resizing 1080p to 720p / SAR | DAR

[lariva]

Here is what I have: 1080P movie in 2.40:1 aspect ratio. Resolution 1920x800. Given the resolution Aspect Ratio signaling is 1:1 (pixels are square)

When converting to 720p, I could:
1) Resize to 1280x534 and keep the pixels square
2) Resize to 1280x720 and set the sample aspect ratio to 27:20

Both will result in "proper" display and 720p compliance (right?)

Here is where things begin to drift into the area I don't fully understand:

If I reduce vertical resolution from 800 to 720 pixels there will be more data within the frame then if resizes to 534 pixels.

During the display process the player will enforce proper aspect ratio and if viewed on 1280x1024 monitor display 1280 horizontal and (automatically?) resize to 534 vertically to maintain the AR.

However if viewed on 1600x1200 monitor - the entire 720 vertical lines will be displayed and some stretching (with some obvious quality loss) will occur horizontally. However, the quality loss will be smaller then if both horizontal and vertical resize is taking place.

To that end - the question is: which is the "proper" way of encoding?
Keep the maximum number of vertical lines and control aspect ratio through SAR or control aspect ratio based on the number of pixes available?

Thanks.

[Dark Shikari]

Generally the "correct" way to encode 720p is to in fact have 720 lines, meaning 1280x720. That's what aspect ratio signaling is for, basically.

[lariva]

Thanks, thats what I thought, but I got really confused by a lot of 1080 & 720p movies floating around with square pixes (i.e. arbitrary vertical resolution based on the original aspect ratio).




Generally the "correct" way to encode 720p is to in fact have 720 lines, meaning 1280x720. That's what aspect ratio signaling is for, basically.

[Dark Shikari]

Thanks, thats what I thought, but I got really confused by a lot of 1080 & 720p movies floating around with square pixes (i.e. arbitrary vertical resolution based on the original aspect ratio).That's generally popular because its a nice way to save bitrate while still being able to claim the encode is 720p/1080p. But if you have the space, its better to save the full 720 lines.

[CruNcher]

Thanks, thats what I thought, but I got really confused by a lot of 1080 & 720p movies floating around with square pixes (i.e. arbitrary vertical resolution based on the original aspect ratio).

yep but it isn't the correct way and for sure won't be played back by all the chips that support only the standard resolutions (even if nowdays it's from a technical standpoint not important anymore (PC) ) it says 1080, 720 and 480 nothing else ;) so for best device interoperability it's better to be correct use those resolutions and ar signaling as the standard defines for HD-DVD and Blu-Ray else you risk playback problems. This allready starts with Nvidia and ATI Hardware accelleration try to playback the 500 kbit ED stuff non 480p here on doom9 with it you gonna see some nice problems (chroma shift and padding) :P

[lariva]

Thanks everyone.

Where is the SAR information is stored? Is it metadata within the video container? A part of the video stream?

P.S. Netgear 8000 does play it both ways but I'm in a total agreement that things should be done the right way or not at all.

What I'm curious about is how much stretching is going on when 720p video stream hits the TV screen. The TV is 1080p compliant, does that mean that it is equivalent of 1920 pixels horizontally? The PC has a locked resolution, the TV however.... not sure.




That's generally popular because its a nice way to save bitrate while still being able to claim the encode is 720p/1080p. But if you have the space, its better to save the full 720 lines.

[Brother John]

Where is the SAR information is stored?
MPEG-4 (both ASP and AVC) can have AR signalling in the video stream. Modern containers like Matroska and MP4 also support AR signalling. In fact, even AVI does, but there's no real world implementation that uses this feature.

I'm in a total agreement that things should be done the right way or not at all.
Yep, but the right way is subjective. It's always the right way for your specific playback requirements.

[akupenguin]

yep but it isn't the correct way and for sure won't be played back by all the chips that support only the standard resolutions
How many chips support only standard resolutions but allow arbitrary aspect ratios? Isn't the "standard" way to use square pixels and letterbox (Not that you'd ever catch me encoding something like that)? The reason you see files claiming to be 1080p but actually containing 1920x800 is that standard HD broadcasts/disks also often contain only 1920x800 of real pixels, and the encoder just removed the black borders.

[CruNcher]

hmm aku you sure a HD-DVD, Blu-Ray mastered this way would work ?, i mean their must be a reason why Studios still encode with the letterbox in the full resolution (firmware maybe doesn't accept any other resolution, whyever that might be the case), you could maybe croop and then resize to the full 1920x1080p and use AR then but that's not really optimal to circumvent such restrictions :P, but i dunno if this restrictions exist for HD-DVD and Blu-Ray players (Broadcom Chip) i didn't tested it yet, but i really doubt a HD-DVD or Blu-Ray player would playback 1920x800 somehow, most proably such resolutions won't be even accepted with the Authoring programms allready, for Asia players like Sigma Chip based ones i see no problem supporting such encodes tough at least they are mod16 i think.

[akupenguin]

I'm not saying that 1280x534 would work. I'm saying that 1280x720 anamorphic might not work either. Afaik, HD-Blu-DVD-Ray allows neither nonstandard resolutions nor HD with nonsquare pixels.
However, you suggestion is a bit safer in that a player that doesn't support anamorphic would probably play the 1280x720 version with the wrong aspect ratio, while a player that doesn't support arbitrary resolutions wouldn't play the 1280x534 at all.