[opendtv] Re: Popular screen aspect ratios

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sun, 16 Jan 2011 11:50:10 -0500

At 5:43 AM -0800 1/11/11, Ron Economos wrote:

Take a look at section 6.3.3 in H.262.


Thanks for the pointer. But this raises more questions than it answers.



***************************************************************************
If sequence_display_extension() is not present, then it is intended
that the entire reconstructed frame is intended to be mapped to the
entire active region of the display. The sample aspect ratio may be
calculated as follows:

SAR = DAR × horizontal_size / vertical_size

NOTE 1 - In this case horizontal_size and vertical_size are
constrained by the SAR of the source and the DAR selected.

If sequence_display_extension() is present then the sample aspect
ratio may be calculated as follows:

SAR = DAR × display_horizontal_size / display_vertical_size
These equations were not visible in my copy of the standard. I messed with Word a bit and they showed up. 


***************************************************************************

The SAR of a 720x480 stream with aspect_ratio_information
set to 16:9 and sequence_display_extension() NOT present
is 0.84375.


Correct. I would add:

The SAR of a 720x480 stream with aspect_ratio_information
set to 3:4 and sequence_display_extension() NOT present
is 1.125
This is what one would assume from your original question since you did not state that the sequence_display_extension is present. Remember you stated that the aspect_ratio_information was set to 3:4.
Thus I interpreted the use of display_horizontal_size and display_vertical size were being used to define a pan and scan window within a 4:3 raster.
This raises the question of how you signal the use of these values for pan and scan. I believe that this requires the use of picture_display_extension defined in 6.3.12. 




The SAR of a 720x480 stream with aspect_ratio_information
set to 4:3, sequence_display_extension() present,
display_horizontal_size = 540 and display_vertical_size = 480
is 0.84375.

Clearly, they are equivalent. The equation in the DVB specification
is just restating this section in H.262
I know this is a trick question, but assuming that sequence_display_extension() IS present, why would you do this? The 540 x 480 data is meaningless other than to use display_horizontal_size and display_vertical size to tell the encoder that the 3:4 aspect_ratio_ information is actually 9:16 at the display. Just weird...
It would be easier to set aspect_ratio_information the 9:16 and let the decoder place this in letterbox for its 4:3 output. 

Am I missing something here?
Or was this entire exercise just to demonstrate how easily one can be confused when implementing a standard? 

Regards
Craig


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