On Sat, 18 May 2002 08:25:08 +0300, in rec.video.desktop
"Jukka Aho"
>Nermin Tanovic
>
>> When I move the MPG file from SD drive to my PC using USB
>> port, the movie becomed much darker that when I play it on
>> camcorder.
>Computers typically process graphics in RGB color space
>(i.e. each primary color has its own channel), whereas
>a digital camcorder does it in YCbCr space (there is a
>brightness part of the signal and two color difference
>channels with lower resolution.)
>
>The thing is, the YCbCr format in the camcorder has some
>headroom below "black" and above "white" for additional
>processing (for example, to correct overexposured parts
>of the image). On a scale from 0 to 255, black is defined
>as "16" and white as "235".
>
>Computer RGB formats do not have the same kind of concept
>- they use the full scale from 0 to 255, where black is 0
>and white 255.
>
>* * *
>
>Now we have a dilemma: when converting YCbCr video data
>to RGB for computer processing, how can we retain the
>extra headroom and footroom in the values? If we simply
>clip them off and rescale the 16...235 range to 0...255,
>we do not any longer have that extra data to work with.
>
>Usually, this is solved simply by using the same limited
>brightness range on a computer, too. Even though the image
>will look dark and washed out when processing it on the
>computer, it looks fine when put back on video tape. And
>you get to keep that extra image data for editing. As
>almost all video capturing/processing works this way,
>it is hard to avoid. The same problem exists with playing
>DVDs on a computer, too - this is why the DVD player
>applications usually have contrast and brightness controls.
>
>However, if your final target _is_ computer displays and
>applications, not video devices, you should correct the
>brightness range for better viewing on a computer monitor.
>One tool for doing this is Avery Lee's VirtualDub and its
>"levels" filter. See
>
>VirtualDub can also frameserve to other applications,
>such as MPEG encoders.
>
>Unfortunately, you must re-encode your MPEG video if you
>want to make any permanent changes to it, and this will
>always reduce quality. Another option would be simply
>using a media player application that has brightness
>controls, such as Zoom Player:
>
>
>You could also be interested in Ben Rudiak's lossless
>HuffYUV codec for storing intermediate video files,
>should you need them at any point:
>
>
>For some background reading, see Charles Poynton's
>excellent Color FAQ:
>
>