[opendtv] Re: Popular screen aspect ratios

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sat, 8 Jan 2011 09:49:53 -0500

I wrote this response before reading Rons explanation. I believe what I wrote here is correct, and that Ron's explanation is lacking some specifics. (Craig) 



At 5:06 AM -0800 1/7/11, Ron Economos wrote:


Now for the aspect ratio bonus round question!

If a 720x480 MPEG-2 video bitstream with
aspect_ratio_information set to 4:3,
display_horizontal_size set to 540 and
display_vertical_size set to 480 is received
by a decoder, how should it display the
image on  a 16:9 output?
Good trick question. I had to research exactly what display_horizontal_size and 
display_vertical_size mean in MPEG-2 syntax and how they are used.
What is happening here is that the decoder is being asked to pan and scan the normal 4:3 source from the 720 x 480 anamorphic bitstream inside a 1.125:1 window (540 x 480 square pixel).
First step is to decode the bitstream and scale it to square pixel 640 x 480, to achieve the desired 4:3 aspect ratio. This would normally be presented in pillarbox on a 16:9 display.
But the use of the display size syntax, together with frame_centre_horizontal_offset and frame_centre_vertical_offset will cause the decoder to pan and scan the source within a 540 x 480 window at the center of the 854 x 480 widescreen output. The offset data determines how to move the source relative to the 540 x 480 window.
All that being said, there is still a bit of ambiguity in the MPEG-2 spec. The spec says that display_horizontal_size should be in the same units as horizontal_size, the syntax for defining the number of samples per line. But in the case Ron has presented, there are more samples than in the decoded image (i.e. 720 is scaled to 640 for presentation). So the ambiguity is whether the 540 display horizontal size is square pixels for the display or squeezed pixels like the source.
I believe my answer above is the correct one, but it is easy to see how an author could get confused when creating the bitstream and adding the pan and scan offsets. 

Regards
Craig


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