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I do know that black bars appear (like letter- and pillar- boxes) when video's aspect ratio doesn't match output device's aspect ratio. My questions are:

  1. How does these black bars get created? What creates them (in all possible cases)? Does software video player create them? Or television set? Are they part of transmitted signal? Or are they added at client's/viewer's end? Does VCR/DVR have something to do with them?

  2. On one occasion, I saw white bars (in pillarboxing)! What determines the color? How?

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Black is the preferred color because on a CRT display this "color" uses the least amount of energy (i.e. the electron beams are turned off for black, that is no color). Black is also the "natural" color where no image is scanned on a color CRT display. Plasma displays may use grey to match phosphor usage of the used versus unused display areas. White bars would probably only be acceptable on LCD, DLP and LCoS displays, which employ backlights and are effectively shutters. –  sawdust Apr 2 '12 at 1:13
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@sawdust: black is also to a great extent preferred since it distracts the least (in a subjective manner, for sure, but as a general statement) from the material shown. It is intuitive that the absence of image should be the same color as the absence of light (which, as you say, comes "free" with CRT). –  Daniel Andersson Apr 2 '12 at 7:49

2 Answers 2

  1. In all possible cases? Anything. They can be added at any point to make the video match the expected/desired output aspect ratio.

  2. The device/process doing the letterboxing.

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When resizing or encoding, or editing a video at a specific aspect
or
when outputting a file or to tape out of a video project
The operator selects the resolutions , and can have choices to fill, or maintain , or set specific aspects.

The standard fill of the pillars or letterboxing is black, some programs allow for easy choice of any color.

When editing or composing a video, a picture , or moving video even, could be applied to box an internal video at a different or same aspect. Somewhere around this process, a pillar box or letter boxing of the picture, could have ANYthing in it. You could literally put a picture of pillars on both sides of the picture, have scrolling texts or dancing pigs.

Take for example a video encode, with pillarboxed 3x4 inside of a 16x9 (wider) aspect frame, the whole video data does take the whole frame, and it does waste some data to have a larger frame around it, but it can help to maintain the correct aspect on todays usual wider screen displays. The pillarboxing might be applied there, The black edges of the screen compress very well using compression algorithms.

When authoring a DVD the software will send a Data Tag to the Disk that is being written to inform the DVD player what the aspect is intended to play back. This can often be overridden by consumer DVD players, televisions that can adjust aspect, and Computers which can be very flexable with aspects and resolutions.
The pillarbox or letterboxing can be added here to the video (during dvd encode) by resize (if needed) and box frame fill. Same basic thing applies with any black parts of the video frame requiring some data, but minimal data.
Some DVD creation programs can set a color for the box frame

After whatever methods were chosen for the resolution, and boxing, on the original, or sent via a transmission. It is time to display

Computer:
The display software (file or dvd), can be adjusted for a specific aspect.
Some display softwares allow for choosing the color on the blank rendered portions of the boxing. This would only apply to boxing of the frame that was not encoded in the original video. The whole window or screen is always refreshed, but any areas that are boxed at this point do not take a lot of effort to render.

DVD:
The consumer DVD can send out a signal of the picture it is decoding in a few different aspects. Remember the Tag that we put on, back when we made the DVD. The disk has a tag that can tell the DVD to output in 3x4 or 16x9. this can usually be overridden.
When the DVD boxes up the picture, it would be done by the video display chip in the dvd player, requiring very little effort to display the frame around the video. The DVD could also have video that is already boxed.

TVs Monitors
Older CRT tvs, are mostly stuck at 3-4 , so when a 16x9 signal is sent to them, it is the DVD player , or computer, that would box the frame up as needed to get to a "wide" aspect.
Most of the newer TVs can change the aspect , manually, and many can change it automatically. and can make a few changes in resolution. Any boxing that is needed or desired here can also be applied, any boxing that existed in the rest of the processes would also be there.

Monitors
The resolution of the computer monitor can change, and can have different aspects the software can be used to create any boxing, and alter the aspect to work with the aspect of the computer output and monitor, any original boxing that existed on the video would still exist.

Got it? confused yet?

Ok so lets take this really fast , to conclude.

The original camera source [ ] can be shot in different aspects [{ }] and be box framed itself [[]] Manipulations of size [ ͏ ] and res {͏ʬ} in editing can change all that any way it wants [[ ]] Output encodes can be sent -> out in different aspects [[[]]] , some things can be told [=] what do do /* but they can also be set manuel ? , The display can change it all around again [ ] [[ ]] . The viewers , the players , the encoders, the editers , the original source.

And You just want it to play right :-)

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Re: "When resizing or encoding, or editing a video at a specific aspect" While it does happen, it shouldn't except as a natural result of splicing together video clips with different aspect ratios (in which case *boxing is better than stretching, though cropping may sometimes be preferred). Otherwise, you end up with a 4:3 video that gets pillarboxed by a DVD publisher, and then gets letterboxed when played back on a 4:3 display... There's just no reason not to let the displaying device (or user) choose how to present a non-native aspect ratio picture. –  Lèse majesté Apr 3 '12 at 0:06
    
@Lèsemajesté Yes it would be a bad idea in most cases to lock in a 3-4 as 16-9 to a dvd, but I do it all the time for the portables, so everything is same always. What about 1-2.3 type though? the superwide stuff. –  Psycogeek Apr 3 '12 at 1:45

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