[Tig] HD monitor vs projector grading

Bob Friesenhahn bfriesen
Fri Mar 17 16:30:19 GMT 2006


--
Thanks to oktobor for supporting the TIG
--
On Fri, 17 Mar 2006, Adrian Thomas wrote:
>> 
>>                    As BOB pointed out, if all current TFT and or LCD 
>> monitors are working in a 8 bit space, than how do we see them as an 
>> option while grading on a GUI monitor?
>
> UP TO 8 bits, many are 6 bit precision.

A very good point.  Existing LCD panels are designed to emulate the 
transfer characteristics of a CRT (sRGB or Rec.709) with a built-in 
gamma curve.  A gamma curve of 2.2 is bit-wise most efficient for 
tranferring intensity information since each step is a visually-linear 
step in terms of intensity.  Often the gamma curve may be 
selected/tailored using the LCD's menu, and some LCDs allow loading a 
curve via proprietary means.  LCD technology does not provide a 
natural gamma curve so the panel needs to include a mapping table 
between "CRT" 8-bit values and the values required by the LCD panel 
driver circuitry.  Depending on what the LCD panel drivers are capable 
of, there may be a mismatch between the 8-bit gamma-corrected codes 
and the values the panel supports internally.  This can result in 
repeated (or lost) codes or mapping to the wrong value.  So more than 
one 8-bit value could map to the same displayed intensity level, or 
perhaps the panel actually supports some intermediate levels but the 
gamma table does not make use of them because they are not a good 
match.

While gamma-corrected codes are the most bit-wise efficient way to 
transfer intensity values to the display, it is likely that some other 
transfer function (using more bits) would allow much better use of the 
LCD transfer curve if the LCD panel drivers and display interface 
allowed for it.

The approach that the eCinema Systems folks are using is restricted to 
making use of commercial LCD panels since manufacturing these panels 
requires a billion dollar investment.  They (eCinema) can QC panels 
and discard ones with defective pixels, change to an improved 
backlight, and provide a superior mapping between the codes sent for 
display and the LCD driver electronics codes.  For motion video, 
pre-compensation of displayed values based on inter-frame analysis may 
offer improved results.

Bob
======================================
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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