

--
Ben Hutchings
The generation of random numbers is too important to be left to chance.
                                                            - Robert Coveyou


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[Lawrence D'Oliveiro-2]

Ben Hutchings wrote:
 >
> On Tue, 2009-11-10 at 14:32 +1300, Lawrence D'Oliveiro wrote:
 >
>> Ben Hutchings wrote:
>>
>>> - least number of chroma samples per line, c2
 >
>> But this is never zero. Unless you truncate it to an integer.
>
> Per scan-line, not per line of chroma samples.  To a first
> approximation, the sample sites for 2x subsampling look like:
>
> YCY YCY  4:2
> Y Y Y Y   :0

But those chroma samples are shared between the lines. So the value is
0.5, not 0. Unless you truncate it to an integer.

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[Ben Hutchings-3]

On Tue, 2009-11-10 at 20:27 +1300, Lawrence D'Oliveiro wrote:

> Ben Hutchings wrote:
>  >
> > On Tue, 2009-11-10 at 14:32 +1300, Lawrence D'Oliveiro wrote:
>  >
> >> Ben Hutchings wrote:
> >>
> >>> - least number of chroma samples per line, c2
>  >
> >> But this is never zero. Unless you truncate it to an integer.
> >
> > Per scan-line, not per line of chroma samples.  To a first
> > approximation, the sample sites for 2x subsampling look like:
> >
> > YCY YCY  4:2
> > Y Y Y Y   :0
>
> But those chroma samples are shared between the lines.

Kind of.  This gets complicated when you consider how it interacts with
interlacing.

> So the value is 0.5, not 0. Unless you truncate it to an integer.

Yes, well, maybe it's not so useful to treat the third number as part of
a ratio.

Ben.

--
Ben Hutchings
The generation of random numbers is too important to be left to chance.
                                                            - Robert Coveyou


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