[opendtv] Re: Popular screen aspect ratios

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 11 Jan 2011 06:07:45 -0500

At 9:34 AM -0800 1/10/11, Ron Economos wrote:

Here's the text of the original 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?

In the DVB specification, there's an equation:

display_horizontal_size = 4 / 3 * horizontal_size / source aspect ratio

For 2.21:1 source aspect ratio, display_horizontal_size would
be 434.

Ron


Not certain what Ron is getting at with the equation and 2.21:1 info...
The equation may be correct, but the display_size data is there to set the size of the mask into which the source is moved during a pan and scan operation using the H&V centering metadata.
The aspect_ratio_information, which I missed when responding to the original question, is not ambiguous; it says that the 720 x 480 samples are to be scaled to a 4:3 aspect ratio. So Ron was incorrect in an earlier post that the defined stream would be shown as 16:9 on a widescreen display. 


For a 16:9 display, nothing needs to be done. The original
source is 16:9. The physical dimension of the 16:9
display will "stretch" the 720x480 decoded image.
Since the aspect ratio metadata specifies that the content is 4:3, the original source cannot be 16:9. 

So here is my final answer.

For a widescreen 16:9 display.

Decode the source to 720 x 480 samples.

Scale the source to 640 x 480

Mask the source to 540 x 480 and place it centered in a 854 x 480 container.

Display in pillarbox on a 16:9 display.
For a 4:3 display the only difference is that the 540 x 480 active image is placed into a 640 x 480 container with slight pillarbox. 

Regards
Craig




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