[linuxtv-softmpeg] Re: Chroma artifacting

[Peter Urbanec <linuxtv.org.dvb@urbanec.net>]

Peter Urbanec wrote:

> I'll have a read of the standard to see if I can get a clear answer on 
> how the streams are supposed to be encoded.

OK. Got a bit more info. It would appear that the coding of fields and 
frames can pretty much switch at any point in the video sequence. The 
chroma and luma ordering is different depending on whether the picture 
is progressive, or top_field_first=0 or 1. In particular, half the 
chroma samples for the first field actually reside in the second field 
which is yet to be seen. The top field will always contain the first 
half of chroma samples - irrespective of whether it comes first or second.

I have not had a chance to actually take a look at what the libsoftmpeg 
code does. I figured I'll share the information I have, just in case 
someone else gets a chance to investigate the source code before I can. 
I probably won't have any time to look at the code for the next month or 
so - I am flat out getting our product finished for NAB.

Excerpts from the Standard:

6.1.1.1   Progressive and interlaced sequences

This Specification deals with coding of both progressive and interlaced 
sequences. The output of the decoding process, for interlaced sequences, 
consists of a series of reconstructed fields that are separated in time 
by a field period. The two fields of a frame may be coded separately 
(field-pictures). Alternatively the two fields may be coded together as 
a frame (frame-pictures). Both frame pictures and field pictures may be 
used in a single video sequence. In progressive sequences each picture 
in the sequence shall be a frame picture. The sequence, at the output of 
the decoding process, consists of a series of reconstructed frames that 
are separated in time by a frame period.

6.1.1.8   4:2:0 format

In this format the Cb and Cr matrices shall be one half the size of the 
Y-matrix in both horizontal and vertical dimensions. The Y-matrix shall 
have an even number of lines and samples.

NOTE - When interlaced frames are coded as field pictures, the picture 
reconstructed from each of these field pictures shall have a Y-matrix 
with half the number of lines as the corresponding frame. Thus, the 
total number of lines in the Y-matrix of an entire frame shall be 
divisible by four.

The luminance and chrominance samples are positioned as shown in Figure 6-l.

X Represents luminance samples
0 Represents chrominance samples

----------------
|X X|X X|X X|X X
|0  |0  |0  |0
|X X|X X|X X|X X
----------------
|X X|X X|X X|X X
|0  |0  |0  |0
|X X|X X|X X|X X
----------------
|X X|X X|X X|X X
|0  |0  |0  |0
|X X|X X|X X|X X
----------------

Figure 6-l - The position of luminance and chrominance samples - 4:2:0 data


In order to further specify the organisation, Figures 6-2 and 6-3 show 
the vertical and temporal positioning of the samples in an interlaced 
frame. Figure 6-4 shows the vertical and temporal positioning of the 
samples in a progressive frame. In each field of an interlaced frame, 
the chrominance samples do not lie (vertically) mid way between the 
luminance samples of the field, this is so that the spatial location of 
the chrominance samples in the frame is the same whether the frame is 
represented as a single frame-picture or two field-pictures.

Top     Bottom
Field    Field
 X
 0
          X
 X
          0
          X
 X
 0
          X
 X
          0
 --------->
    Time

Figure 6-2 - Vertical and temporal positions of samples in an interlaced 
frame with top_field_first = 1



Bottom    Top
Field    Field
          X
          0
 X
          X
 0
 X
          X
          0
 X
          X
 0
 --------->
    Time

Figure 6-3 - Vertical and temporal positions of samples in an interlaced 
frame with top_field_first = 0



Frame
  X
  0
  X
  X
  0
  X
  X
  0
  X
  X
  0
  X
------->
Time

Figure 6-4 - Vertical and temporal positions of samples in a progressive 
frame


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   "Beauty lies in the hands of the beer holder."




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