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Old 20th October 2009, 03:56   #1  |  Link
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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?
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Old 20th October 2009, 04:11   #2  |  Link
Dark Shikari
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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.
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Old 21st October 2009, 03:09   #3  |  Link
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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 14:22.
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bitrate, letterbox, quality

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