Welcome to Doom9's Forum, THE in-place to be for everyone interested in DVD conversion.

Before you start posting please read the forum rules. By posting to this forum you agree to abide by the rules.

 

Go Back   Doom9's Forum > General > Newbies

Reply
 
Thread Tools Search this Thread Display Modes
Old 20th October 2009, 04:56   #1  |  Link
montython
Registered User
 
Join Date: Oct 2007
Posts: 22
letterbox - bitrate

Does letterboxing add to bitrate requirements assuming a constant visual quality?

For example, let's assume that we have a 512x288 source.
Using common codecs such as xvid, x264, wmv3:

1) Encode to 512x288 output at 1000 kbps.

2) Letterbox (not resize, just add black borders) the source to 1024x576. Encode at 1000 kbps.


I suppose the second one will have exactly the same visual quality, since the black borders are constant throughout the video. The information in the first frame (for black borders) will be replicated for the remaining frames without extra bitrate requirement.

Is this correct?
montython is offline   Reply With Quote
Old 20th October 2009, 05:11   #2  |  Link
Dark Shikari
x264 developer
 
Dark Shikari's Avatar
 
Join Date: Sep 2005
Posts: 8,689
No. For one, even signaling skip blocks costs bits, especially in video formats without arithmetic coding (everything but H.264). Usually it's one bit per 16x16 block of pixels.

Secondly, it completely breaks off-frame motion vectors, increasing the bit cost any time that an object or part of the scene moves in from outside the frame.

Third, if the black borders aren't on mod-16 boundaries, that further worsens compression on the edges.

Fourth, it breaks motion vector prediction for the top row of the frame in basically every video format newer than MPEG-2.
Dark Shikari is offline   Reply With Quote
Old 21st October 2009, 04:09   #3  |  Link
montython
Registered User
 
Join Date: Oct 2007
Posts: 22
Thank you for the brief but explanatory answer.

My example was an extreme one for demonstrative purposes. In fact, I noticed that I didn't use the correct terminology. My example is not letterboxing, it is windowboxing. About your first point: Even if we assume a non-H.264 codec, in practice, signaling skip blocks will only cost a few kilobits per second. In my extreme example, this effect is about 15 kbps.

About the motion compensation side of the story, I found a rather technical article about this. More or less I understand the effect of black borders. Thank you for pointing out.




*****
For those who are interested, the following are related topics.

Compression of black bars in AVC, does it take a lot of space?
Why does x264 waste bits on the borders?

Last edited by montython; 21st October 2009 at 15:22.
montython is offline   Reply With Quote
Reply

Tags
bitrate, letterbox, quality

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +1. The time now is 20:17.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2020, vBulletin Solutions Inc.