[Tig] LUTs for Video File Codecs - bigger problems

glenn chan glennchan
Thu May 4 10:36:28 BST 2006


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Thanks to oktobor for supporting the TIG
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This is somewhat of a digression, but one very common encoding mistake that
I see is an encode does not properly convert from studio RGB (16-235) to
computer RGB (0-255) color space.  There are a lot of colorist reels like
this... which is a little unfortunate.  When it happens, the blacks are
elevated/milky/grey-ish, and the whites are too dim.  Contrast and
saturation will be reduced.  The difference is much, much more visible that
slight errors/imprecisions in dithering (the numerical difference is up to
around 40 times greater).

Of much lower importance, you can also make an improvement by changing the
"surround" that a web encode is shown in.  A dark background color will best
approximate the grading environment.  This is particularly relevant for
colorist reels.  A white surround will make differences in white balance
very obvious... a dark surround will hide these differences (because your
eye's white balance will drift).  When intercutting various pieces together
into a reel, there will be many very rapid changes in white balance.  These
changes will be a lot more obvious than in the grading environment... which
may be undesireable if you did not want jarring jumps in white balance.
Jarring jumps in white balance is one reason why people want to color
correct their footage in their first place... although in the context of
music videos, you can get away with anything (i.e. jump cuts... normally
considered an "error" is not an error in music videos).

2- If you want good encodes, the most pragmatic way of going about it is to
do a better encode.  The colorist can't really do all that much to improve
the encode, while better encoder can (i.e. a better quality encoder, manual
keyframes).  You can generally encode something well enough that you do not
see artifacts.
Perhaps one thing worth doing is knowing what to avoid.  i.e. a subtle grain
or noise will usually be lost on the audience, as many encoders will smooth
out the video at lower bitrates.  The resolution is typically lowered,
hiding fine detail like grain.
You could also be aware that a white surround (common in webpages) will
cause white balance to be a lot more noticeable.


Glenn
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